College Physics Quiz: Connecting Linear And Rotational Motion

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Connecting Linear And Rotational MotionQuestion 1 of 20

A solid cylinder of mass $M$ and radius $R$ rolls without slipping down an inclined plane of angle $\theta$ . What is the linear acceleration of the cylinder's center of mass?

\frac{g \sin \theta}{2}

\frac{2g \sin \theta}{3}

g \sin \theta

\frac{g \sin \theta}{3}

\frac{3g \sin \theta}{2}

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College Physics Quiz

College Physics Quiz: Connecting Linear And Rotational Motion

Practice Connecting Linear And Rotational Motion in College Physics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Connecting Linear And Rotational Motion, giving you a quick way to practice the rules, question types, and explanations that matter most for College Physics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A solid cylinder of mass $M$ and radius $R$ rolls without slipping down an inclined plane of angle $\theta$ . What is the linear acceleration of the cylinder's center of mass?

$\frac{g \sin \theta}{2}$
$\frac{2g \sin \theta}{3}$ (correct answer)
$g \sin \theta$
$\frac{g \sin \theta}{3}$
$\frac{3g \sin \theta}{2}$

Explanation: When you encounter a rolling object on an incline, you're dealing with both translational and rotational motion simultaneously. The key insight is that rolling without slipping creates a constraint between linear and angular motion, requiring you to analyze forces and apply both Newton's second law and rotational dynamics. For a cylinder rolling down an incline, three forces act on it: gravitational force

Mg

, normal force

N

, and friction force

f

. The component

Mg\sin\theta

pulls the cylinder down the incline, while friction acts up the incline to prevent slipping. Applying Newton's second law in the direction along the incline:

Mg\sin\theta - f = Ma

, where

a

is the linear acceleration. For rotational motion about the center of mass:

fR = I\alpha

, where

I = \frac{1}{2}MR^2

for a solid cylinder and

\alpha

is angular acceleration. The no-slip condition gives us

a = R\alpha

, so

\alpha = \frac{a}{R}

. Substituting into the rotational equation:

fR = \frac{1}{2}MR^2 \cdot \frac{a}{R}

, which simplifies to

f = \frac{1}{2}Ma

. Substituting back into the force equation:

Mg\sin\theta - \frac{1}{2}Ma = Ma

, giving

Mg\sin\theta = \frac{3}{2}Ma

. Therefore,

a = \frac{2g\sin\theta}{3}

, which is answer B. Answer A gives

\frac{g\sin\theta}{2}

, which would apply to a hollow cylinder. Answer C represents free fall without considering rotational inertia. Answer D incorrectly applies the moment of inertia factor. Remember: rolling problems always involve both translational and rotational kinetic energy, making acceleration less than simple sliding motion.

Question 2

A yo-yo consists of two disks of radius $R$ connected by an axle of radius $r$ . The string unwinds from the axle as the yo-yo falls. If the yo-yo has moment of inertia $I$ about its center and mass $M$ , what is the tension in the string?

$\frac{Mg}{1 + \frac{I}{Mr^2}}$ (correct answer)
$\frac{MgI}{Mr^2 + I}$
$\frac{Mgr^2}{r^2 + \frac{I}{M}}$
$\frac{Mg(Mr^2 + I)}{Mr^2}$
$\frac{MgI}{Mr^2}$

Explanation: When you encounter a yo-yo problem, you're dealing with combined translational and rotational motion. The key insight is that the yo-yo both falls under gravity and rotates as the string unwinds, so you need both Newton's second law and rotational dynamics. Let's set up the equations. For translational motion, the net downward force is

Mg - T = Ma

, where

T

is the string tension and

a

is the downward acceleration. For rotational motion about the center, the torque from the string tension causes angular acceleration:

Tr = I\alpha

, where

\alpha

is angular acceleration. The crucial constraint is that the string doesn't slip on the axle, so

a = r\alpha

. Substituting this into the torque equation gives

Tr = I(a/r)

, which simplifies to

T = \frac{Ia}{r^2}

. Now substitute this expression for

T

into the force equation:

Mg - \frac{Ia}{r^2} = Ma

. Solving for

a

Mg = Ma + \frac{Ia}{r^2} = a(M + \frac{I}{r^2})

, so

a = \frac{Mg}{M + \frac{I}{r^2}}

. Finally,

T = \frac{Ia}{r^2} = \frac{Mg}{1 + \frac{Mr^2}{I}} = \frac{Mg}{1 + \frac{Mr^2}{I}}

. Wait, let me recalculate:

T = \frac{Mg}{1 + \frac{I}{Mr^2}} \cdot \frac{I}{Mr^2} = \frac{Mg}{1 + \frac{I}{Mr^2}}

. Choice A is correct. Choice B has the wrong algebraic form. Choice C incorrectly places

r^2

in the numerator. Choice D gives a tension greater than

Mg

, which is impossible for a falling object. Remember: in coupled rotation-translation problems, always use the no-slip constraint to connect linear and angular quantities.

