AP Physics 1 — Unit 5: Torque and Rotational Dynamics
Practice questions, answers, and key terms for Unit 5, aligned to the College Board CED.
AP Physics 1 Unit 5 practice questions
A skater spins through 3 full turns. Your friend writes the angular displacement as 1080 — degrees. Why does that number break every rotational kinematics equation you're about to use?
Angular displacement must be measured in radians, not degrees. 3 turns = 6π rad ≈ 18.85 rad, and 1080° breaks the equations.
A wheel speeds up from rest at a steady angular acceleration. Someone asks which equation to grab. You already know the linear constant-acceleration set — what's the trick?
Rotational equations are exact analogs of linear ones — swap x→θ, v→ω, a→α. So ω = ω₀ + αt mirrors v = v₀ + at.
A fan's blades go from 120 rad/s to rest in 4 s when you cut the power. You want the angular acceleration. Which relationship gets you there?
α = (ω − ω₀)/t = (0 − 120)/4 = −30 rad/s². The negative sign means it's slowing down (deceleration).
A spinning frisbee and a single dot of paint on its edge are tracked at the same instant. Why can the dot be treated with kinematics but the whole frisbee can't be modeled as one moving object?
A rigid body's points all move differently, so it isn't one particle. But a single point on it can be tracked individually.
A disk starts from rest and reaches 40 rad/s after turning through 100 rad. You don't know the time. Which angular kinematics equation skips time entirely?
ω² = ω₀² + 2αθ, the time-free equation here. So 40² = 2α(100), giving α = 8 rad/s².
On a merry-go-round, a kid near the center and a kid at the edge complete each loop at the exact same time. The edge kid insists they're moving way faster. Who's right?
Both are right about different things — same ω, but their speeds differ. Linear speed v = rω is larger at the edge where r is bigger.
A bike wheel of radius 0.35 m spins at 20 rad/s. A pebble stuck in the tread — how fast is it actually moving through the air?
v = rω = (0.35)(20) = 7 m/s. The tangential linear speed of a point depends on its distance from the axis.
Two ants ride a rotating record, one at 5 cm and one at 15 cm from the spindle. Which quantity is identical for both, and which is triple for the outer ant?
Both share the same angular velocity ω and angular acceleration α (rigid body). The outer ant's linear speed is triple because v = rω and its radius is 3× larger.
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Key terms in Unit 5
These 12 terms show up in the Unit 5 cards. Each one links to its definition in the AP Physics 1 key-term reference.
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