# An accelerating car

1. Nov 1, 2012

### davon806

1. The problem statement, all variables and given/known data
When the wheels rotate backwards,a backward force is exerted on the ground by the tyres.
By 3rd law,a equal but opposite force acts on the tyre by the ground,so the car
accelerates.
But I have also found some websites that states static friction is the only external
force causing the car to move forwards.
In some typical physics questions,the driving force and friction are two different
forces act on a car,where friction is in the opposite direction of displacement.
I would like to ask which force is actually accelerating the car?Friction or reaction
force by the ground(due to the engine)?
If it is friction,why backward arrow is drawn in some textbooks to represent friction while
the car is accelerating forwards?

2. Relevant equations

3. The attempt at a solution

2. Nov 1, 2012

### PhanthomJay

yes, this is a friction force
yes, by friction
that is correct
the driving force (or traction force) is the friction force on the driving tyres acting forward; friction on the non driving tyres acts backwards
friction is the reaction force, accelerating the car
maybe they are showing the forces acting on the ground by the tyres and not the force on the tyres from the ground..?

3. Nov 1, 2012

### AJ Bentley

That's probably showing the other friction forces like air resistance and rolling friction that are tending to slow the car down and so act backwards.

That's separate from the reactionary friction force on the wheels driving the car forward.