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# Uniform circular motion help please ! watch

1. I'm a bit confused about something in my textbook.

Consider a vehicle of mass m moving at speed v in a circle of radius r as it moves round a roundabout on a level road.

The centripetal force is provided by the force of friction between the vehicles tyres and the road surface.

So, force of friction F = mv^2/r

For no skidding or slipping, the force of friction between the tryes and the road surface must be less than a limiting value Fo which is proportional to the vehicles weight.

My problem is here ! So after watching a YouTube clip of a car skidding around a round a bout I would think that the frictional force has to be as high as it can be in order to keep the car moving in a circular path. So why is there a limiting value of the frictional force?

Also how is this "limiting value" of friction proportional to the vehicle's weight.
If friction acts parallel to the surface and the weight acts downwards how did my book manage to relate the two ?
2. (Original post by Olive123)
I'm a bit confused about something in my textbook.

Consider a vehicle of mass m moving at speed v in a circle of radius r as it moves round a roundabout on a level road.

The centripetal force is provided by the force of friction between the vehicles tyres and the road surface.

So, force of friction F = mv^2/r

For no skidding or slipping, the force of friction between the tryes and the road surface must be less than a limiting value Fo which is proportional to the vehicles weight.

My problem is here ! So after watching a YouTube clip of a car skidding around a round a bout I would think that the frictional force has to be as high as it can be in order to keep the car moving in a circular path. So why is there a limiting value of the frictional force?

Also how is this "limiting value" of friction proportional to the vehicle's weight.
If friction acts parallel to the surface and the weight acts downwards how did my book manage to relate the two ?

The book is kind of wrong with that example. Friction is proportional to the reaction force, this doesn't always equal the weight, but in this example it does. Think of it as the harder you press down on a surface the larger the friction becomes. You can test it with your finger now

Friction has to have a limiting value because otherwise nothing could ever slip. It is to do with the coefficient of friction of the surfaces (obviously a smooth table will have a lower coefficient of friction to a rough road). The limiting value is basicaly the maximum force you can apply to something before it slips. You could think of it as being similar to how the floor can collapes under us if we have too large a weight
3. (Original post by hello calum)
The book is kind of wrong with that example. Friction is proportional to the reaction force, this doesn't always equal the weight, but in this example it does. Think of it as the harder you press down on a surface the larger the friction becomes. You can test it with your finger now

Friction has to have a limiting value because otherwise nothing could ever slip. It is to do with the coefficient of friction of the surfaces (obviously a smooth table will have a lower coefficient of friction to a rough road). The limiting value is basicaly the maximum force you can apply to something before it slips. You could think of it as being similar to how the floor can collapes under us if we have too large a weight
Thank you very much !! Loved the examples, really helped

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