Newtonian Mechanics

2007-12-26 9:43 pm

A ball of mass m is projected vertically downwards with speed v from a certain height and rebounds from the ground back to the same height.

(1) The collision between the ball and the ground is not perfectly elastic.
(2) The loss in energy of the ball in the collision is 1/2 mv^2.
(3) If the ball is progected vertically upwards from the same height with the same speed, it would rebound to a greater height.
Ans is (1) and (2)
Explain briefly. thx

回答 (1)

myisland8132

2007-12-26 9:56 pm

✔ 最佳答案

(1) 正確
因為若是perfectly elastic﹐則該球回到原高處時速率是v﹐所以一定可以繼續升高﹐現在只可以去到原高處﹐證明碰撞過程中有能量損失
(2) 正確
最先物體擁有的能量是1/2 mv^2+mgh﹐碰撞完回到原高處的能量是mgh (v=0)﹐所以碰撞過程中損失的能量是1/2 mv^2
(3) 不正確
因為將個球向上拋﹐球回到原高處時速率一樣是v﹐其後的事與上述情況一樣﹐所以只可彈回同原先一樣的高度。

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