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If only an external force can change the state of motion of the center of mass of a body, how does it happen that the internal force of the brakes can bring a car to rest?

If only an external force can change the state of motion of the center of mass of a body, how does it happen that the internal force of the brakes can bring a car to rest?

Grade:11

2 Answers

Kevin Nash
askIITians Faculty 332 Points
8 years ago

The internal forces of the brake come into action only when the driver put his foot on the braking pedal provided in the car. By doing so, the driver applies an external force which is responsible for moving the braking pedal from its stable position.
Therefore, the deceleration of the center of mass of the car is caused by the external force provided by the driver, when he/she presses the braking pedal.
Mihir
69 Points
8 years ago
Here, the car plus the driver are a system. Now consider two situations-
  1. When the road is frictionless.
  2. When road has friction.
    Considering the first case, suppose the car moves intially with velocity ‘v’. Now if the driver applies the brakes instantaneously, the friction between the brake and the wheel will only prevent the car from rolling, however the car will continue to slide along the path and never stop. So we can realise that the force applied by the driver cannot be responsible for slowing down the car.

    In the second scenario, the moment brakes have been applied, the car will experience a forward force due to friction. This force will offer opposite torque to the wheel of the car/vehicle and hence reduce the angular velocity.
    To conclude, the force responsible for slowing down a vehicle is friction and not the force applied by the driver on the brakes!

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