# Wheels turning on a car

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1. Oct 17, 2015

### Michhcim

When a car moves forward, the wheel turns it exerts a force on the road in the opposite direction of travel, and friction counter acts it. the wheel turns and the car moves forward This implies that friction on the wheel is acting in the direction of the car moving. Though I thought that friction always acts in the opposite direction of travel?

2. Oct 17, 2015

### Staff: Mentor

Imagine that you applied enough power to lose traction and spin the wheels. Which direction is the tire's contact patch moving, relative to the road surface?

3. Oct 17, 2015

### HallsofIvy

The bottom of the wheel is moving back, toward the back of the car (assuming the car in not in reverse!). The friction on the ground acts opposite to that, toward the front of the car.

4. Oct 17, 2015

### Michhcim

I think i can answer my own question, the static friction between the wheel and the road acts in the direction of travel. The dynamic friction between the wheel and the car acts opposite the direction of travel. This is the friction shown in the free body diagram of a car moving.

5. Oct 17, 2015

### A.T.

Only for the driven wheels (connected to the engine)

What do you mean here. Axle friction?

6. Oct 17, 2015

### Michhcim

Point 1 Does that mean that the non driven wheels, do not experience any static friction with the road?

Point 2 Yes, the dynamic friction between all the moving parts, from the engine to the axle. That is countering the driving force of the car

7. Oct 17, 2015

### A.T.

They do, because of axle friction, but in the opposite direction than the driven wheels.

8. Oct 18, 2015

### vela

Staff Emeritus
To keep things simple, let's neglect internal friction in the car, e.g., axle friction, and rolling friction. Let's also assume the car is moving on a straight path do we can assume there are no forces causing the direction of the car's motion to change.

If the wheels are speeding up or slowing down, there's static friction between the wheels and the road. It's the friction that supplies the torque needed to change the rotational speed of the non-driven wheels. If the wheels turn at a constant rate, there's no friction.

If you look at the car a single object, these are internal forces that don't affect the overall motion of the car directly. What they do is they tend to slow down the rotation of the wheels. If the tire is not to skid, this torque causing the wheels to slow must be opposed by a torque due to friction between the road and the wheel. That's the external force acting on the car that causes it to slow down.