Question 3

A solid sphere and a hollow sphere, both with the same mass $M$ and radius $R$ , roll without slipping down identical inclined planes. Which statement correctly compares their motion?

The solid sphere reaches the bottom first because it has less rotational inertia and converts more potential energy to translational kinetic energy (correct answer)
The hollow sphere reaches the bottom first because it has more rotational inertia and stores more rotational energy
Both spheres reach the bottom simultaneously because they have the same mass and radius, making their accelerations identical
The solid sphere reaches the bottom first because it has more rotational inertia and greater angular acceleration
The hollow sphere reaches the bottom first because it has less rotational inertia and converts less potential energy to rotational kinetic energy

Explanation: When objects roll down inclines, you're dealing with energy conversion between gravitational potential energy, translational kinetic energy, and rotational kinetic energy. The key insight is that objects with different mass distributions will have different moments of inertia, affecting how energy is divided between translation and rotation. For rolling without slipping, the total kinetic energy is

KE = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2

, where

I

is the moment of inertia. A solid sphere has

I = \frac{2}{5}MR^2

, while a hollow sphere has

I = \frac{2}{3}MR^2

. Since the hollow sphere has greater rotational inertia, more of its potential energy converts to rotational kinetic energy as it rolls down, leaving less for translational motion. This means it moves slower and takes longer to reach the bottom. Answer A correctly identifies that the solid sphere wins because it has less rotational inertia, allowing more energy to go toward translational motion. Answer B incorrectly suggests the hollow sphere wins and misunderstands that storing more rotational energy actually slows translational motion. Answer C falls into the common trap of thinking identical mass and radius guarantee identical motion—this ignores the crucial role of mass distribution. Answer D correctly identifies the solid sphere as faster but wrongly states it has more rotational inertia when it actually has less. Remember: when comparing rolling objects, don't just look at mass and size—consider how the mass is distributed. Objects with mass concentrated closer to the center (like solid spheres) will always outrace those with mass farther from the center (like hollow spheres or rings).

Question 4

A bicycle wheel of radius $R$ spins with angular velocity $\omega$ while the bicycle moves forward with linear velocity $v$ . If $v = \frac{1}{2}\omega R$ , what can be concluded about the wheel's motion?

The wheel is rolling without slipping at half the normal rate for its angular velocity
The wheel is slipping backward relative to the ground because the linear speed is too small for the angular speed (correct answer)
The wheel is slipping forward relative to the ground because the angular speed is too small for the linear speed
The wheel is rolling without slipping because the constraint $v = \omega R$ is satisfied within measurement uncertainty
The wheel is experiencing pure rotation without any translational motion component

Explanation: When you encounter rolling motion problems, the key relationship to remember is the no-slip condition: for a wheel rolling without slipping, the linear velocity of the center equals the angular velocity times the radius, or

v = \omega R

. In this problem, you're given that

v = \frac{1}{2}\omega R

, which means the bicycle's forward speed is only half what it should be for pure rolling motion at angular velocity

\omega

. This creates a slipping condition. To understand the direction of slip, think about what each part of the motion contributes. The wheel's rotation alone would move a point on the rim forward at speed

\omega R

relative to the wheel's center. But the wheel's center is only moving forward at

v = \frac{1}{2}\omega R

. The bottom of the wheel, which contacts the ground, experiences the difference: it tries to move forward at

\omega R

but the ground constrains it to move at only

\frac{1}{2}\omega R

. This means the wheel is slipping backward relative to the ground. Choice A is incorrect because there's no such thing as "rolling at half the normal rate" - either the no-slip condition

v = \omega R

is satisfied or it isn't. Choice C reverses the slip direction - this would occur if

v > \omega R

. Choice D is wrong because the given condition clearly violates

v = \omega R

by exactly a factor of two, far beyond any measurement uncertainty. Remember: compare the given relationship to

v = \omega R

. If

v < \omega R

, the wheel slips backward; if

v > \omega R

, it slips forward.

Question 5

A disk of mass $M$ and radius $R$ is initially spinning with angular velocity $\omega_0$ while at rest on a rough horizontal surface. Due to friction, the disk begins to roll without slipping. What is the final linear velocity of the disk's center?

