al physics

2010-06-20 2:33 am

At a construction site, workman A is in an elevator moving upward at a constant speed u. When the elevator is level with the first floor, he lets fall a spanner. At the same instant, a second workman B standing on the first floor throws a hammer upward with initial speed u.

Which of the following statements correctly describes the motion of the hammer and the spanner as seen by the workman on the first floor?

A)Spanner falls downward and the hammer initially moves upward.

B)The spanner lands on the first floor at the same time as the hammer does.

C) The spanner lands on the floor before the hammer does.

D) The spanner and the hammer both move upward. But, the spanner rises more slowly.

Ans: B

I choose the answer D but of course is wrong...as I believe there should have the upward inertia for the spanner...would you mind to tell me why my concept is wrong? Thanks so much...

Please teach me with explanation...Thanks a lot !!!

回答 (1)

六呎將軍

2010-06-20 2:39 am

✔ 最佳答案

At the moment when workman A releases the spanner, the spanner starts to perform free fall motion, i.e. a motion with acceleration = g downward until it reaches the ground of the first floor.

So, we have, for the spanner:

Initial velocity of the spanner = u upward

Acceleration = g downward

Then for the hammer thrown by workman B, we also have, according to the given information:

Initial velocity of the spanner = u upward

Acceleration = g downward

Since the spanner and the hammer have the same initial velocity and acceleration, also start their free fall motion at the same time instant, they should fall back to the ground of the first floor at the same time.

參考: Myself

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