A body is thrown vertically up with a velocity of 100 m/s and another one is thrown 4 sec after the first one. How long after the first one is thrown will they meet?

Explanation

Let them meet after t sec.
`S_1=100t-1/2 g t^2` and `S_2=100(t-4)-1/2 g (t-4)^2`
`therefore 100t-1/2 g t^2=100(t-4)-1/2 g(t-4)^2`
`therefore 400=1/2g[t^2-(t-4)^2]=1/2 g . 4(2t-4)`
`therefore 2t-4=800/(4g) =20` , if `g=10m//s^2`
`therefore` t=12 sec