phy ce mc 87 急

2008-04-17 4:47 am

A ball falls freely from rest and hits the ground.It then rebounds 10 1/4 of its original height. ( neglecting air resistance)

"Its PE just before collision is four times its PE just after collision."
Why this statement is incorrect?

Thank you!

回答 (1)

天同

2008-04-17 5:01 am

✔ 最佳答案

Notice the use of the word [JUST] before collision, and [JUST] after collision.
That is, the question asks if the PE at the time before striking the ground equals to the PE immendiately after leaving the ground.

Since at these times, the ball is in contact with the ground, its PE is zero with reference to the ground.

2008-04-16 21:04:50 補充：
The second sentence of my post should read "That is, the question asks if the PE at the time immediately before striking the ground is four time to the PE immediately after leaving the ground"
Sorry for the mistake in the original post.

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