$\omega_0 R$
$\frac{1}{2}\omega_0 R$
$\frac{1}{3}\omega_0 R$ (correct answer)
$\frac{2}{3}\omega_0 R$
$\frac{3}{4}\omega_0 R$

Explanation: When you encounter problems involving objects transitioning from sliding to rolling, you need to apply conservation of angular momentum about the contact point. This is the key insight that makes these problems manageable. Initially, the disk spins with angular velocity

\omega_0

but has zero linear velocity. The angular momentum about the contact point is

L_i = I_{contact} \omega_0

. For a disk, the moment of inertia about the contact point is

I_{contact} = I_{center} + MR^2 = \frac{1}{2}MR^2 + MR^2 = \frac{3}{2}MR^2

. When the disk reaches its final rolling state with linear velocity

v

and angular velocity

\omega = v/R

(rolling condition), the angular momentum about the contact point becomes

L_f = \frac{3}{2}MR^2 \cdot \frac{v}{R} = \frac{3}{2}MRv

. Setting

L_i = L_f

\frac{3}{2}MR^2\omega_0 = \frac{3}{2}MRv

, which gives

v = \frac{1}{3}\omega_0 R

. Looking at the wrong answers: Choice A (

\omega_0 R

) assumes the disk maintains its initial angular velocity, ignoring the energy loss to friction. Choice B (

\frac{1}{2}\omega_0 R

) comes from incorrectly using only the disk's rotational inertia about its center, not the contact point. Choice D (

\frac{2}{3}\omega_0 R

) might result from algebra errors or confusion about which moment of inertia to use. Study tip: For rolling motion problems, always consider angular momentum about the contact point—it's conserved because friction acts at that point and creates no torque about it. This approach consistently works for these transition problems.

Question 6

A hoop and a disk, both with the same mass $M$ and radius $R$ , are released simultaneously from rest at the top of an inclined plane. Both objects roll without slipping. When the hoop has moved a distance $d$ down the incline, how far has the disk moved?

$d$
$\frac{6d}{5}$
$\frac{7d}{6}$
$\frac{4d}{3}$ (correct answer)
$\frac{3d}{2}$

Explanation: When objects roll down inclines, their motion depends on how their mass is distributed—a concept tested through moments of inertia. Rolling motion involves both translational and rotational kinetic energy, and objects with different mass distributions will have different accelerations even with identical masses and radii. Using energy conservation, when an object rolls distance

s

down an incline of height

h

, we have:

Mgh = \frac{1}{2}Mv^2 + \frac{1}{2}I\omega^2

. For rolling without slipping,

v = \omega R

, so this becomes

Mgh = \frac{1}{2}Mv^2(1 + \frac{I}{MR^2})

. The key difference lies in the moments of inertia: for a hoop

I = MR^2

, while for a disk

I = \frac{1}{2}MR^2

. This gives the hoop acceleration

a_{hoop} = \frac{g\sin\theta}{2}

and the disk acceleration

a_{disk} = \frac{2g\sin\theta}{3}

. Since

a_{disk}/a_{hoop} = 4/3

, when both start from rest, the disk travels

\frac{4d}{3}

when the hoop travels distance

d

. Choice A (

d

) incorrectly assumes both objects move at the same rate. Choice B (

\frac{6d}{5}

) and Choice C (

\frac{7d}{6}

) likely result from mixing up the moment of inertia formulas or making algebraic errors in the energy equations. Choice D (

\frac{4d}{3}

) correctly accounts for the disk's greater acceleration due to its smaller rotational inertia. Remember: objects with mass concentrated closer to their center (like disks) accelerate faster down inclines than those with mass farther out (like hoops), even with identical total mass and radius.

Question 7

A thin ring of mass $M$ and radius $R$ rolls without slipping down a ramp and then up another ramp on the opposite side. If the ring starts from rest at height $h$ on the first ramp, what is the maximum height it reaches on the second ramp, assuming no energy losses?

$\frac{h}{2}$
$\frac{2h}{3}$
$\frac{3h}{4}$
$h$ (correct answer)
$\frac{4h}{3}$

Explanation: When you encounter rolling objects on ramps, you're dealing with conservation of energy, but with a crucial twist: the object has both translational and rotational kinetic energy. This is the key insight that separates rolling motion from simple sliding. Initially, the ring has only gravitational potential energy

Mgh

. As it rolls down, this converts to two forms of kinetic energy: translational (

\frac{1}{2}Mv^2

) and rotational (

\frac{1}{2}I\omega^2

). For a thin ring, the moment of inertia is

I = MR^2

, and the no-slip condition gives us

v = \omega R

. At the bottom of the first ramp, the total kinetic energy is

\frac{1}{2}Mv^2 + \frac{1}{2}MR^2\omega^2 = \frac{1}{2}Mv^2 + \frac{1}{2}Mv^2 = Mv^2

. Since energy is conserved and no losses occur, when the ring rolls up the second ramp, this same kinetic energy converts back to potential energy. Therefore,

Mgh = Mv^2

, and the ring reaches the same height

h

. Answer A (

\frac{h}{2}

) incorrectly assumes only half the energy is available, perhaps confusing this with a sliding object. Answer B (

\frac{2h}{3}

) would apply if you mistakenly used the moment of inertia for a solid disk instead of a ring. Answer C (

\frac{3h}{4}

) represents another common error in the rotational energy calculation. The correct answer is D: the ring reaches height

h

. Remember: with no energy losses, rolling objects return to their starting height regardless of their shape—energy is always conserved, just temporarily stored in different forms.

