Rolling
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AP Physics 1 › Rolling
A cylinder of radius $R$ rolls without slipping. If its center moves a distance $x$ along the floor, what is its angular displacement?
$\theta = 2\pi x$
$\theta = R/x$
$\theta = x/R$
$\theta = xR$
Explanation
This question examines angular displacement in rolling without slipping for AP Physics 1. Rolling without slipping equates linear distance x to arc length θR, yielding θ = x/R. Here, θ is angular displacement and R is radius. This relationship ensures consistent motion without slippage. Choice A, θ = xR, might be selected by swapping the formula incorrectly. A transferable strategy is to relate linear and angular displacements via radius in rolling problems.
A cylinder rolls without slipping at speed $v$. What is the instantaneous speed of the point in contact with the ground?
$\dfrac{v}{R}$
$2v$
$0$
$v$
Explanation
This problem examines the instantaneous velocity of the contact point in rolling without slipping motion. The key insight is that rolling without slipping means the contact point has zero velocity relative to the ground at each instant. This occurs because the point's rotational velocity (ωR = v backward) exactly cancels its translational velocity (v forward). The velocities add vectorially: v forward + v backward = 0. Choice A incorrectly considers only the translational motion, while choice B considers only the rotational motion. To understand rolling motion, remember that the contact point acts as an instantaneous pivot point with zero velocity.
A cylinder rolls without slipping at speed $v$ on a horizontal surface. What is the instantaneous speed of the point in contact with the ground?
$0$
$v$
$2v$
It depends on the coefficient of kinetic friction
Explanation
This question tests the fundamental principle of rolling without slipping. When an object rolls without slipping, the point in contact with the ground must be instantaneously at rest - this is the definition of "no slipping." The contact point has two velocity components that exactly cancel: the forward velocity v from the center's motion and the backward velocity -v from rotation about the center. These add to give zero instantaneous velocity at the contact point. Choice B incorrectly assumes the contact point moves with the center, choice C doubles the speed incorrectly, and choice D confuses static friction (which enables rolling) with kinetic friction. Remember that "rolling without slipping" means the contact point has zero velocity relative to the ground.
A wheel of radius $R$ rolls without slipping. If its angular speed doubles, what happens to its center-of-mass speed?
It is unchanged
It halves
It quadruples
It doubles
Explanation
This question examines the relationship between angular and linear speeds in rolling motion. For rolling without slipping, the condition v = ωR must always be satisfied, where v is the center-of-mass speed, ω is angular speed, and R is radius. If the angular speed doubles from ω to 2ω while the radius R remains constant, then the center-of-mass speed must change from v = ωR to v' = (2ω)R = 2(ωR) = 2v. Choice A incorrectly assumes the speeds are independent, choice C reverses the relationship, and choice D incorrectly squares the factor. The key insight is that v and ω are directly proportional for rolling without slipping, so doubling one doubles the other.
A hoop rolls without slipping so that $\omega$ increases. What must happen to the center-of-mass speed $v$ (radius $R$ constant)?
$v$ must equal $\dfrac{\omega}{R}$
$v$ must increase proportionally with $\omega$
$v$ must decrease as $\omega$ increases
$v$ can stay zero while $\omega$ increases
Explanation
This question tests understanding of the constraint imposed by rolling without slipping. The fundamental relationship v = ωR must be maintained at all times during rolling motion. If ω increases while R remains constant, then v must increase proportionally to maintain this equality. For example, if ω doubles, v must also double. Choice C violates the rolling constraint by suggesting v can remain zero while ω increases - this would cause slipping. The key insight is that rolling without slipping creates a rigid mathematical relationship between translational and rotational motion that cannot be violated.
A disk rolls without slipping. If its center-of-mass speed doubles, what happens to its angular speed $\omega$?
It doubles
It quadruples
It stays the same because rolling implies constant $\omega$
It halves
Explanation
This question explores how changes in linear speed affect angular speed in rolling motion. Since the rolling constraint v = ωR must always hold (with constant R), if v doubles, then ω must also double to maintain the relationship. Mathematically, if v₂ = 2v₁ and v = ωR, then ω₂R = 2ω₁R, giving ω₂ = 2ω₁. Choice A incorrectly assumes rolling motion requires constant angular speed, when it actually requires a constant ratio between v and ω. When one quantity in the rolling relationship changes, the other must change proportionally to maintain the constraint.
A ball rolls without slipping with angular speed $\omega$. What is the center-of-mass speed in terms of $\omega$ and radius $R$?
$v=\omega R$
$v=2\omega R$
$v=\dfrac{\omega}{R}$
$v=0$ because it does not slip
Explanation
This problem requires applying the rolling without slipping condition when angular speed is given. For rolling without slipping, the linear speed v of the center of mass equals the product of angular speed ω and radius R: v = ωR. This relationship ensures that the distance traveled by the center equals the arc length swept by any radius during rotation. The contact point travels zero distance relative to the ground, maintaining the no-slip condition. Choice B incorrectly inverts the relationship, while choice C doubles it unnecessarily. To convert between linear and angular quantities in rolling motion, always use v = ωR as your fundamental equation.
A solid disk of radius $R$ rolls without slipping on a level floor at constant speed $v$. What is the disk’s angular speed?
$\omega=\dfrac{v}{2R}$
$\omega=\dfrac{2v}{R}$
$\omega=0$ because there is no slipping
$\omega=\dfrac{v}{R}$
Explanation
This problem tests understanding of the rolling without slipping condition for rotational motion. When an object rolls without slipping, the linear speed of its center of mass is directly related to its angular speed by the equation v = ωR, where v is the center-of-mass speed, ω is the angular speed, and R is the radius. This relationship exists because the arc length traveled by a point on the rim during one rotation must equal the linear distance traveled by the center. Solving for ω gives ω = v/R. Choice D incorrectly assumes that no slipping means no rotation, when actually it means the rotation and translation are perfectly synchronized. To solve rolling problems, always start with the fundamental constraint v = ωR.
A wheel rolls without slipping so that $v=\omega R$. If the wheel’s radius is tripled while $\omega$ stays the same, what happens to $v$?
It triples
It is unchanged
It becomes one-third as large
It doubles because the rim speed is always $2v$
Explanation
This problem examines how radius affects linear speed when angular speed is constant. From v = ωR, we see that linear speed is directly proportional to radius when angular speed is fixed. Tripling the radius triples the linear speed because each point on the larger rim must travel three times as far in each rotation. The wheel covers three times the distance per revolution while maintaining the same rotation rate. Choice B incorrectly assumes v is independent of R, while choice C suggests an inverse relationship. When analyzing rolling motion with changing parameters, identify which variables are held constant and apply v = ωR to find the proportional changes.
A ball rolls without slipping with center-of-mass speed $v$. What is the magnitude of the angular acceleration if $v$ is constant?
$\alpha=\dfrac{g}{R}$
$\alpha=\dfrac{v}{R}$
$\alpha=\dfrac{v^2}{R}$
$\alpha=0$
Explanation
This question tests understanding of angular acceleration in rolling motion. Angular acceleration α is the rate of change of angular velocity with respect to time: α = dω/dt. Since the ball rolls with constant center-of-mass speed v, and the rolling condition gives ω = v/R, the angular velocity ω must also be constant (as both v and R are constant). When angular velocity is constant, its time derivative is zero, so α = 0. Choice A incorrectly confuses angular acceleration with angular velocity, choice B incorrectly uses a centripetal acceleration formula, and choice D introduces gravity unnecessarily. Remember that constant linear speed in rolling motion means constant angular speed, which means zero angular acceleration.