物理力學兩條問題__就來搞死我啦

First of all, by the formula s=ut+1/2at^2, sub u = 0 since the body is dropped from rest.

Then s=1/2at^2 and when t = 1, s = 4
So a = 8 m s^-2.

點解唔可以用v^2=u^2+2as?
因為 v 係指個 final velocity. 在題目中個 v 根本就無比到. 所以就會變了一條式有兩個 unknown 而 solve 唔到.

How far below the starting point will it be after a further 4s?
Sub t = 5 into the original equation:
s=1/2at^2
= (1/2) x 8 x 5^2
= 100 m
So it will be 100 m below its starting point after a further 4s.

留意番4m/s係average velocity......
姐係當t=0.5s既時候,v=4m/s
但係final velocity係咩,題目無講到

唔可以，因為星球的地心吸力，會令 free falling 的物體進行加速運動，因此不可以用等速運動的公式！你話 v=4m/s 就係當左佢等速運動！

你應該當 u=0 t=1 s=4 去計 a，即該星球的加速度，才可計到第 5s，物體去到邊！