Tyre friction

[Gareth56]

I came across this in my physics book when discussing friction:- "If the wheels of a car are rolling normally without slipping or sliding, the bottom of the tyre is at rest against the road at each instant; so the friction force the road exerts on the tyres is static friction."

 

Does that mean that when I'm travelling at say 70mph in my car the bottom of each tyre just for a fraction of a second is at rest with respect to the road surface? If so it must be for a very very short period of time!!

[insane_alien]

yes, that is exactly what it means. how did you think wheels worked?

 

it is very obvious when things are moving slowly(say a bicycle) so why would it be different when it is faster?

 

also, if the wheels were slipping you'd get tyre squeal everytime you went fast and it'd be very very noisy.

[Gareth56]

Thank you. It may have been obvious to you buy sadly not that obvious to me hence the reason I asked.

 

However thank you for taking the time and trouble to indulge such a dimwit!

[iNow]

Just to be fair, it actually WAS obvious to you since you got the answer right away with no help from others. You just needed confirmation to feel sure, which is perfectly okay.

[Sisyphus]

And similarly, the top of the wheel is always moving at twice the speed as the vehicle as a whole. Wheels are fun.

[Gareth56]

Just to be fair, it actually WAS obvious to you since you got the answer right away with no help from others. You just needed confirmation to feel sure, which is perfectly okay.

 

Also to be fair there is a "nice" way to give confirmation and a somewhat condescending way to do it. IMHO the latter was the case here.

 

However I'm obliged for the confirmation. I don't think we need dwell on the matter any further.

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And similarly, the top of the wheel is always moving at twice the speed as the vehicle as a whole. Wheels are fun.

 

If you'll excuse my ignorance could you expand on that?

[insane_alien]

i didn't intend to be condesending. by using the word obvious i was meaning 'easily observable' as the tyre typically moves slowly enough to be seen clearly rather than just a blur.

 

as to the top of the wheel moving faster than the vehicle over all, the 'back' part of the wheel has to rotate to the front. in order to do this it must go faster than the vehicle itself.

 

the top rim has to rotate at the same relative velocity to the axle as the bottom rim so the top gets to wice the speed of the axle relative to the ground.

[John Cuthber]

Don't get too upset about the scientist's odd use of the word obvious. There's a famous observation that mathematicians spend all their time "proving the obvious". The joke being that once something has been proved, then (for those who can follow the proof) it's "obviously" true.