If the accelerator changes a body's velocity and turning it and bringing it a fixed point results in a constant velocity, this means that the force being exerted is equal to the kinetic frictional force. However, if the scooter goes from a smooth surface to a rough surface, keeping the accelerator at the same fixed point, the velocity reduces but still remains constant after that. But the friction offered by the rough surface is more, therefore to maintain a constant velocity, the force exerted needs to be increased. But since we haven't made any changes on our part, (we're keeping the the accelerator at the same point) this means that the scooter recognized the new friction itself and increased the force itself to maintain constant velocity. Does this mean that the scooter can recognize friction offered itself? Or maybe the accelerator doesn't exactly control the velocity but some other quantity? Or is my observation wrong and when we go on to the rough surface, the velocity decelerates to zero?


closed as unclear what you're asking by sammy gerbil, ZeroTheHero, Jon Custer, John Rennie, Kyle Kanos Aug 25 '17 at 10:01

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    $\begingroup$ Your question is rather confusingly written. Would help to rewrite it for clarity. $\endgroup$ – Samuel Weir Aug 24 '17 at 19:28

When analyzing vehicle dynamics, we often assume that the friction is static friction.

That represents a scenario where the wheels are in "pure rolling" (they don't slide/slip). The point of contact with the ground is stationary relative to the ground, and movement comes from the wheel turning, and having a new part of the wheel in contact with the ground. There is never slipping between the two, so it is purely static friction.

If we assume this is pure rolling, the tires do not slip. In that scenario, the friction has to be high enough so that we don't start slipping instead of rolling.

The static friction doesn't actually need to change when going between surfaces as long as both can provide enough static friction to avoid slipping.

Therefore, as long as the wheels stayed in approximately pure rolling, the surface should not make it any more difficult to drive.

If the surface had too little friction, you might notice.

  • $\begingroup$ I interpreted the "kinetic friction" to be some kind of rolling resistance - perhaps just the roughness of the ground. $\endgroup$ – sammy gerbil Aug 24 '17 at 20:56
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    $\begingroup$ @sammygerbil I believe he is mixing up rolling resistance and static friction, which is why I wanted to point this out. He seems to have gone from a smoother surface to a rougher one, and expected velocity to decrease because the surface would have greater kinetic friction. What he doesn't account for is that a vehicle in pure rolling only experiences static friction, so ground roughness will not change how the surfaces interact; unless it is too rough to maintain constant contact, or smooth enough to allow slipping. $\endgroup$ – JMac Aug 24 '17 at 21:00

No your scooter cannot detect rough ground - unless it is 'smart' like your 'phone.

You seem to be confused about what the accelerator does. It does not keep the scooter's velocity constant, and it does not exert a constant force. It controls the power output of the engine.

If the accelerator setting is fixed, then the power output of the engine is (approximately) constant. Engine power = resistance force x velocity, provided that the velocity is not changing. If the resistance force doubles then the velocity of the scooter halves.

The force opposing the motion of rolling wheels is usually called rolling resistance. It is not strictly kinetic friction, which applies to sliding objects.

  • $\begingroup$ I think his confusion stems from the fact that he expects the resistance to change due to the new surface. Regardless, this does still answer the question (as far as I can tell from the question). $\endgroup$ – JMac Aug 24 '17 at 20:26
  • $\begingroup$ I think i was confused because i just studied friction for the first time that day and have still not learned about rolling friction. But i understand now. So what you and JMac are saying is that 1) The velocity would change if the resistance was actually increasing but 2) the resistance isn't changing because this is rolling friction? $\endgroup$ – worldlier9 Aug 27 '17 at 11:45
  • $\begingroup$ 1) and 2) contradict. What I am saying is that resistance increases (because the ground is rougher) and therefore velocity decreases (because engine power is approximately constant). What you call the resistance doesn't change the fact that it increases. $\endgroup$ – sammy gerbil Aug 27 '17 at 18:54
  • $\begingroup$ But in JMac's answer he said in case of rolling friction, the rough surface wouldn't really increase the resistance much. (as far as i interpreted it) $\endgroup$ – worldlier9 Aug 28 '17 at 16:12
  • $\begingroup$ JMac is saying that friction is not requred if the wheel is rolling with constant speed. If the coefficient of friction changed then the scooter would not change speed. He is not talking about rolling resisitance, which is not the same as friction. Gravel provides more rolling resistance than tarmac, so the scooter would slow down. $\endgroup$ – sammy gerbil Aug 28 '17 at 16:54

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