The discussion centers around two main topics: the minimum potential energy gained by a string hanging over the edge of a table and a conceptual question regarding the momentum of a trolley colliding with a spring. The first part involves theoretical reasoning about potential energy, while the second part explores momentum conservation and energy transfer during a collision.
Participants do not reach a consensus on the minimum potential energy gained by the string or the fate of the trolley's momentum during the collision. Multiple competing views remain regarding the interpretation of the exam question and the nature of momentum in this context.
Unresolved aspects include the assumptions made about the string's center of mass and the definitions of momentum storage and destruction. The discussion also highlights the lack of clarity regarding the role of friction in the trolley's motion.
Students preparing for exams in physics, particularly those interested in potential energy, momentum conservation, and conceptual problem-solving in mechanics.
Well, what do you think? Yes, C is nonsense. What about the other choices?abcd8989 said:Besides, while I am drilling for my exam, I find I don't know how to think of this conceptual past paper question:
A trolley travels with uniform velocity to the right on a horizontal ground and collides with a light helical spring attached to a wall fixed to the ground (earth). At the instant the trolley becomes momentarily at rest during collision, what has happened to the initial momentum of the trolley?
A. It has been transferred to the earth.
B. It has been stored in the spring.
C. It has been dissipated as sound and heat ( I guess this one is nonsence as only energy can be dissipated as sound and heat, right?)
D. It has been destroyed by the friction due to the ground.
For A and B, I am not sure which is correct. Both seem to be reasonable. For D, I also don't know whether momentum can be "destroyed", but I think it is not the answer. As it seems to be quite irrelevant with the question. The trolley stopped because all the KE is converted to elastic PE which is temporarily stored in the spring, but not stopped by "the friction due to the ground". However, I don't know whether "a little part of the momentum" has really been destroyed.Doc Al said:Well, what do you think? Yes, C is nonsense. What about the other choices?
How does one 'store' momentum?abcd8989 said:For A and B, I am not sure which is correct. Both seem to be reasonable.
No, momentum cannot be 'destroyed'. And there's no friction mentioned in the problem anyway.For D, I also don't know whether momentum can be "destroyed", but I think it is not the answer. As it seems to be quite irrelevant with the question. The trolley stopped because all the KE is converted to elastic PE which is temporarily stored in the spring, but not stopped by "the friction due to the ground". However, I don't know whether "a little part of the momentum" has really been destroyed.