Rotational Kinematics
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AP Physics 1 › Rotational Kinematics
A fan starts at $\theta=0$ and rotates with constant $\omega=6,\text{rad/s}$ for $5,\text{s}$. What is $\Delta\theta$?
$6,\text{rad}$
$30,\text{rad}$
$1.2,\text{rad/s}^2$
$0,\text{rad/s}$
Explanation
This problem involves calculating angular displacement for constant angular velocity motion. For rotational motion with constant angular velocity, the angular displacement is Δθ = ωt, where ω is the angular velocity and t is the time interval. With ω = 6 rad/s and t = 5 s, we get Δθ = 6 × 5 = 30 rad. The motion starts at θ = 0, so the final position is θ = 30 rad, making Δθ = 30 - 0 = 30 rad. Choice B (1.2 rad/s²) incorrectly suggests an acceleration value, while choice D (0 rad/s) confuses angular velocity with displacement. For constant angular velocity problems, remember that displacement equals velocity times time.
A record’s angular velocity changes from $+8$ to $+2,\text{rad/s}$ in $3,\text{s}$. What is $\alpha$?
$+2,\text{rad/s}^2$
$-2,\text{rad/s}^2$
$-6,\text{rad}$
$+6,\text{rad/s}$
Explanation
This problem requires finding angular acceleration from a change in angular velocity. Angular acceleration is defined as α = Δω/Δt = (ωf - ωi)/Δt. The angular velocity changes from +8 rad/s to +2 rad/s over 3 seconds, so α = (2 - 8)/3 = -6/3 = -2 rad/s². The negative sign indicates the object is slowing down (decelerating) even though it continues rotating in the positive direction. Choice C (+6 rad/s) has incorrect units for acceleration, while choice A (+2 rad/s²) has the wrong sign. When velocity decreases (even if still positive), acceleration is negative.
A turntable has constant angular acceleration $\alpha=1.5,\text{rad/s}^2$ from rest. What is $\omega$ at $t=4,\text{s}$?
$24,\text{rad/s}^2$
$3.0,\text{rad/s}$
$6.0,\text{rad/s}$
$6.0,\text{rad}$
Explanation
This problem tests applying constant angular acceleration from rest. For rotational motion starting from rest ($\omega_0 = 0$) with constant angular acceleration, the angular velocity at time t is $\omega = \omega_0 + \alpha t = 0 + \alpha t$. With $\alpha = 1.5 , \text{rad/s}^2$ and $t = 4 , \text{s}$, we get $\omega = 1.5 \times 4 = 6.0 , \text{rad/s}$. Choice A ($6.0 , \text{rad}$) has incorrect units for velocity, while choice D ($24 , \text{rad/s}^2$) might come from incorrectly multiplying all values together. For constant acceleration from rest, final velocity equals acceleration times time.
A wheel’s angular position is $\theta(t)=2.0t^2$ (rad) for $0\le t\le3,\text{s}$. What is its angular acceleration?
$2.0,\text{rad/s}^2$
$4.0,\text{rad/s}^2$
$4.0,\text{rad/s}$
$12,\text{rad}$
Explanation
This problem tests understanding of rotational kinematics relationships between angular position, velocity, and acceleration. Angular acceleration $α$ is the second derivative of angular position with respect to time: $α = \frac{d^2θ}{dt^2}$. Given $θ(t) = 2.0t^2$, we first find angular velocity by taking the first derivative: $ω = \frac{dθ}{dt} = 4.0t , \text{rad/s}$. Then, taking the derivative of $ω$ gives us $α = \frac{dω}{dt} = 4.0 , \text{rad/s}^2$. Choice C ($12 , \text{rad}$) has incorrect units for acceleration, while choice D ($4.0 , \text{rad/s}$) would be the angular velocity at t = 1 s, not the acceleration. When given position as a function of time, always differentiate twice to find acceleration.
A disk rotates with constant $\omega=-4,\text{rad/s}$ for $5,\text{s}$. What is its angular displacement?
$20,\text{rad}$
$-0.8,\text{rad/s}$
$-20,\text{rad}$
$4,\text{rad/s}$
Explanation
This problem involves angular displacement with constant angular velocity in rotational kinematics. For constant angular velocity, the angular displacement is Δθ = ωt. With ω = -4 rad/s (negative indicating clockwise rotation) and t = 5 s, we get Δθ = (-4 rad/s)(5 s) = -20 rad. The negative sign indicates the displacement is in the clockwise direction. Choice A (20 rad) has the correct magnitude but wrong sign, ignoring the direction of rotation. Always maintain the sign of angular velocity to correctly determine the direction of angular displacement.
