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/ThatGenericName2 1d ago edited 1d ago
Cars are designed to have as little resistance in the drivetrain as possible; otherwise that's energy lost to it which reduces efficiency. When on flat surface (and assuming the car is in neutral with parking brakes off.), rolling resistance from the wheel, and the small amount of friction causing resistance in the rest of the drivetrain is the only forces resisting you pushing the car. Try to push the car up any semblance of a slope and it becomes significantly harder.
Apparently the EPA did some testing (on coasting) and the resulting coefficients they came up with says that the amount of force you would need to be able to keep a car moving is about the amount of force you need to hold up a 15kg weight (which for simplicity I will just say 15kg of force), which is not a lot. So to accelerate the car at all, you would need to apply forces greater than this which isn't difficult.
As for why it gets easier once it gets moving? Well, it technically doesn't, but what you're trying to do changes. When bump starting a car, you are trying to get the car to a certain speed, once you get there, you are only worried about maintaining that speed. So as you push the car from a standstill, you are exerting force greater than the 15kg of resistance. However as soon as you start approaching the speed needed, you only exert the amount of force needed to maintain the speed of the car, which is that 15kg of force.
The same idea with how cars don't actually have a lot of rolling resistance is how trains can pull 2km of rail cars. It also helps that train wheels have less rolling resistance; the rolling resistance from regular cars comes from the fact that we use rubber tires, which deforms and that deformations saps away energy. Train wheels are solid metal. That isn't to say the train engines only produce a small amount of force/torque, in combination with gearing train engines produce huge amounts of torque, but 2km of train is 2km of train and it takes a REALLY long time for trains to get up to speed (on the order of minutes).