# Static versus kinetic friction

1. Jul 29, 2007

### MIA6

In my book, static friction means The frictional force that must be overcome to start one surface moving over another. Kinetic friction means the force that opposes the motion of one surface over another. My teacher said that a wheel rolling is a static friction, and it's weaker than sliding friction. She likely said that sliding friction was kinetic friction. I don't understand why rolling is static friction. In my opinion, I think static friction is a friction when an object just starts to move, and kinetic friction is when the object is moving. ANother thing is if rolling is static friction, then it will not be weaker than kinetic but stronger. Hope you can explain to me. thanks.

2. Jul 29, 2007

### mgb_phys

Friction is caused by bond sforming between the two surfaces. Think of putting two pieces of metal together, an atom on the surface is just as strongly attracked to another atom in the opposite peice of metal as it is to it's neighbours in the same piece. In fact if you make very smooth, very clean pieces of metal in a vacuum and put them together they will instantly weld into a single piece!

Rolling friction is generally called static friction because the points of the wheel and road that are in contact aren't moving relative to each. Rolling a wheel is equivalent to picking each point up and putting the next point down - you can imagine this more easily if you think of a wheel as like a catepillar track with very many small catepillar plates.

Sliding friction is stronger because you are constantly making many new bonds and breaking others - friction depends on the surface area because that gives you more bonds to make / break.

3. Jul 30, 2007

### Staff: Mentor

Careful here. The force that must be overcome to just start something sliding is the maximum value of the static friction. But static friction can certainly be lower than the maximum value. Imagine a block resting on a table. You push it sideways with a force of 0.1 N and it doesn't move. The static friction is thus 0.1 N. Push it harder and the static friction will increase to compensate--up to some maximum value.
Friction opposes slipping between surfaces. If the surfaces slip, then you have kinetic friction. (If they don't slip, it's static.)
As long as the wheel rolls without slipping, the friction is static. And that can be quite low.
The friction will be static as long as the surfaces do not slip or move with respect to each other; Realize that the bottom of the wheel (where it makes contact with the ground) is not moving with respect to the ground.
Generally, between dry surfaces, the value of the kinetic friction is less than the maximum value of static friction. But realize that static friction can have a value from zero to the maximum, depending upon the situation.

For example: If you had a perfect nondeformable wheel rolling with constant velocity on a perfect, horizontal surface the amount of static friction acting would be zero! Can't get much lower than that.

Another example: In your car you step on the brakes, trying to stop as quickly as you can. If you can keep your brakes from locking, and the tires rolling without slipping, you can use the maximum value of static friction to slow down the car. But if you lock the brakes and the tires start to slip, the weaker kinetic friction acts--now you have less force with which to slow down. (That's why locking the brakes is bad!)

4. Jul 30, 2007

### rcgldr

In the real world with automotive tires, some slippage is better than none, although I'm not sure how sophisticated racing car ABS systems are, since braking is done on all 4 tires, so there's no rotational telemetry to use as a "reference". On the other hand, traction control for rear wheel drive race cars can use the rotation telemetry from the front wheels to optimize the slippage factor at the rear. In the case of Indy Racing League race cars, this slippage factor is driver adjustable while racing.

Although popular terms in physics, the terms static and dynamic are more commonly used to describe friction in reference to cars and racing games.

Rolling "friction" is misleading. There is rolling resistance, due to deformation of the moving parts (wheel surface, bearing surface, and axle).

In the case of automotive tires, there's significant slippage at the outer edges of the contact patch due to deformation ("squirm").

A bit off topic, but a tidbit of knowledge here, since there was a mention of "dry surfaces":

Other than some combinations of teflon on teflon, in most cases the dynamic friction is significantly less than maximum static friction (for some teflon on teflon surfaces, it's virtually the same, but extremely low). For precise control movements, fluid is used to reverse this situation. One example is a track shaped like an inverted "V", with fluid flowing through numerous holes in the track. The control device, also shaped like an inverted "V", rests on top of the track. The result is a situation where resistance to movement increases with speed, ideal for controlling precise movements.

Last edited: Jul 30, 2007