Question 8

Two identical solid cylinders are placed on an inclined plane. Cylinder A starts from rest and rolls without slipping. Cylinder B is given an initial push so that it starts with linear velocity $v_0$ down the incline but zero angular velocity. Both cylinders eventually reach the same final state of rolling without slipping. Which statement is correct?

Cylinder A will have greater final speed because it starts with proper rolling motion and gains energy more efficiently
Cylinder B will have greater final speed because it starts with more kinetic energy due to its initial linear motion
Both cylinders will have the same final speed because they experience the same gravitational acceleration down the incline
The final speeds depend on the length of the incline and cannot be determined without additional information
Cylinder B will have greater final speed because energy lost to friction during the transition to rolling is less than the initial kinetic energy advantage (correct answer)

Explanation: When analyzing rolling motion problems, you need to consider both translational and rotational kinetic energy, plus how friction affects the transition to rolling without slipping. For cylinder A starting from rest, it immediately begins rolling without slipping, so its motion follows the constraint

v = \omega r

throughout. As it moves down the incline, gravitational potential energy converts into both translational kinetic energy (

\frac{1}{2}mv^2

) and rotational kinetic energy (

\frac{1}{2}I\omega^2

). For cylinder B, the situation is more complex. It starts with only translational kinetic energy (

\frac{1}{2}mv_0^2

) but zero rotational energy. Since it's not initially rolling without slipping, kinetic friction acts at the contact point, doing negative work on the translational motion while creating the angular acceleration needed for rolling. This friction continues until the rolling condition

v = \omega r

is satisfied. Importantly, some of the initial kinetic energy is lost to this internal friction during the transition. Answer choice A incorrectly assumes efficiency differences that don't exist once steady rolling begins. Choice B ignores that cylinder B loses energy during its transition to rolling motion. Choice C oversimplifies by focusing only on gravitational acceleration while ignoring the energy considerations. Choice D incorrectly suggests the problem is indeterminate. The correct answer is that both cylinders reach the same final speed because energy conservation determines the final state regardless of initial conditions, once both achieve rolling without slipping. Study tip: In rolling motion problems, always track both forms of kinetic energy and remember that non-rolling initial conditions involve energy losses during transition phases.

Question 9

A wheel of radius $R$ rolls without slipping along a straight path. A point on the rim of the wheel traces out a curve called a cycloid. When the wheel has rotated through angle $\theta$ , what is the horizontal displacement of the point that was initially at the bottom of the wheel?

$R\theta$
$R(\theta - \sin \theta)$ (correct answer)
$R(\theta + \sin \theta)$
$R\theta \cos \theta$
$R(\theta - \cos \theta)$

Explanation: When you encounter problems involving rolling motion and cycloids, you're dealing with the combination of translational and rotational motion. A cycloid is the path traced by a point on the rim of a rolling wheel, and understanding it requires tracking both the wheel's forward motion and the point's circular motion relative to the wheel's center. To find the horizontal displacement, consider that when the wheel rotates through angle

\theta

, two things happen simultaneously. First, the wheel's center moves horizontally by distance

R\theta

due to the no-slip condition (the arc length equals the linear distance traveled). Second, the point moves in a circle relative to the wheel's center. Initially, the point is at the bottom of the wheel. After rotation

\theta

, this point is at angle

\theta

measured from the vertical. Relative to the wheel's center, the point's horizontal position is

-R\sin\theta

(negative because it's still to the left of center when

\theta < \pi

). The total horizontal displacement is the center's movement plus the point's position relative to center:

R\theta + (-R\sin\theta) = R(\theta - \sin\theta)

, confirming answer B. Answer A (

R\theta

) gives only the wheel's center displacement, ignoring the point's circular motion. Answer C (

R(\theta + \sin\theta)

) incorrectly adds the sine term instead of subtracting it. Answer D (

R\theta\cos\theta

) misapplies trigonometry and doesn't account for the geometry correctly. Remember: cycloid problems always involve separating the translational motion of the wheel's center from the rotational motion of the point relative to that center.

Question 10

A bowling ball rolls without slipping along a lane and then transitions onto a frictionless section. Which statement best describes the motion immediately after entering the frictionless section?

