Question

a stone is dropped from a height of 20m. how long will it take to reach the ground. at what speed will it hit the ground?

Answer 1

h=20m/s
u=0
v=?
t=?
v²-u²=2gh
v²-o²=2*10m/s * 20m
v=√400
v=20m/s[velocity with which stone hit the ground]
v=u+gt
20=0+10*t
t=2s[time to reach the ground] 

Answer 2

Given:

Stone dropped from height (h)= 20m

To Find:

Time taken to reach the ground (t)

Final velocity at which stone hits the ground (v)

Solution:

We can solve this numerical problem by the following method.

We know,

Height (h)= 20m

Initial velocity (u)= 0m/s

To find 'v' we can use the formula-

$v^{2}-u^{2}=2gh$

Assuming acceleration due to gravity 'g'= 10m/s,

$v^{2}-0^{2}=2 X 10 X 20$

$v^{2}=400$

$v=\sqrt{400}$

$v=20m/s$

To find the time taken for the stone to reach the ground,

$v=u+gt$

$20=0+10(t)$

$t=\frac{20}{10}$

$t=2s$

Hence, the stone reaches the ground in 2 seconds at a speed of 20m/s.