phy~~~~~~~

Hkpoly

2006-12-19 10:45 pm

在一場排球賽中，珍妮在發球時把排球拋到半空，當排球在最高點時拍擊它。珍妮把球拋起的速度為 6 m s1。設向上為正。
(a)試求排球到達最高點所需的時間。

回答 (1)

YEUNG Tim Wai

2006-12-20 12:06 am

✔ 最佳答案

The direction of acceleration is always downward.
The velocity of the ball is strictly decreasing.(assumed the upward direction is positive)

Before the velocity reached zero, the velocity is positive, the ball goes upward.
After that, the velocity is negative, the ball goes downward.

The ball reached the maxium height when the velocity is zero.

By the equation of motion in constant acceleration,
v=u+at, a=-10, u=6,
when v=0, t=6/10=0.6.

So, the ball takes 0.6s to reach the maxium height.

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