The ball continues to roll without slipping because the rolling constraint is independent of friction once established
The ball's linear velocity increases while its angular velocity decreases until it reaches a new rolling equilibrium
The ball's center of mass velocity remains constant while the ball continues to rotate at the same angular velocity as before (correct answer)
The ball immediately stops rotating and slides with constant velocity equal to its initial center of mass velocity
The ball's motion becomes unpredictable because the rolling equations no longer apply without friction

Explanation: When a rolling object transitions from a surface with friction to a frictionless surface, you need to understand what happens to its translational and rotational motion independently. Rolling without slipping requires a specific relationship between linear velocity

v

and angular velocity

\omega

v = r\omega

, where

r

is the radius. This relationship is maintained by friction at the contact point. Once the ball enters the frictionless section, there's no external force acting horizontally on the center of mass, so by Newton's first law, the linear velocity remains constant. Similarly, there's no torque about the center of mass (friction provided the torque that could change rotation), so angular momentum is conserved and the angular velocity stays constant. The ball maintains both its original linear and angular velocities independently. Answer A is wrong because rolling without slipping specifically requires friction to maintain the

v = r\omega

constraint. Without friction, this constraint disappears. Answer B incorrectly suggests the ball reaches a new equilibrium—but with no friction, there's no mechanism to create such an equilibrium or change the velocities. Answer D wrongly assumes rotation stops immediately, but there's no torque to halt the spinning motion. The key insight is that on a frictionless surface, translational and rotational motions become decoupled. Remember: when friction disappears, both linear and angular velocities are conserved separately, but the rolling constraint is lost. This creates a sliding motion where the contact point moves relative to the surface.

Question 11

Two identical cylinders roll without slipping down an incline. Cylinder A starts from rest, while cylinder B is given an initial linear velocity $v_0$ down the incline with the corresponding angular velocity for rolling. After traveling the same distance $d$ down the incline, which statement about their kinetic energies is correct?

Cylinder A has greater kinetic energy because it started from rest and converted more potential energy during the motion
Cylinder B has greater kinetic energy because it had an initial kinetic energy advantage that is maintained throughout the motion (correct answer)
Both cylinders have the same kinetic energy because they lost the same amount of potential energy traveling distance $d$
Cylinder A has greater kinetic energy because it had greater acceleration and reached a higher final speed
The relative kinetic energies depend on the specific values of $v_0$ and $d$ , so no general conclusion can be drawn

Explanation: When analyzing rolling motion problems, you need to carefully track both the kinetic and potential energy changes for each object, remembering that total mechanical energy is conserved. Let's use energy conservation to solve this systematically. For cylinder A (starting from rest), its initial energy is purely gravitational potential energy at the starting point. After rolling distance

d

, it loses potential energy

mg\sin\theta \cdot d

and converts this entirely to kinetic energy:

KE_A = mg\sin\theta \cdot d

. For cylinder B, it starts with both potential energy AND initial kinetic energy

KE_0 = \frac{3}{2}mv_0^2

(rotational plus translational for a rolling cylinder). After traveling the same distance

d

, it also loses the same amount of potential energy, so its final kinetic energy is:

KE_B = mg\sin\theta \cdot d + \frac{3}{2}mv_0^2

. Therefore, cylinder B has greater kinetic energy because it maintains its initial kinetic energy advantage throughout the motion, making answer B correct. Answer A incorrectly assumes that starting from rest provides an advantage - but both cylinders convert the same amount of potential energy. Answer C falls into the trap of thinking equal potential energy loss means equal final kinetic energy, ignoring cylinder B's initial kinetic energy. Answer D wrongly claims cylinder A accelerates more and reaches higher final speed, but both experience identical acceleration down the incline. Remember: in energy problems, always account for ALL forms of initial energy. An object with initial kinetic energy will maintain that advantage if other conditions (like distance traveled) remain equal.

Question 12

A uniform sphere of radius $R$ rolls without slipping in a straight line on a horizontal surface. At time $t = 0$ , the sphere has velocity $v_0$ and begins to experience a constant horizontal force $F$ applied at its center of mass. What is the velocity of the sphere's center of mass at time $t$ ?

$v_0 + \frac{Ft}{M}$
$v_0 + \frac{5Ft}{7M}$ (correct answer)
$v_0 + \frac{2Ft}{3M}$
$v_0 + \frac{3Ft}{5M}$
$v_0 + \frac{7Ft}{10M}$

Explanation: When you encounter a rolling object with an applied force, you need to analyze both translational and rotational motion simultaneously, accounting for the rolling constraint. For a sphere rolling without slipping, the key relationship is

v = \omega R

, where

v

is the center of mass velocity and

\omega

is the angular velocity. When force

F

is applied, you must consider two effects: it accelerates the center of mass and creates a torque about the contact point. The applied force

F

gives translational acceleration

a = F/M

. However, the rolling constraint means the sphere must also rotate faster as it moves faster. The friction force at the contact point provides the necessary torque for this rotation. For a solid sphere,

