AP Physics 1 Question 96: Answer and Explanation
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Question below refers to the following information:
A rigid rod of length L and mass M sits at rest on an air table with negligible friction. A small blob of putty with a mass of m moves to the right on the same table, as shown in overhead view in the figure. The putty hits and sticks to the rod, a distance of 2L/3 from the top end.
How will the rod-putty system move after the collision?
- A. The system will have no translational motion, but it will rotate about the rod’s center of mass.
- B. The system will move to the right and rotate about the rod-putty system’s center of mass.
- C. The system will move to the right and rotate about the rod’s center of mass.
- D. The system will have no translational motion, but it will rotate about the rod-putty system’s center of mass.
Correct Answer: B
B-Conservation of linear momentum requires that the center of mass of the system continue to move to the right after the collision. The rotation will be about the combined rod-putty center of mass. To understand that, imagine if the putty were really heavy. Then after the collision, the rod would seem to rotate about the putty, because the center of mass of the rod-putty system would be essentially at the putty's location. In this case, you don't know whether the putty or the rod is more massive, but you do know that when the two objects stick together, they will rotate about wherever their combined center of mass is located.