[Physics] Braking car and the force of friction – front-wheel-drive

frictionnewtonian-mechanicsrigid-body-dynamicsrotational-dynamics

There are a few things in car Physics that keep confusing me and I hope you can help me improve my understanding.
The first thing I would like to discuss is braking.
I understand that when a car is accelerating, the wheels are pushing backwards on the asphalt which results in a towing force speeding the car up (N3 pair). However, when the brakes are applied and the braking force does not exceed a certain value, the wheels are still rotating – and so still pushing back on the asphalt – the same story. Yet all of the diagrams I found on the Internet depict the force vector with its head against the movement of the car, which makes perfect sense intuitively, but I would really like to know what this change in direction is caused by.

My hunch is that it must have something to do with the braking pads being engaged, they generate a lot of friction on the rotor, changing the car's kinetic energy into heat. But, when I attempt to draw a free-body diagram, there is nowhere the inner force could be drawn.

Thanks for investing your time in advance!

Best Answer

When a car is braking, the wheels are pushing on the asphalt in opposite direction then when accelerating. So it is not the same story.

Acceleration: the force is generated by the engine which is applying torque on the shaft, which is turning the wheels. The friction at the contact point between the wheels and the road gives rise to the forward force.

Braking: during braking, the engine is usually not actively applying torque to the shaft, so the forward force is gone. On the other hand, the brake pads are applying force on the rotor, slowing it down. This has the opposite effect on the wheel rotation: they are not forced to turn faster, but slower. This (+ friction) gives rise to decelerating force at the contact point with the road.