momentum的問題

2009-08-11 11:18 pm

1. a 7-g bullet is fired towards a wooden block by a 3-kg gun.
the bullet embeds in the block after traveling 15cm inside the block.
on average, the blck applies a resisting force of 280N on the bullet

what is the velocity of the bullet before it hits the block?

2.pls refer to the picture.

http://i159.photobucket.com/albums/t152/roy219_2007/8d0b608e.jpg

PLS ans. with explain
thanks.!

更新1:

statement(3)唔明呀... 點睇個net force 係一樣架?@@可否再解釋?? 向下的時間g=10ms^-2, 咁和上都冇a.. f=ma..個force 不是細左咩?

回答 (1)

Prof. Physics

2009-08-12 4:01 am

✔ 最佳答案

1. Work done by the resisting force = K.E. lost by the bullet

Fs = 1/2 mv2

(280)(0.15) = 1/2 (0.007)v2

Velocity of the bullet, v = 110 ms-1

2. The answer is C. (2) and (3) only.

For (3), the net force acting on the ball during the two journeys are the same, the weight of the ball, which acts downwards.

For (2), by energy conservation, K.E. and G.P.E. interchange, but the total mechanical energy conserves.

So, the speeds are the same at the two aspects.

For (1), momentum is a vector, which has direction.

So, the direction of the velocity of the ball in the two circumstances are opposite, so the momentum are not the same. In fact, they are equal in magnitude but opposite in direction.

2009-08-14 13:44:15 補充：
你畫free-body diagram就會知向上向下時都只有weight作用於物件上，即mg。所以是一樣的。

參考: Physics king

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