(a) From your frame of reference:
- Motion diagram: The pie appears to be at rest on the roof of the van.
- Force diagram: There are two forces acting on the pie - gravity (downward) and the normal force from the roof of the van (upward).
From the driver's frame of reference:
- Motion diagram: The pie appears to be moving forward with the van.
- Force diagram: There are two forces acting on the pie - the force from the van pushing it forward (in the direction of motion) and the normal force from the roof of the van (upward).
(b) From your frame of reference:
- Motion diagram: The pie remains at rest on the roof of the van.
- Force diagram: Same as in part (a).
From the driver's frame of reference:
- Motion diagram: The pie appears to move slightly backward and then settle back to its original position on the roof of the van.
- Force diagram: There are two forces acting on the pie - the force from the van pushing it forward (in the direction of motion) and the normal force from the roof of the van (upward).
(c) Yes, the motion diagrams are consistent with the force diagrams for each case in both frames of reference. In both cases, the normal force balances the force due to gravity to keep the pie in place on the roof of the van. The additional force in the driver's frame due to the van's acceleration in the forward and backward directions causes the slight movement of the pie observed in that frame.
Your friend has a pie on the roof of his van. You are standing on the ground and observe the van stopping abruptly for a red light. The pie does not slip off the roof (a) Construct a motion diagram and a force diagram for the pie as the van approaches the red light, from your frame of reference and from the driver's frame of reference. (b) Repeat part (a) for the case when the light turns green. Be sure to specify the observer and identify the other object causing each force. (c) Are the motion diagrams consistent with the force diagrams for each case?
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