EXAPLES OF MOTIONS

1. A golf ball rolls across a smooth grass ‘green’ towards a hole. It is heading straight for the hole, The hole is 24m distance. The aim is to make the ball fall in the hole. If it is travelling faster than 2ms-1 when it reaches the hole, it will not fall in.

Question:

If the ball is slowing down at 0.7ms-2, will it fall in the hole? (original speed is 6ms-1)

Answer:

此题属于对公式v2-u2=2ax的基本概念考核，一般性学生只会记住a，Vi，Vf，t，而忽略了该公式，这类公式在VCE考试中出现频率并不高，所以很容易被学生遗忘。

(The main point of this question is to calculate the speed of the golf ball when it reaches the hole)

v2-u2=2ax

u2= v2-2ax=36-2*0.7*24

u=1.5m/s

So，the speed less than 2m/s，it will not reach the hole。

2. Fred is riding his bike on a level road at a speed of 10.0m s-1. The tail-light breaks off. It takes 0.45s to reach the ground. How far above the ground was the tail-light when it was attached?

(VVAA, 2007 examination)

Answer:

(Note that the speed of bike has nothing to do with the vertical motion of tail-light)

此题其实过程非常简单，而同学们容易犯错的地方在于，他们只会对x=ut+0.5at2，而在这题中在纵方向（vertical direction）没有原始速度，所以u=0，只存在重力加速度a，同学们在做类似题目的时候一定要搞清题目所问的物体所在受力的方向再做判断。

x=0.5*10*0.452

x=1m