I know that rolling friction is almost always less than the maximum static friction, but why is this? In particular I am researching why ball bearings have much less friction. It has been said that the ball bearings come in contact with much less of the metal, but this does not make sense to me as friction is not proportional to surface area. So if it has nothing to do with surface area, then why is rolling friction generally much less than sliding friction?
[Physics] Why is rolling friction less than the maximum static friction
frictionrotational-dynamics
Related Solutions
The problem with this question is that static friction and kinetic friction are not fundamental forces in any way-- they're purely phenomenological names used to explain observed behavior. "Static friction" is a term we use to describe the observed fact that it usually takes more force to set an object into motion than it takes to keep it moving once you've got it started.
So, with that in mind, ask yourself how you could measure the relative sizes of static and kinetic friction. If the coefficient of static friction is greater than the coefficient of kinetic friction, this is an easy thing to do: once you overcome the static friction, the frictional force drops. So, you pull on an object with a force sensor, and measure the maximum force required before it gets moving, then once it's in motion, the frictional force decreases, and you measure how much force you need to apply to maintain a constant velocity.
What would it mean to have kinetic friction be greater than static friction? Well, it would mean that the force required to keep an object in motion would be greater than the force required to start it in motion. Which would require the force to go up at the instant the object started moving. But that doesn't make any sense, experimentally-- what you would see in that case is just that the force would increase up to the level required to keep the object in motion, as if the coefficients of static and kinetic friction were exactly equal.
So, common sense tells us that the coefficient of static friction can never be less than the coefficient of kinetic friction. Having greater kinetic than static friction just doesn't make any sense in terms of the phenomena being described.
(As an aside, the static/kinetic coefficient model is actually pretty lousy. It works as a way to set up problems forcing students to deal with the vector nature of forces, and allows some simple qualitative explanations of observed phenomena, but if you have ever tried to devise a lab doing quantitative measurements of friction, it's a mess.)
"Rolling friction" is a misnomer. Never be confused by it. Its not friction. Its ROLLING RESISTANCE-the correct name. It comes into play because objects are not perfect rigid in real life. It comes into play because of the deformation of shape of objects when in contact. When an object rolls on another WITHOUT slipping, there is a surface contact between them.
It is the normal reaction that is responsible look at the diagram. A body rolling like this (in the diagram) is deformed and the normal forces on the front parts of the surface are always more (whether road is hard or soft) and results in a net backward force that will gradually stop the rolling body. The deformation depends on the nature of the two bodies, depending on their rigidity. Rolling resistance is usually less than static friction. Both are expressed in the same form where the coefficients are called coefficient of rolling resistance and coefficient of static friction. Experimentally, coefficient of rolling resistance is found to be smaller than coefficient of static friction for bodies with same mass. Coefficients are determined experimentally.
Best Answer
You are confusing "rolling resistance" and friction. These are two different unrelated phenomena, so there is no reason why one should be less than the other.
When the ball bearing rolls with constant velocity without sliding, there is no static or kinetic friction with the surface. The ball is not accelerating so there is no net force on it due to friction.
The ball bearing is slowed down because of rolling resistance, not friction. Rolling resistance arises from the deformation of the surfaces which are in contact, so unlike friction it does depend on the area of contact.