Question

A force of 20 N acts on a body of mass 2kg at rest for 4seconds.How much force is required to bring the body into rest in 2seconds?

Answer 1

a = F/m = 20/2 = 10
v = u+at = 0+ 10×4 = 40 m/s
Now, for second part of motion initial velocity becomes 40m/s and final velocity becomes 0. And t= 2s.
So,
0 = 40 + a'×2
》a'= -20( retardation)
So force(retarding) = m×a' = 2×20
= 40 N