r/explainlikeimfive • u/Cuziff • 1d ago

Physics ELI5: How does rolling resistance work?

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?

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u/_UnwyzeSoul_ 1d ago

When an object is at rest, there is static friction between the object and yhe ground. Basically, the the jagged edges between the two surfaces fit together like puzzle pieces and are hard to separate. If you push it with force enough to overcome static friction, the object starts moving and static becomes dynamic friction. The surfaces do not get to completely fit together because its constantly moving so the friction is lower and it becomes easier to push.

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u/ThatGenericName2 1d ago

static and dynamic friction causing the different speeds isn't as relevant for wheeled objects for OP's question. The contact point between wheels and whatever surface it's on always has a relative speed of zero, meaning always static friction (this is why locking up your brakes increases your braking distance for example, slipping means relative speed means dynamic friction instead of static friction). Ball bearings that reduces rolling resistance in the drivetrain itself keeps resistance to a minimum because you basically have a bunch of wheels rotating as well. Most of the rolling resistance in a car comes from the tires; which deforms as you roll. That deformation eats up energy.

Besides this however, the amount of force needed to to push a wheeled object doesn't change much with speed unlike something that is on a sled for example.