# phy!!!CE !急!!!

A student throws a ball vertically upwards.The speed of the ball is 3ms^-1 at both instants t1 and t2.Assuming that air resistance is negligible,which of the following statements is/are correct?

1)The time inerval between t1 and t2 is 0.6s

2)The distance travelled bu the ball between t1 and t2 is 0.9s

3)The acceleration the ball is zero at a certain instant in between t1 and t2

Ans: 2 and 3 is correct. WHY!?!?!?!

其實我都係計到1同2岩...但係對答案話錯-_-呢份係mock卷..

### 1 Answer

- 1 decade agoBest Answer
見你搵英文問, 我就用英文答啦。

First, I will like to explain to you why 2 is correct.

Let the distance between the highest point reached and the point reached at t1 be s

by v^2-u^2=2as

0-(3)^2 = 2(-10)(s)

s = 0.45m

But the distnace travelled between t1and t2 is equal to 2s

Theresore, the distance travelled between t1 and t2 is 0.9m

Secondly, I will like to point out that 3 is wrong. In fact, the ball is under the effect of gravity during the whole process.

Finally, I will calculate out the value of t2-t1 for you.

From the previous steps, we know that the distance between the highest point and the point at t1 is 0.45m and let the time between the highest point and t1 be t

By s = ut+(1/2)at^2

0.45 = 3t+(1/2)(-10)t^2

5t^2-3t+0.45 = 0

100t^2-60t+9 = 0

(10t-3)^2 = 0

Therefore , t=0.3

But t2-t1 = 2t

Therefore, 1 is correct.

In conclusion, 1 and 2 are correct, but 3 is wrong.