AP Physics C Mechanics 4.4: Elastic and Inelastic Collisions

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.

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4.4.A—Describe whether an interaction between objects is elastic or inelastic question 1