AP® Physics C: Mechanics · Unit 5 · Topic 5.1
Constant Angular Acceleration
When angular acceleration is constant, angle, angular velocity, and time obey the same equations as linear motion, with theta, omega, and alpha replacing x, v, and a.
The linear kinematic equations come from integrating a constant acceleration twice: once to get velocity, once more to get position. Rotational motion follows the exact same logic, because angular position, angular velocity, and angular acceleration are defined by the same calculus relationships as their linear cousins, just measured in radians instead of meters. If alpha is constant, integrating it once gives omega = omega0 + alpha t, and integrating again gives theta = theta0 + omega0 t + (1/2) alpha t2. Combine those two to eliminate t and you get omega2 = omega02 + 2 alpha(theta - theta0). Same structure, new letters, same reason it works.
Because the equations are structurally identical, you solve rotational problems exactly like linear ones: identify what you know, pick the equation missing the unknown you don't care about, and solve. Revolutions are just angle in radians divided by 2 π, so converting between the two is a unit step, not a new concept.
This topic is the foundation for the rest of the unit. Once you can describe how fast something spins and how that spin rate changes, you're ready to ask why it changes, which is where torque, moment of inertia, and the rotational version of Newton's second law come in.
Key terms
- Angular displacement (theta)
- The angle through which an object has rotated, measured in radians.
- Angular velocity (omega)
- The rate of change of angular displacement, in radians per second.
- Angular acceleration (alpha)
- The rate of change of angular velocity, in radians per second squared.
- Revolution
- One full turn, equal to 2 π radians.
Watch it explained
- How it worksHow does integrating a constant angular acceleration build the rotational kinematic equations?
- Worked exampleFind how many revolutions a wheel makes spinning up from rest at 3 rad/s2 for 8 s.
- Worked exampleFind the angular acceleration of a wheel that slows from 40 rad/s to rest in 120 radians.
- Common mistakeWhy doesn't a wheel spinning at constant angular speed have zero acceleration everywhere on it?
Check yourself
End of topic.