(a) For t < 0. No force as Particles is at rest. For t > 4s, No force again particle comes at rest.
For 0 < t < 4s, as slope of OA is constant so velocity constant i.e ., a = 0, so force must be zero.
(b) Impulse at t = 0
Impulse = change in momentum
I = m(v - w) = 4(0 - 0.75) = 3kg ms-1
Impulse at t = 4s
1 = m(v - u) = 4 (0 - 0.75) = -3 kg ms-1