Question

a body is projected with an initial velocity of 20 m/sec.it will return to its starting point after how many sec​

Answer 1

method 1 : A body is projected vertically upwards with velocity 20m/s. Let it will return to its starting point after time T.

time of ascending , t = u/g

where u is initial velocity of body, u = 20m/s

and g is acceleration due to gravity.

now time of ascending , t = 20/10 = 2sec

total time taken to return the starting poi t = 2 × time of ascending

so, T = 2 × 2 = 4sec.

method 2 :

at maximum height, where velocity becomes zero, i.e., v = 0

so, 0 = 20 - 10t [ as we know, v = u + at]

=> t = 2sec

now, finding height reached by body, h

h = ut - 1/2gt²

= 20 × 2- 1/2 × 10 × (2)²

= 20m

so, find time taken to fall by body from 20m,

i.e., 20 = 0 + 1/2 (10)t'²

or, t' = 2sec

hence, total time taken = t + t'

= 2 + 2 = 4sec.