Question

A motorboat of 1200kg with velocity of 90 km /hr to 18 km /hr in 4 s . Find cgange in momentum , acceleration and magnitude of the force required

Answer 1

Since it is not mentioned about the nature of motion, let's assume that the boat was traveling in the same direction and speed changes from 90 km/hr to 18 km/hr in 4 s.

So initial speed, u =  90 kmph = 25 m/s.
final speed, v = 18 kmph = 5 m/s.
Change in momentum = final momentum - initial momentum
  = mv -mu = m(v-u)
 = 1200(5-25) = 1200 x (-20) = -24000 kgm/s.
The negative sign indicates a reduction in the momentum.

Acceleration is the rate of change in momentum, given by a =$\frac{v-u}{t}$.

 Therefore, a = $\frac{(5-25)}{4} = \frac{-20}{4} = -5 ms^{-2}$.
The negative sign indicates a reduction in the velocity.

Force is mass x acceleration.
Therefore, F =ma = 1200 x (-5) = -6000N.
The negative sign indicates that this force is opposing motion of the boat.