# Static Friction of a Car's Tyre

1. Sep 18, 2014

### Jimmy87

1. The problem statement, all variables and given/known data
I'm trying to understand how static friction opposes the motion of a moving car. From what I have read, the car tyre doesn't move relative to the ground. The car tyre pushes on the ground and the ground pushes back - this is how the car moves. The force of friction is in the direction of movement. However, there are also frictional forces (which must also be static) trying to slow the car down otherwise the engine would not need to provide a force. So are there two static friction forces on a car; one trying to slow if down and opposing the motion of the car and the other providing the equal and opposite force to propel the car forwards? That doesn't really make any sense to me.

Also we are doing this experiment where you change the material of a ramp that a car goes down and measure the acceleration using light gates. Would a higher frictional surface (e.g. sandpaper) decrease the acceleration? How do you explain this in terms of static friction?

2. Relevant equations
None

3. The attempt at a solution
Had a look on previous threads. I understand that it has to all do with static friction but I can't make sense of fricitonal forces which oppose and help motion.

2. Sep 18, 2014

### Staff: Mentor

The force that opposes the motion is not static friction. There is air drag on the car. These are the only external horizontal forces acting on the car in flat terrain.

Chet

3. Sep 18, 2014

### Jimmy87

So when a freebody diagram shows that a car is going at a constant speed because the driving forces of the engine are matching the opposing forces of motion, these opposing forces of motion do not include friction? What then happens when a car brakes, surely that must have something to do with friction?

4. Sep 18, 2014

### tjmiller88

Hi Jimmy,

That's correct for the most part, assuming the car's transmission and wheel bearings are perfectly frictionless. This actually isn't true in real life, so you can have a small amount of internal friction from the mechanics of the car itself, but no the road friction does not slow the car down at all unless the wheels stop rolling.

Yup, brakes do work due to friction, but again that is not road friction. The brakes apply friction to the wheel rotors, providing "internal" friction within the car itself.

Now, you can slam on the brakes, which will then create kinetic friction between the wheel rubber and the road, which will indeed resist the direction of motion.

Make sense?

5. Sep 18, 2014

### Staff: Mentor

When the brakes are applied, drag from the car chassis is applied to the disk rotor, and the torque delivered from the engine to the wheels decreases. This causes the forward frictional force propelling the car forward to decrease. If the brakes are applied firmly enough, the wheels will lock, and the frictional force on the tires will actually be backwards. Since, until the wheels lock, there is no relative motion between the tire surface and the road surface, even before the wheels lock, the frictional force will be backwards.

Chet

6. Sep 18, 2014

### Jimmy87

Perfect sense, thank you.

7. Sep 18, 2014

### Staff: Mentor

In my judgement, this is totally incorrect. If you look at the car as a free body, there are only two external forces acting on it, the frictional force from the ground acting on the tires and the force from air drag. (This is why using free body diagrams is so important). If we neglect the effect of air drag during braking, the only important external force acting on the car during braking is the frictional force from the ground acting on the tires. In order for the car to slow down, this force must be acting in the direction opposite to the motion. Therefore, even when the tires are rolling, the friction from the road on the tires must slow the car down.

When the brakes are applied, the brakes are trying to make the tangential velocity of the tire surface with respect to the road surface less than the speed of the car. But, unless the car is skidding, this will not happen. However, what will happen is that the tangential force exerted by the road on the tire will be opposite to the motion of the car. The car will not skid (slip with respect to the road) as long as the tangential frictional force at the road surface is less than the normal force times the coefficient of static friction.

So, in summary, the car will not burn rubber (from trying to accelerate too fast) or skid (from trying to decelerate too fast) as long as the magnitude of the tangential friction force is less than the normal force times the coefficient of friction. The direction of the frictional force will be in the forward direction if the wheels are trying to rotate faster than the car speed (divided by the radius of the tire), and in the backward direction if the wheels are trying to rotate slower than the car speed (divided by the radius of the tire).

Chet

8. Sep 19, 2014

### Jimmy87

Thanks for your detailed answer. So why does a toy car rolling down a slope of sandpaper go slower once the car is rolling. If friction only opposes rolling motion when braking why does it go slower down a rougher slope? What extra forces are making it go slower?

