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I Braking forces in a car

  1. Nov 9, 2017 #1
    I understand intuitively that an automobile is set in motion because the wheels apply a force on the road in a backwards direction. Using Newton's 3rd law, the car gets propelled because the road applies a reaction force in the opposite direction.

    Before starting to ask my full question, I want to ask this: Is this forward reaction force considered friction? It's in opposition to the motion of the wheels. Therefore, I want to say yes it is.

    Okay, now for the important part, I know that there is rolling resistance because rolling objects eventually come to a stop. When the brakes are applied, the kinetic energy is dissipated into heat and the kinetic energy is reduced. What I don't understand is the contact happening between the contact of the road and car. There must be a force acting against the forward velocity. Is this static friction and rolling resistance (the same possibly?)? The tires should be slowing down and the static friction which prevents slipping would possibly come stop the car from sliding across the road. It needs to act in the opposite direction of motion of the car. How is my reasoning?

    The wheels take time to slow down but does this change their relative motion on the road? Is it a matter of the forces acting in opposition of the car now "win out" causing it to slow down?
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  3. Nov 9, 2017 #2
    While writing this, I think I decided it's at least air resistance and rolling resistance. I'm just unsure about static friction which I believe in a non-slip scenario is the only other friction. I guess I'll ask: is static friction while the car is in motion acting in the opposite direction of the car's motion? Also, let's call left on your 2D screens moving forward since PoV matters.
    Last edited: Nov 9, 2017
  4. Nov 9, 2017 #3


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    It is static friction between tires and road that is responsible for the forward force when you press on the accelerator. It is static friction between tires and road that is responsible for the rearward force when you apply the brakes.

    Rolling resistance tends to act like a rearward torque -- like pressing very lightly on the brakes. By itself, this does not slow the car down. (Think about a car with square wheels on ice -- there is plenty of rolling resistance, but the car just skids). You need friction for rolling resistance to do anything. In practice, rolling resistance is usually much less than the available frictional force, so this caveat is of little importance.
  5. Nov 9, 2017 #4
    Does the relative motion cause the static friction to change direction? I guess what's causing the change in direction?

    I'm definitely thinking too far into this but I see the wheels are rotating but slowing down so still pushing the road as I said before so I guess I want to know what changed exactly.
  6. Nov 10, 2017 #5


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    When a car is rolling (as opposed to skidding) the rubber touching the road is not moving. If the car is moving at 100 k/hr then a point at the center of the wheel is moving forward at that speed, a point at the top of the wheel is moving forward at 200 km/hr (the forward speed of the car plus the speed of the edge of the rotating wheel), and a point on the bottom is moving at zero km/hr (the speed of the car minus the speed of the edge of the rotating wheel).

    Because the contact patch is not moving relative to the road, static friction will oppose any attempt to push the patch in any direction. When you apply the brakes you're reducing the rotational speed of the wheel. But reducing that speed has the effect of pushing the contact patch forward, because we're subtracting a smaller rotational speed from the forward speed of the car. Static friction opposes this forward push with a backwards force, and slows the car. Acceleration is the exact opposite: trying to increase the rotational speed of the wheel is trying to push the contact patch backwards, so the static friction force points forward and accelerates the car.
  7. Nov 10, 2017 #6


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    No, it's called static, because there is no relative motion. But there is shear deformation of the contact patches which can have different directions, even without relative motion.
  8. Nov 10, 2017 #7


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    Correct. As others have said it's static friction because the tyres don't slide over the surface of the road.

    If you accelerate too fast and the tyres slip then there is still friction but we call it kinetic friction rather than static friction. Usually kinetic friction is less than static friction so for a fast take off you want to avoid spinning the wheels. Likewise if you need to stop in an emergency you want to avoid locking the wheels.

    For maximum friction you want to be operating close to the point where the wheels slip but don't actually slip. ABS brake systems allow you to stand on the brake without fully locking the wheels. They modulate the brake so for at least some of the time you get the slightly higher static friction force rather than kinetic friction.
  9. Nov 10, 2017 #8


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    As a car tyre rotates the flat spot in contact with the road moves around the wheel. So the rubber is constantly deforming and relaxing. This heats up the tyre and that energy has to come from somewhere. It makes the wheel harder to rotate and that's one component of rolling resistance.

    In summary, when driving along you typically have:

    Static friction pushing you forwards
    Air resistance pushing you backwards
    Rolling resistance pushing you backwards
    A bunch of other friction forces in the engine and transmission that effectively push you backwards.

    and if you are travelling along a flat road at a constant velocity they all add up to zero.

    When you hit the brakes static friction reverses direction and pushes you backwards. So all the forces push you backwards and you slow down.
  10. Nov 10, 2017 #9
    Thank you all that responded! I intuitively thought it was relative to the road for a stop but I wanted to be sure. It's the same for an acceleration but I got tripped up watching cars rolling but slowing down. I understood that a lock would definitely be similar to pushing a block along the road.
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