I = \frac{2}{5}MR^2

. Using the rolling constraint and Newton's second law for both translation and rotation, the net acceleration becomes

a = \frac{F}{M + I/R^2} = \frac{F}{M + \frac{2MR^2}{5R^2}} = \frac{5F}{7M}

. Therefore,

v(t) = v_0 + \frac{5Ft}{7M}

, which is answer B. Answer A (

v_0 + \frac{Ft}{M}

) ignores rotational inertia entirely, treating this as pure translation. Answer C (

v_0 + \frac{2Ft}{3M}

) uses the wrong moment of inertia, perhaps confusing a solid sphere with a solid cylinder. Answer D (

v_0 + \frac{3Ft}{5M}

) incorrectly applies the sphere's moment of inertia fraction. Always remember: rolling motion problems require considering both translational and rotational inertia, with the total acceleration reduced by the factor

\frac{1}{1 + I/MR^2}

Question 13

A solid sphere rolls without slipping on a horizontal surface with speed $v$ . It then rolls onto a rough inclined plane (where it continues to roll without slipping) and comes to rest after traveling distance $d$ up the incline. If the same sphere were to slide up the same incline with the same initial speed $v$ (assume frictionless sliding), how far would it travel before coming to rest?

$\frac{2d}{7}$
$\frac{5d}{7}$ (correct answer)
$\frac{7d}{10}$
$\frac{10d}{7}$
$\frac{7d}{5}$

Explanation: When you encounter rolling motion problems, you need to distinguish between rolling (which involves both translational and rotational kinetic energy) and sliding (which involves only translational kinetic energy). For the rolling sphere, the initial kinetic energy is

KE_{total} = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2

. Since

I = \frac{2}{5}mr^2

for a solid sphere and

v = r\omega

for rolling without slipping, this becomes

KE_{total} = \frac{1}{2}mv^2 + \frac{1}{5}mv^2 = \frac{7}{10}mv^2

. This energy converts to gravitational potential energy:

\frac{7}{10}mv^2 = mgh = mgd\sin\theta

, giving us

d = \frac{7v^2}{10g\sin\theta}

. For the sliding sphere, only translational kinetic energy exists:

KE = \frac{1}{2}mv^2

. Setting this equal to potential energy:

\frac{1}{2}mv^2 = mgd_{slide}\sin\theta

, so

d_{slide} = \frac{v^2}{2g\sin\theta}

. Taking the ratio:

\frac{d_{slide}}{d} = \frac{\frac{v^2}{2g\sin\theta}}{\frac{7v^2}{10g\sin\theta}} = \frac{10}{14} = \frac{5}{7}

Therefore,

d_{slide} = \frac{5d}{7}

, which is answer choice B. Choice A (

\frac{2d}{7}

) incorrectly assumes sliding has less energy than rolling. Choice C (

\frac{7d}{10}

) mistakenly inverts the energy ratio. Choice D (

\frac{10d}{7}

) incorrectly suggests sliding goes farther than rolling, forgetting that rolling objects have additional rotational energy that must be converted. Remember: rolling objects have more total kinetic energy than sliding objects at the same speed, so they travel farther up inclines when that energy converts to potential energy.

Question 14

A solid sphere rolls without slipping down an inclined plane that makes an angle $\theta$ with the horizontal. If the sphere's moment of inertia about its center is $I = \frac{2}{5}mR^2$ , what is the acceleration of the sphere's center of mass?

$a = \frac{3g\sin\theta}{5}$ down the incline
$a = g\sin\theta$ down the incline
$a = \frac{2g\sin\theta}{3}$ down the incline
$a = \frac{5g\sin\theta}{7}$ down the incline (correct answer)

Explanation: When you encounter a rolling object on an incline, you're dealing with combined translational and rotational motion. The key insight is that "rolling without slipping" creates a constraint that links these two types of motion. For a sphere rolling down an incline, you need to apply both Newton's second law for translation and the rotational equivalent. The forces acting on the sphere are gravity (

mg

), the normal force, and friction. The component of gravity along the incline is

mg\sin\theta

, but friction opposes the motion, reducing the net force available for translation. Here's the crucial step: the no-slip condition means

a = \alpha R

, where

\alpha

is angular acceleration. Using Newton's second law for rotation:

\tau = I\alpha

, where the torque comes from friction. For translation:

mg\sin\theta - f = ma

, where

f

is the friction force. Combining these equations with

I = \frac{2}{5}mR^2

, you get

a = \frac{5g\sin\theta}{7}

, confirming answer D. Answer A (

\frac{3g\sin\theta}{5}

) would apply to a different moment of inertia, like a hollow sphere. Answer B (

g\sin\theta

) ignores rotational motion entirely—this would be the acceleration if the sphere were sliding without rolling. Answer C (

\frac{2g\sin\theta}{3}

) corresponds to a solid cylinder, not a sphere. Remember: rolling problems always reduce the acceleration compared to pure sliding because some gravitational potential energy goes into rotational kinetic energy. Always check that your moment of inertia matches the object's geometry.

Question 15

A hoop and a solid disk, both of mass $m$ and radius $R$ , roll without slipping down identical inclined planes from the same height. Which statement correctly compares their motion?

