Question

A stone of mass `5kg` falls from the top of a cliff `50m` high and buries `1m` deep in sand . Find the average resistance offered by the sand and the time it takes to penetrate .

Answer 1

Correct Answer - `- 2450 N; 0.064 s` .
Velocity on striking the sand,
`u = u = sqrt(2 gh) = sqrt(2xx9.8xx50) = sqrt(980) m//s`
`upsilon = 0, s =1 m, a =` ?
From `upsilon^(2) - u^(2) = 2 a s, 0 - 980 = 2xx a xx 1 `
`:. A = - 490 m//s^(2)`
Average resistance,
`F = ma = 5 (-490) = - 2450 N`
From `upsilon = u+ at`
`0 = sqrt(980) - 490t , t = sqrt(980)/(490) = 0.64 s` .