9. Sep 19, 2014

### rcgldr

In addition to losses in the drivetrain, there's also rolling resistance (not friction) that opposes rolling motion since the forces involved during contact patch deformation are greater than the forces involved during contact patch recovery (hysteresis), and sliding friction losses as the contact patch deforms. Wiki article:

http://en.wikipedia.org/wiki/Rolling_resistance

For the toy car, friction in the axle bearings is probably larger factor than rolling resistance, since the "tires" are too stiff to deform much.

10. Sep 19, 2014

### CWatters

I would be surprised if it did this in practice. Normally static friction is higher than dynamic so once rolling I would expect it to accelerate or perhaps reach a terminal velocity. It would only slow down if something changed.

If it encountered a layer of snow or mud that would probably increase rolling resistance. See other replies.

11. Sep 19, 2014

### Staff: Mentor

I really don't understand the physical situation described in this question. Can you elaborate a little?

What may be happening is that the sandpaper is acting to reduce the torque that the toy car motor can deliver to the wheels to propel it forward. This must be related to the interaction between the wheel material and the sandpaper. This is just speculation, but as the tire rolls into contact with the sandpaper, the sand particles are very sharp, and the tire material deforms to fill the asperities in the sandpaper. When the rubber (or whatever material the tires are made of) rolls out of the contact patch, the material filling the asperities has difficulty separating from the asperities. This causes a torque in the direction opposite to the direction that the tire is rolling. This reduces the torque available from the motor to drive the car forward. Not sure about this, but it is certainly possible.

Chet

Last edited: Sep 19, 2014
12. Sep 19, 2014

### BvU

So, Jimbo, what do the measurements show ? You are in a postion to say something relevant here !
How about sharing the raw data ?

13. Sep 24, 2014

### Jimmy87

Hi guys, sorry for delay only collected the data today! You guys were right, the roughness of the surface does not seem to effect the speed of car down a ramp!! My teacher even thought that the sandpaper would slow the car down! Just so that I am understanding this correctly please could someone tell me if the following is correct:

Imagine you are going along a flat road in your car at 70mph and you then press your clutch in so that the engine is disengaged from the wheels. Are you saying that if there was no air resistance and no friction on the internal workings of the car that the car would keep going forever? As long as the friction is static friction (no skidding) friction never opposes motion? Is that right?

14. Sep 24, 2014

### olivermsun

For a real car that would absolutely not be the case. The rolling resistance of the tires which rcgldr mentioned above is pretty huge for a car with typical weight and tires. (Unless you discount it as part of the internal workings of the car.)

15. Sep 24, 2014

### vela

Staff Emeritus
Think about Newton's first law. If you remove all of the external forces, the car must move at constant velocity.

You should recognize that there would be no static friction in this idealized case. Static friction would only appear when the car accelerates or decelerates.

16. Sep 24, 2014

### Staff: Mentor

This is only if there is no air drag on the car. Otherwise, friction with the road would have to push the car forward enough to overcome the air drag.

Chet

17. Sep 24, 2014

### vela

Staff Emeritus
Which is what I took the OP to mean when he wrote "If there was no air resistance" and why I said "idealized case."

18. Sep 24, 2014

### Staff: Mentor

Oops. Sorry. My mistake.

Chet

19. Dec 26, 2015

### Vibhor

Hi vela ,

When the car decelerates (brakes are applied) and doesn't skid ( i.e rolls without slipping) , static friction on rear tires should be in backward direction and . On front tires it should be in forward direction .

Is my understanding correct ?

Thanks

20. Dec 27, 2015

### haruspex

That would be true if the brakes are only applied on the rear tyres. Usually all four wheels brake together, so once you disengage the engine they should all behave the same way.

One point seems to have been glossed over in the prior discussion. As stated, particularly at low speeds, a free-wheeling car would be slowed by rolling resistance. But how does that become a retardant force, i.e. horizontal?
Rolling resistance arises because, at a given stage of compression, the rubber takes more force to compress it than it delivers when allowed to decompress. This results in a greater upward force at the leading part of contact than at the trailing part, so creates a retardant torque. But still, a torque is not a horizontal force.
Without friction, the torque would make the tyre rotate more slowly and the car would slide as the wheels are no longer matching the linear speed. So, with friction, a retardant frictional force arises to maintain rolling contact.