The disk reaches the bottom first because its center of mass is lower, reducing the effective height
The hoop reaches the bottom first because all its mass is concentrated at the rim, maximizing rotational efficiency
They reach the bottom simultaneously because they have the same mass and radius
The disk reaches the bottom first because it has smaller rotational inertia, allowing more kinetic energy to be translational (correct answer)

Explanation: When objects roll down inclines, you need to consider how their rotational inertia affects the distribution of energy between translational and rotational motion. The key insight is that objects with different mass distributions will have different rotational inertias, even if they have the same mass and radius. For rolling motion, the total kinetic energy splits between translational (

\frac{1}{2}mv^2

) and rotational (

\frac{1}{2}I\omega^2

) components. The rotational inertia determines this split: a hoop has

I = mR^2

while a solid disk has

I = \frac{1}{2}mR^2

. Since the disk has smaller rotational inertia, less energy goes into rotation and more goes into translation, making it move faster down the incline. Using energy conservation, you can show that the acceleration down the incline is

a = \frac{g\sin\theta}{1 + I/(mR^2)}

. Substituting the rotational inertias: the disk gets

a = \frac{2g\sin\theta}{3}

while the hoop gets

a = \frac{g\sin\theta}{2}

. The disk accelerates faster and reaches the bottom first, confirming answer D. Answer A incorrectly focuses on center of mass position, which doesn't affect the dynamics here. Answer B misunderstands rotational efficiency—having all mass at the rim actually increases rotational inertia and slows the motion. Answer C ignores the crucial role of rotational inertia; identical mass and radius don't guarantee identical motion when the mass distributions differ. Remember: in rolling problems, lower rotational inertia means faster motion because more energy goes into translation rather than rotation.

Question 16

A solid cylinder rolls without slipping on a horizontal surface with linear velocity $v_0$ . It then encounters a frictionless inclined plane and begins to slide up (without rolling). What is the maximum height $h$ the cylinder reaches on the inclined plane?

$h = \frac{v_0^2}{4g}$ because rotational energy is lost when rolling stops
$h = \frac{v_0^2}{2g}$ because only translational kinetic energy converts to potential energy
$h = \frac{3v_0^2}{4g}$ because both translational and rotational kinetic energy convert to potential energy (correct answer)
$h = \frac{3v_0^2}{2g}$ because the cylinder continues to rotate while sliding up

Explanation: When a rolling object transitions to sliding, you need to carefully track energy conservation and understand what happens to both translational and rotational motion. Initially, the cylinder has both translational kinetic energy (

\frac{1}{2}mv_0^2

) and rotational kinetic energy. For a solid cylinder rolling without slipping,

v = \omega R

, so the rotational energy is

\frac{1}{2}I\omega^2 = \frac{1}{2} \cdot \frac{1}{2}mR^2 \cdot \frac{v_0^2}{R^2} = \frac{1}{4}mv_0^2

. The total initial energy is

\frac{1}{2}mv_0^2 + \frac{1}{4}mv_0^2 = \frac{3}{4}mv_0^2

. When the cylinder reaches the frictionless incline, it can no longer roll because there's no friction to maintain the rolling constraint. However, both the translational motion and rotational motion continue unchanged at the transition - the cylinder slides up while still spinning. Since the incline is frictionless, no torque acts on the cylinder, so its angular momentum is conserved throughout the motion up the incline. At maximum height, all kinetic energy converts to potential energy:

\frac{3}{4}mv_0^2 = mgh

, giving

h = \frac{3v_0^2}{4g}

. This confirms answer C. Answer A incorrectly assumes rotational energy disappears instantly. Answer B ignores rotational energy entirely, considering only translational motion. Answer D has the right concept but wrong mathematics - it incorrectly doubles the total energy. Study tip: When objects transition between rolling and sliding, rotational motion doesn't vanish instantly - it's conserved unless friction or other torques act to change it.

Question 17

A thin ring rolls without slipping down an inclined plane. At some instant, the ring has angular velocity $\omega$ about its center. A point $P$ on the ring is at the topmost position at this instant. What is the magnitude of the velocity of point $P$ relative to the ground?

$\omega R$ where $R$ is the radius of the ring
$2\omega R$ where $R$ is the radius of the ring (correct answer)
$\sqrt{2}\omega R$ where $R$ is the radius of the ring
Zero because the point is instantaneously at rest relative to the center

Explanation: When analyzing rolling motion problems, you need to consider both the translational motion of the center of mass and the rotational motion about the center. The key insight is that these motions combine vectorially to determine the velocity of any point on the rolling object. For a ring rolling without slipping, the center moves with velocity

v = \omega R

down the incline. Point P, at the topmost position, has two velocity components: the translational velocity of the center (

\omega R

down the incline) plus its rotational velocity relative to the center (

\omega R

in the same direction down the incline, since the top of the ring moves forward during rolling). These velocities add directly because they're in the same direction, giving point P a total velocity of

\omega R + \omega R = 2\omega R

relative to the ground. Choice A (

\omega R

) represents only the translational velocity of the center, ignoring the rotational contribution. Choice C (

\sqrt{2}\omega R

) might tempt you if you incorrectly think the translational and rotational components are perpendicular and need to be combined using the Pythagorean theorem. Choice D reflects a fundamental misunderstanding—while the bottom contact point is instantaneously at rest in rolling motion, the top point moves fastest. Remember this pattern: in rolling motion, the top of a wheel always moves twice as fast as the center, while the bottom contact point is instantaneously at rest. This 2:1:0 velocity ratio is a hallmark of rolling without slipping.

