# What is the direction of friction

1. Jul 24, 2014

### Greg Bernhardt

Definition/Summary

The direction of the force of dynamic friction is along the plane of contact, and is opposite to the direction of relative motion.

The direction of the force of static friction is along the plane of contact, and is opposite to the direction in which there would be relative motion if there was no friction (for example, if one of the surfaces suddenly turned to ice).

In particular, the friction from the road on the driving or braking wheels of a car is in the same direction as the acceleration or braking, but the friction on the non-driving or non-braking wheels of a car is in the opposite direction.

Equations

Extended explanation

On the wheels of a non-skidding car:

The wheels are rolling, and so the point of contact of each tyre with the road is instantaneously stationary, and static friction applies.

There is also the "rolling friction", or rolling resistance, an additional small force, caused by deformation of the tyre, which in exam questions can usually be ignored.

to find the direction of static friction, always ask "what would happen if there was no friction?" (for example, if one surface was ice) …

If a brake is being applied to a particular pair of tyres (on the same axle), imagine that the road under those tyres suddenly becomes ice (but the road under the non-braked tyres remains normal): the braked tyres will go slower, but the car will stay the same speed (because there are no external braking forces on it): looking down from the window, you see the road going backward faster than the braked tyres are going backwards, so the road will try to drag those tyres backward with it

In other words: the tendency is for the road to move backward relative to the braked tyres, so the friction on the road is forward, and the friction on the braked tyres is backward

However, the non-braked tyres (usually the front, steering, tyres) will have a forward friction force from the road at the same time …

To see this, imagine instead that the non-braked tyres (only) are on ice: then they will go at the same speed, but the car will go slower: looking down from the window, you see the road going backward slower than the non-braked tyres are going backward, so the road will try to drag those tyres forward.

In other words: the tendency is for the road to move forward relative to the non-braked tyres, so the friction on the road is backward, and the friction on those tyres is forward.

This of course is why 4-wheel braking slows a car faster than 2-wheel!

Similarly, for a car being accelerated by its engine, the friction from the road on the driving tyres is forward, but on the non-driving tyres is backward.

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