Question

A stone is thrown upwards with initial velocity at 20m/s. Air resistance is ignored and g=10m/s2.
(a) calculate how far it went to the top
(b) how long does it take to reach the top
(c) what is it's velocity just before it reaches the ground
(d) how long does it take to return to the ground

Answer 1

(a)
v^2=u^2-2gs
but v=0
0=20^2-(2×10×s)
0=400-20s
20s=400
s=20m

(b)
v=u-gt
but v=0
0=20-(10×t)
0=20-10t
10t=20
t=2 seconds

(c)
v=u+gt
when the stone reaches the maximum height, it returns and experiences free fall and u becomes zero (u=0m/s)
v=0+10×2
v=20m/s

(d)
time of flight= 2×2
=4 seconds