AP Physics C Mechanics 4.4 Collisions Overview

Classify collisions by kinetic-energy changes and analyze object velocities using conservation of momentum for an isolated system.

[Maximum number: 2]

A horizontal circular platform with rotational inertia $I_{P}$ rotates freely without friction on a vertical axis. A small motor-driven wheel that is used to rotate the platform is mounted under the platform and touches it. The wheel has radius r and touches the platform a distance D from the vertical axis of the platform, as shown above. The platform starts at rest, and the wheel exerts a constant horizontal force of magnitude F tangent to the wheel until the platform reaches an angular speed $\omega_{P}$ after time $\Delta t$ . During time $\Delta t$ , the wheel stays in contact with the platform without slipping.

The kinetic energy of the spinning platform before the object is dropped on it is $K_{i}$ . The total kinetic energy of the platform-object system when it reaches angular speed $\omega_{f}$ is $K_{f}$ . Which of the following expressions is true? $K_{f}<K_{i}$ $K_{f}=K_{i}$ $K_{f}>K_{i}$
Justify your answer.

LO CON-5.D.c, SP 7.A, 7.C 2 points

The kinetic energy of the spinning platform before the object is dropped on it is $K_{i}$ . The total kinetic energy of the platform-object system when it reaches angular speed $\omega_{f}$ is $K_{f}$ . Which of the following expressions is true?

K_{f}<K_{i}

\ldots K_{f}=K_{i}

\ldots K_{f}>K_{i}

Justify your answer.

For selecting $K_{f}<K_{i}$ with an attempt at a relevant justification

1 point

For a correct justification

1 point

Example: Because the two disks will be rotating with the same final angular speed, this
is an inelastic collision, and kinetic energy will be lost during the collision.

All question bank results loaded

Covered syllabus points

4.4 Elastic and Inelastic Collisions question 1