What is difference between rolling, kinetic and static friction?

Question

If there is a toy train on the floor moving linearly then would the friction acting be rolling or static?

Answer 1

There exist only two types of friction:

1. Static friction, which holds on to an object if necessary to prevent it from starting to slide (therefore static, since there is no sliding), and
2. kinetic friction, which tries to stop an ongoing sliding.

These two are never acting simultaneously. Static is there before sliding starts (think of a parked car on a sloped San Francisco road, or think of you pushing a table that just won't move), and kinetic after sliding has started (think of a curling stone thrown across the ice, or think of the table finally starting to move at your hard push).

A rolling wheel is not sliding. It may be moving, but it isn't sliding (unless you force wheel spin). The point that touches the ground is momentarily stationary in that very moment of contact. In the next moment a new point takes over and is momentarily stationary. There is no sliding taking place - meaning, there is static and not kinetic friction. This is the case with your toy train in a rail.

Now, that toy train may not be very efficiently made. There may be a lot of kinetic friction in gears, around axles, it may even touch the rail sides and experience kinetic friction there as well. On car rubber wheels the rubber may be expanding and contracting which causes heat generation and thus energy loss from the motion. And the surface may be soft and be deforming while rolling, such as a sandy beach, which will suck out even more energy from the motion and furthermore cause scewed normal force that cause counteracting torques.

All such factors will slow down the train - you may say that they together feel like a "friction" pointing backwards. The term rolling friction has therefore been invented to encompass all those effects. But rolling friction is not a friction, though, but rather the effect of energy losses due to all those real-life non-ideal causes.

Ideally rolling friction is zero in any motion - realistically it is not. Your toy train most likely experiences quite some rolling friction.

In any case, don't think of it as an actual friction, because it clearly is not. It is just a lot of factors that together are called a friction because they together seem to work that way.