# How does friction help the car to move forward? [duplicate]

Below is the extract from my book.

There is a car with four wheel drive system and during acceleration, the engine causes the tires to push backward on the road surface. This push produces frictional forces $\vec{f}$ that act on each tire in the forward direction. The net external force $\vec{F}$ from the road, which is the sum of these frictional forces accelerates the car, increasing its kinetic energy.

Here the author says frictional force produce acceleration. How can a frictional force produce acceleration in the car?

The only possible explanation I can think that cause the car to moves forward in this case is when the tire pushes the road in backward direction and the friction force is the reaction force acting as mentioned in Newton's Third law.

• possible duplicate of Does a car use friction to move? Jan 7, 2015 at 23:57
• @DavidHammen That question you mentioned doesn't tell about the frictional force acting in forward direction. Jan 8, 2015 at 0:16
• Have you drawn a free-body diagram? Jan 8, 2015 at 0:27
• @KyleKanos No, but in the book, diagram of car with friction force pointing in forward direction of car is given. Jan 8, 2015 at 0:32
• Can you reproduce it here? Is it any different from the 2nd answer in the proposed duplicate? Jan 8, 2015 at 0:52

Here the author says frictional force produce acceleration. How can a frictional force produce acceleration in the car?

I think that as the car moves forward the tire pushes the road in backward direction and the friction force is the reaction force acting as mentioned in Newton's Third law.

You phrase this in a way that you think it is one or the other. Can they not both be true simultaneously?

Imagine a puck on a frictionless surface with a compressed spring against a wall. As the spring expands, the puck accelerates away. The force of the spring on the wall and the force of the wall on the spring are a pair of action/reaction forces.

So the force from the wall onto the spring is simultaneously a reaction force and is the net force on the puck that is causing acceleration.

• The second part is the only possible explanation I can think of that can cause acceleration in this case. I will edit it to make it clear for others. Jan 8, 2015 at 1:10

When your car sits at a traffic light and you rev the engine, then speed away with tires screaming - you create a lot of friction between wheels and road, and the result is forward motion.

So yes your hunch was right:

The only possible explanation I can think that cause the car to moves forward in this case is when the tire pushes the road in backward direction and the friction force is the reaction force acting as mentioned in Newton's Third law.