A coconut is hainging on a tree at a height of 15 m from the ground. A boy launches a projectile vertically upwards with a velocity of 20 m/s. After what time will the projectile pass by the coconut? Explain the two answers that you get in this problem.

Explanation

He `S=+15m`
`u=+20m//s` (upwards)
`g=-10m//s^(2)`
`t=?`
using `S=ut+1/2"gt"^(2)` we get
`15=-20xxt+1/2(-10)t^(2)`
or `15=20xxt-5t^(2)`
or `3=4t-t^(2)`
or `t^(2)-4t+3=0`
or `(t-1)(t-3)=0`
`:.` Either `t-1=0impliest=1s`
or `t-3=0impliest=3s`
At t=1s, projectile passes the constant while going upwards
and at t=3s the projectile passes the coconut while coming downwards.