How do brakes bring a car to rest?

[sparkle123]

Homework Statement

If only an external force can change the state of motion of a body, how can the internal force of the brakes bring a car to rest?

The Attempt at a Solution

I think that it may be because friction between brakes and wheels converts kinetic energy into heat. Or perhaps friction between brakes and wheels somehow (?) leads to friction between wheels and the road, causing a retarding force.

Thank you!

 

[technician]

Your last statement about wheels and road is the important thing.
Brakes stop wheels going round they do not stop the wheel moving along the ground... think about brakes and car on ice... does it stop?

 

[sparkle123]

Thanks technician!

So Brakes lock wheels? I thought that was decrease the amount of friction since coefficient of kinetic friction is usually lower.

 

[Delphi51]

Essentially, the wheels push against the road and the road pushes back on the wheels - that's the external force. The wheels don't need to lock; just a matter of some resistance to turning so there is a push against the road.

The same thing happens in reverse when a car accelerates from rest. It's wheels push on the road and the road pushes back on the car, causing it to accelerate.

 

[sparkle123]

Thanks Delphi51!

Say the car is moving to the right. When it brakes, is it just that the rightward force from road decreases or is there actually a leftward force from the road?

Thanks again!

 

[Delphi51]

There is no (horizontal) force at all when the car is moving at constant speed.
Yes, when it decelerates, there is a leftward force causing the deceleration.
The car pushes on the road and the road pushes back with an equal and opposite force.
Newton's Third Law.

 

[sparkle123]

OH! Thank you! :)

I have a related question:
When the brakes of an automobile are applied, the road exerts the greatest retarding force:
A) while the wheels are sliding
B) just before the wheels start to slide
C) when the automobile is going fastest
D) when the acceleration is least
E) at the instant when the speed begins to change

I think the brakes eventually lock for this question. Aren't C and E the same (as soon as brakes are applied)? And why isn't C/E the right answer? Thanks!

 

[Delphi51]

It is related to the fact that the kinetic coefficient of friction is smaller than the static one.
The road has a better grip on the car in one case than in the other.

 

[sparkle123]

That would eliminate choice A, but how would you determine the correct answer among the remaining choices?

Thank you!

 

[Delphi51]

There is no retarding force when in constant motion (implied in C).
At minimum acceleration (D), the road/car force is minimum according to F=ma
(E) "speed begins to change" indicates small acceleration because acceleration doesn't change instantly. There will be a smooth downturn in the speed vs time graph and its slope (acceleration) will initially be near zero.

 

[sparkle123]

Thank you! :)