Circular Motion; textbook clarification

[ThomasMagnus]

Hello,

I am currently studying circular motion. I came across something in my textbook that I would like some helP with.

It says:"if the wheels and tires of a car are rolling normally without slipping or sliding, the bottom of the tire is at rest against the road at each instant; so the frictional force the road exerts on the tires is static friction"

My question is: How could the tires be at rest if they are in constant motion? Wouldn't the frictional force be kinetic since the wheels are moving?

Thanks

 

[cupid.callin]

Hello,

I am currently studying circular motion. I came across something in my textbook that I would like some helP with.

It says:"if the wheels and tires of a car are rolling normally without slipping or sliding, the bottom of the tire is at rest against the road at each instant; so the frictional force the road exerts on the tires is static friction"

My question is: How could the tires be at rest if they are in constant motion? Wouldn't the frictional force be kinetic since the wheels are moving?

Thanks

Look at the picture

Here the lower most point has 2 velocities ... the green one due to translation and red due to rotation.

As in case of pure rolling, wr=v so lowermost point has 0 velocity

*this will not be true in case of rolling with slipping, as in that case either v>wr or v<wr

 

[ThomasMagnus]

Hi,

Are you saying the vector sum of the velocities are 0 at the bottom?

Is this because the force the wheel applies to the ground is counteracted by an equal and opposite force of the ground on the wheel?

(I'm really new at this so excuse my ignorance ;)

Thanks

 

[cupid.callin]

Is this because the force the wheel applies to the ground is counteracted by an equal and opposite force of the ground on the wheel?

These forces will be in vertical direction. how can you expect them to change a horizontal velocity?

v_cm is the velocity of each particle towards right, right?
this will b true of the sphere was not rotating and so will remain true even if its ratating

and due to rotation each particle will have speed wr=v in tangential direction ... ie left for lowermost point ...

SO net velocity is v - wr = v -v = 0