A2L Item 065
- Description: Identify conserved quantities when rock is dropped from a rotating platform.
- Goal: Recognize physical conditions under which conservation principles hold.
- Source: UMPERG-ctqpe134
- Keywords: Angular Momentum, Conservation, Energy, Interactions, Mechanics, Rotational Motion
The question for students:
A
child is standing at the rim of a rotating disk holding a rock. The
disk rotates without friction. The rock is dropped at the instant
shown. What quantities are conserved during this process.
- Only angular momentum is conserved.
- Only mechanical energy is conserved.
- Both angular momentum and mechanical energy are conserved.
- Neither is conserved.
- cannot be determined.
Commentary for teachers:
Answer
(3) is the correct response if the rock is simply dropped. Some students may fail to include the rock as part of the system after it is dropped.
Background
Objects traveling in a straight line do have angular momentum with respect to any origin that is not on the path of the object. The rock does not cease to have angular momentum with respect to the center of the disk when it is dropped. Although the angular momentum and energy of the rock will change as the rock falls, its angular momentum and energy just after it is dropped are the same as just before.
Questions to Reveal Student Reasoning
Does the rock have angular momentum (or energy) just before it is dropped? just after it is dropped?
If energy (angular momentum) is lost, what happens to it?
Changes in angular momentum are caused by a net torque. What torques act on the system?
Suggestions
Have students relate their answer to this question to the previous one.