# Friction on the wheels of car

1. Feb 12, 2015

### lavankohsa

My book says option a,b,c is correct. I am able to understand the option a and b but why the friction has larger magnitude on rear wheels than the front wheels.

2. Relevant equations

3. The attempt at a solution
According to me it should be same because weight of the car is equally distributed on front and rear wheels.

2. Feb 12, 2015

### Staff: Mentor

It is not (see motorbikes for extreme examples), but that is not the point: the car is accelerating, so the forces are not balanced.

3. Feb 12, 2015

### lavankohsa

Ok. and what about if the car is moving with uniform speed then front and rear wheel should have equal friction ?

4. Feb 12, 2015

### Staff: Mentor

If you neglect air resistance (something you cannot do for real cars), yes.

5. Feb 12, 2015

### lavankohsa

But if car is in pure rolling motion, then the friction is zero i guess .

6. Feb 12, 2015

### Staff: Mentor

In an even more hypothetical example where there is no friction at all and the car is going at constant speed, friction is zero.

7. Feb 12, 2015

### haruspex

Even if the weight were equally distributed, that only says the maximum frictional force is the same on each. In rolling motion, the actual frictional force is anything from 0 to maximum (static) frictional force (in any direction).
The actual frictional force on the non-driving wheels is typically very low, since it only needs to be sufficient to overcome rolling resistance of the tyres and provide their rotational acceleration. The actual frictional force of the driving wheels has to overcome that of the non-driving wheels plus provide for the acceleration of the vehicle.

8. Feb 12, 2015

### haruspex

Only if the car is coasting. If it is accelerating on the flat or maintaining a constant speed on a hill (or moving in any way that an engineless cart would not) then there must be some friction.

Edit: if it is accelerating at all there must be friction to provide the angular acceleration of the wheels, so that includes coasting up or down hill.

Last edited: Feb 12, 2015
9. Feb 12, 2015