Q:

How do internal forces affect motion?

As per newton's first law, only an external force can bring any change in the acceleration of a body, internal forces cannot. So, when we apply brakes to an accelerating car, aren't those brakes(opposing force) part of internal forces? How can they put the car to a halt?

A:

The car is slowing due to the force exerted by the road on the tyres. All the brakes do is transfer that force from the tyres to the body of the car.

