Question

A car moving with a uniform velocity of 30 m s-1. It is stopped in 2s by applying a force of 1500 N through its brakes. Calculate : a) the change in momentum of car, b) the retardation produced in car and c) the mass of car.

If the force comes in negative then we will write it negative or positive in the answer. My teacher says that the force is always in positive form or it is zero ?

Answer 1

The deceleration = Change in velocity / time

Change in speed = Final velocity - initial velocity

= 0 - 30 = - 30m/s

Deceleration = - 30/2 = - 15m/s²

= 15m/s²

Force = mass × Acceleration

1500 = 15 m

M = 1500/15 = 100kgs

Momentum = mass × Velocity

Initial momentum = 30 × 100 = 3000 Kgms⁻¹

Final momentum = 0 × 100 = 0

Change in momentum = Final momentum - initial momentum

0 - 3000 = - 3000

Momentum cannot be negative, the negative sin implies that it is in the opposite direction.

Thus change in momentum is 3000Kgms⁻¹