AL phy spring

還不明白的薑

2009-08-13 6:07 am

Two identical balls A, B with mass m are connected by a light spring, ball A is suspended by a thread from the ceiling and ball B is vertically below ball A. If the thread is suddenly cut, what are the accelerations of ball A and ball B at that instant?

A. 2g , 2g

B. 2g , 0

C. g , g

D. 0 , g

更新1:

Yes, I also get your answer !!! Thx ~

回答 (1)

Prof. Physics

2009-08-13 6:29 am

✔ 最佳答案

Let T be the tension in the thread, T' be the tension in the spring.

Initially the system is in equilibrium, for A: T = T' + mg

For B, T' = mg

So, we have T = 2mg

Now, if the thread is cut, T vanishes.

So, the net force acting on A is T' + mg = 2mg

So, the acceleration of A is 2g downwards (by Newton's 2nd law of motion)

However, for B, the tension in the spring still exists.

So, the net force acting on B at that moment = T - mg = 0

So, there is no acceleration for B.

We choose answer B.

參考: Physics king

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