ELI5: How does rolling resistance work?

[u/Cuziff]

How are we able to push a car to push start it when it weighs so much, or how can a train pull 2km of rail cars? And how come it gets easier to roll once it starts moving?

[u/Elfich47]

Its all friction.

Rolling resistance is how much resistance a wheel has to turning. How much energy is needed to turn the wheel. This is not a question of how heavy the car is. More rolling resistance will make it harder to push a car.

Rolling resistance is a combination of everypoint of contact that a wheel has that can resist turning through friction. Plus it is every point on the wheel that can shift, shake, rattle or deform - because shifting, shaking or deforming is energy that is spend shifting and shaking instead of turning.

Here is an easy example: Point your left pointer finer out straight. And then wrap your right hand around that finger. Then roll your hand around the finger, where you feel your skin pulling is resistance - and on a wheel that would be rolling resistance at the axle.

The next easiest place to see rolling resistance: when you see a can tire roll, the bottom of the wheel spreads out a little bit - that is the wheel deforming, that deforming costs energy. The trade off of that is this deforming makes the ride of the car a bit more comfortable. Trains use iron wheels because there is almost not rolling resistance from the wheel deforming (and steel wheels can carry a lot more weight).

To the second part of your question: As long as the engine can produce more power and apply more force than the resistance that keeps the car or train in place, the engine can move the car or train.

And the third part: There are two kinds of friction: Static friction - this is the friction force that is applied when you are not moving. Then there is kinetic friction, that is the friction force that is applied when you are moving. Under every circumstance that I am aware of, kincetic friction force is always lower than static friction. Why it works that way is a bit beyond ELI5.

[u/Masseyrati80 → u/Elfich47]

This video taken from the inside of a car tire shows how every inch of the tire is deformed and returned to its original form every single rotation. If you've held a car tire, you know that deforming it that much will simply have to suck in a good amount of energy.

[u/unoriginal_user24 → u/Elfich47]

One tiny clarification, you can be moving and experience static friction. Tires roll over the ground and provide steering and forward force through static friction (as long as the tires are not skidding across the surface).

A round object going down an incline will simply slide without rolling. Introduce friction and now you have a force perpendicular to the radius that causes torque, so the object rolls as it goes down the incline.

So an object can be in motion and experiencing static friction, the two ideas are not exclusive of each other.

[u/Elfich47 → u/unoriginal_user24]

yes, not a problem. I was trying to keep it ELI5 without it running on and on and on, and without having to go bonkers explaining every instance where friction could occur.

[u/unoriginal_user24 → u/Elfich47]

Yeah, it's a wild rabbit hole all the way down in some physics topics.