# Why is the force of friction opposing the movement of a moving car static not kinetic

1. Jun 4, 2005

### michaelw

Im very confused as to why..

2. Jun 4, 2005

### whozum

Kinetic means sliding, as in surfaces rubbing together and just moving along with only one point of continuous contact. Static friction means the object is just sitting on the surface. While the tire rolls, it lays down a piece of tire that touches the ground, as the tire rolls, each part of the tire infront of the previous will make contact with the road.

There is no sliding contact.

Last edited: Jun 4, 2005
3. Jun 4, 2005

### michaelw

thanks
so.. the force of friction (static) is always in the same direction as the car is moving?

does that mean that it is the static friction that alone (only force) that makes the car move? if so, then is the force on each tire m(of car)a(of car)/4 if there are 4 tires?

4. Jun 4, 2005

### whozum

No friction is always against the direction of motion.
The power from the engine is what makes the car move.

The force of the tires on the floor is (uniform car) 1/4th of the car's weight at each tire. The tires are part of the car so it is not very useful to interpret their linear acceleration alone.

5. Jun 4, 2005

### michaelw

also, take a look at this picture
it seems in both cases, there is no force shown that makes the car want to go straight (but according to inertia, the car will want to go straight).. in the first, there is static friction (centripital force) that acts to move the car to the left, in the second theres both centripital force as well as gravity..

but what is the force that is countering the forces to the left? why are forces to the left even required, if (according to the diagram) theres no force to the right (or straight).. if there is a force, what is its magnitude?

Last edited by a moderator: May 2, 2017
6. Jun 4, 2005

### Staff: Mentor

The law of inertia states that a body will continue moving in a straight line with constant speed unless a force is imposed on it. No force is needed to make the car "want" to go straight.

In the first picture, the centripetal force is provided by friction; in the second, the centripetal force is provided by the horizontal components of friction and the normal force.

In order to turn to the left, the car must centripetally accelerate. (It's turning in a circle.) And that requires a force. (No force and the car would keep going straight.)

The forces pushing the car to the left are not countered by other forces! That's why the car accelerates to the left! (Since the car is moving forward, it doesn't just slide to the left--it turns in a circle. Review circular motion and centripetal acceleration.)

Last edited by a moderator: May 2, 2017
7. Jun 4, 2005

### Andrew Mason

The force of friction you are referring to is the force of traction: the car tires pushing back on the road. The force of the traction is in the direction of motion of the car. It does no work on the car so it causes no energy to be lost. It doesn't slow the car down or resist its motion.

AM

8. Jun 5, 2005

### al_201314

I read somewhere that the amount of frictional force acting on the same object is not dependent on the surface area of the object in contact with the surface. Why is this so? In that case, can I say that the same amount of force will be required to push a rectangular box when it's standing on the 2 different surfaces of the box?

Can I infer from this that the amount of frictional force acting on a particular object is dependent on the weight on the object itself only? (Assuming the surface is uniform). If yes, why is it so?

Something out of interest, are there any mathematical methods to calculate when will the wheels of a car will skid when the brakes are stepped on? Its common sense that it will skid easier on more slippery surfaces but if anyone could link me to a web that shows explains this scientifically and mathematically it would be much appreciated.

Thanks everyone..