How does something brake.

[/0]

Please explain the mechanicks of braking in terms of forces. I understand that when something accelerates the wheels push back against the ground and by newton's 3rd law blahblahblah. But when you brake from moving at a constant speed what happens?

[enwiabe]

it's just friction again :p

When a car uses its driving force to overcome the road's friction, it exceeds the mu*N stopping force that the surface applies to the wheels.

Once you begin to brake, there is no driving force, so forces in the direction of motion are nil*, hence the frictional force which is acting opposite to the direction of motion decelerates the vehicle bringing it to rest.

Methinks you've overthought the scenario

*Not actually true, as the wheels don't lock up and still use a small amount of rolling friction to keep going forward (slowly) during the braking process. However, these forces are significantly smaller than the frictional force applied in the opposite direction.

[/0]

Why is it though? Isn't that only for sliding objects? When rolling, the static friction helps the car accelerate.
So it is only the rolling friction that slows the car? Then why is there a difference between taking your foot off the pedal and slamming down on the brakes? In the first case you are eliminating the driving force, but you will still decelerate more slowly than in the second example.

[enwiabe]

No, it's not rolling friction, it's kinetic friction that's stopping the car because the rolling motion is opposed. And your scenario makes perfect sense if you consider it like this. I should have explained more clearly, not only is the driving force taken away, the rolling motion (I may be getting out of my knowledge comfort zone here so forewarning if I'm not correct) is opposed by the partial locking up of the wheels, which opposes the motion, which causes a kinetic friction force in the opposite direction.

So if you only take your foot off the pedal, the amount of kinetic friction bringing the vehicle to a stop is much less than the amount than if the brake were applied to actually oppose the rolling motion and allow the kinetic friction to take over.

[/0]

I think I get it now, thanks enwiabe

I wish I could give you karma lol

[enwiabe]

I'd appreciate it if someone more knowledgeable than me in this would check my posts for scientific accuracy, what makes sense in my head might not come out so well in writing and I wouldn't want any VCE'ers to learn the wrong thing.

[/0]

Actually... I think there's something else I don't fully understand

Even if you slam your foot down on the brake, I don't see how the kinetic friction has any effect on the forces acting between the wheel and ground. No matter what kinetic friction is applied to the wheel, there will still be, by Newton's 3rd, a force acting to push the wheel forwards. So at any instant in time when the wheel is still rolling, you will have only 2 forces: friction pushing the wheel forwards, and rolling friction pushng the wheel back.

[Mao]

yes, the dilemma, by Newton's third law, how can anything happen? How can you throw a ball up when there's equal and opposite forces?

The equal and opposite forces act on different objects, in this case, the brake-pads apply kinetic friction to the wheel against the direction of motion, while the wheel apply kinetic friction to the car in the direction of motion. Because of the huge difference in mass, the wheels slow down a lot [small mass --> big deceleration], whereas the car nudges slightly forward [big mass --> small acceleration]

[/0]

Oh so is this like 1 step forward 2 steps back? In the short term you're making the car faster but that's because you're investing in making the wheels slower?

[Mao]

Oh so is this like 1 step forward 2 steps back? In the short term you're making the car faster but that's because you're investing in making the wheels slower?

the car would go marginally faster, but that amount is almost negligible.

Think about when you jump. As the ground exerts a normal force on you which acts upwards [you lifting off], your legs also exerts the force on the Earth which acts to push it away from you. The Earth also accelerates away, but with a magnitude so small it wouldn't matter. A similar thing happens here, the motion of the wheel is significantly reduced, whereas the car only slightly changes its motion.

[/0]

Ok I get that, so let's assume that exchange of momentum has already occurred. Now, the wheel should be slower. However, it still exerts a force backwards as it rolls, so the friction still pushes the wheels forward.
Therefore the net force is still , correct?

As the wheel keeps slowing down, will keep decreasing, so that

  (hurray for subscripts of subscripts)

Therefore the maximum retarding force is provided by rolling friction?

[TrueTears]

I believe this question has already been answered by Mr B.W XDXD