A rotor has $\omega(t)=10-2t$ (rad/s) for $0\le t\le4,\text{s}$. What is its angular acceleration?
$-2,\text{rad}$
$-2,\text{rad/s}^2$
$8,\text{rad/s}$
$2,\text{rad/s}^2$
Explanation
This problem tests finding angular acceleration from a velocity function. Given $\omega(t) = 10 - 2t , \text{rad/s}$, angular acceleration is the derivative: $\alpha = \frac{d\omega}{dt}$. Taking the derivative of $\omega(t) = 10 - 2t$ gives $\alpha = -2 , \text{rad/s}^2$. The negative acceleration indicates the rotor is slowing down from its initial velocity of $10 , \text{rad/s}$. Choice A ($8 , \text{rad/s}$) would be the velocity at t = 1 s, not the acceleration, while choice D (+$2 , \text{rad/s}^2$) has the wrong sign. When given velocity as a function of time, differentiate once to find acceleration.
A wheel starts from rest and reaches $12\ \text{rad/s}$ after $3\ \text{s}$ with constant angular acceleration. What is $\alpha$?
$9\ \text{rad/s}^2$
$0.25\ \text{rad/s}^2$
$36\ \text{rad/s}^2$
$4\ \text{rad/s}^2$
Explanation
This problem tests understanding of rotational kinematics with constant angular acceleration. The wheel starts from rest (ω₀ = 0 rad/s) and reaches ω = 12 rad/s in time t = 3 s. For constant angular acceleration, we use ω = ω₀ + αt, which gives us 12 = 0 + α(3), so α = 12/3 = 4 rad/s². Choice A (36 rad/s²) incorrectly multiplies 12 × 3 instead of dividing. To solve rotational kinematics problems, identify given quantities, select the appropriate equation, and check that units match throughout your calculation.
A disk has $\omega_i=10\ \text{rad/s}$ and $\alpha=-2\ \text{rad/s}^2$ for $3\ \text{s}$. What is $\omega_f$?
$16\ \text{rad/s}$
$4\ \text{rad/s}$
$-6\ \text{rad/s}$
$8\ \text{rad}$
Explanation
This problem requires applying rotational kinematics with constant negative angular acceleration. The disk starts with ω₀ = 10 rad/s and experiences α = -2 rad/s² for t = 3 s. Using ω = ω₀ + αt, we get ω = 10 + (-2)(3) = 10 - 6 = 4 rad/s. The negative acceleration causes the disk to slow down from its initial angular velocity. Choice A (16 rad/s) incorrectly adds the acceleration term instead of subtracting it. To handle problems with deceleration, pay careful attention to the sign of acceleration and ensure it reduces the velocity magnitude.
A fan has $\omega_i=8\ \text{rad/s}$ and constant $\alpha=1\ \text{rad/s}^2$. How long until $\omega_f=11\ \text{rad/s}$?
$\tfrac{3}{11}\ \text{s}$
$3\ \text{s}$
$88\ \text{s}$
$19\ \text{s}$
Explanation
This problem requires finding the time needed to reach a specific angular velocity with constant acceleration. The fan has ω₀ = 8 rad/s, α = 1 rad/s², and we need to find when ω = 11 rad/s. Using ω = ω₀ + αt, we get 11 = 8 + 1(t), which gives t = 3 s. The time is simply the change in angular velocity divided by the angular acceleration. Choice B (19 s) might result from incorrectly multiplying values instead of solving for time. When finding time in kinematics problems, rearrange the velocity equation to isolate time as (ω - ω₀)/α.
A turntable rotates at constant $\omega=5\ \text{rad/s}$ for $4\ \text{s}$. What is the angular displacement $\Delta\theta$?
$1.25\ \text{rad}$
$9\ \text{rad}$
$20\ \text{rad}$
$5\ \text{rad/s}$
Explanation
This problem involves calculating angular displacement for constant angular velocity motion. The turntable rotates at constant ω = 5 rad/s for time t = 4 s. For constant angular velocity, angular displacement is Δθ = ωt = 5 × 4 = 20 rad. Since there's no angular acceleration, the motion follows the simplest rotational kinematics relationship. Choice D (5 rad/s) confuses angular displacement with angular velocity by giving the wrong units. When solving constant velocity problems, remember that displacement equals velocity multiplied by time, whether for linear or rotational motion.