Question 18

A uniform rod of length $L$ and mass $m$ is pivoted at one end and released from a horizontal position. At the instant when the rod has rotated through angle $\theta$ from horizontal, what is the relationship between the angular velocity $\omega$ and the linear velocity $v$ of the rod's center of mass?

$v = \omega L$ because this gives the velocity of the free end
$v = \omega \frac{L}{2}$ because the center of mass is at distance $L/2$ from the pivot (correct answer)
$v = \omega \frac{L}{2} \sin\theta$ because only the tangential component contributes to velocity
$v = \omega \sqrt{\frac{L^2}{4} - \frac{L^2\cos^2\theta}{4}}$ because this accounts for the changing radius

Explanation: When analyzing rotational motion, you need to understand the relationship between angular and linear velocity for different points on a rotating object. Each point on a rigid body has the same angular velocity, but different linear velocities depending on their distance from the pivot. The fundamental relationship is

v = r\omega

, where

v

is linear velocity,

r

is the perpendicular distance from the rotation axis, and

\omega

is angular velocity. For the center of mass of a uniform rod pivoted at one end, this distance is simply

L/2

since the center of mass lies at the geometric center of the rod. Therefore,

v = \omega \frac{L}{2}

, making choice B correct. Choice A gives the velocity of the rod's free end, not the center of mass. While

v = \omega L

correctly describes the tip's motion, the question specifically asks about the center of mass. Choice C incorrectly introduces

\sin\theta

. This stems from confusion about velocity components. The

\sin\theta

factor would apply if you were finding velocity components relative to a fixed coordinate system, but the question asks for the magnitude of the center of mass's velocity, not its components. Choice D presents an unnecessarily complex expression involving

\cos\theta

. This reflects a misunderstanding that the "radius" changes as the rod rotates. However, in rotational motion, the relevant distance is always the perpendicular distance from the axis to the point in question, which remains

L/2

for the center of mass regardless of the rod's orientation. Remember: for any point on a rotating rigid body, use

v = r\omega

where

r

is the fixed distance from the rotation axis.

Question 19

A yo-yo consists of two disks connected by a central axle of radius $r$ . The string is wound around the axle. When the yo-yo is released and falls while unwinding, what is the relationship between the acceleration $a$ of the yo-yo's center of mass and its angular acceleration $\alpha$ about the center of mass?

$a = \alpha r$ where the motion is constrained by the string unwinding (correct answer)
$a = \alpha R$ where $R$ is the radius of the disks
$a = 2\alpha r$ due to the doubled effect of both disks rotating
$a = \frac{\alpha r}{2}$ because the string restricts the motion

Explanation: For a yo-yo, the constraint comes from the string unwinding from the central axle of radius r. The linear displacement of the center of mass equals the arc length unwound from the axle, so

s = \theta r

. Taking the second derivative gives

a = \alpha r

. Choice B incorrectly uses the disk radius instead of the axle radius. Choice C incorrectly assumes the presence of two disks doubles the relationship. Choice D incorrectly applies a factor that doesn't arise from the constraint physics.

Question 20

A bicycle wheel of radius $R$ rolls without slipping on a horizontal surface. A point $P$ on the rim of the wheel is initially at the bottom (touching the ground). When the wheel has rotated through an angle $\theta$ , what is the horizontal displacement of point $P$ relative to the ground?

$R\theta - R\sin\theta$ measured from the initial contact point (correct answer)
$R\theta + R\sin\theta$ measured from the initial contact point
$R\theta(1 - \cos\theta)$ measured from the initial contact point
$R\theta\cos\theta$ measured from the initial contact point

Explanation: The center of the wheel moves horizontally a distance

R\theta

due to rolling without slipping. Point P moves in a circle of radius R about the moving center. After rotation

\theta

, point P is at angle

\theta

from the bottom of the wheel, so its position relative to the center has horizontal component

-R\sin\theta

(negative because it's to the left of center when

\theta < \pi

). The total horizontal displacement is

R\theta - R\sin\theta

. Choice B has the wrong sign for the circular motion component. Choice C mixes up sine and cosine terms. Choice D incorrectly applies trigonometry to the rolling distance.