# I don't quite understand the the friction force in contact force library. friction force in a product of the coefficient of the friction times the Normal force. If the ball purely rolls, the velocity at point of contact is zero. Then,no friction

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Cjjb Gh il 10 Apr 2017
Risposto: Steve Miller il 23 Apr 2023
I mean if the fricton actuate the motion.there would be some problems using the model
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David Goodmanson il 11 Apr 2017
Modificato: David Goodmanson il 13 Apr 2017
I can't comment on the contact force library but certainly you can have frictional force with no relative velocity between the two objects at the contact point or contact surface. When an initially stationary ball rolls down an inclined plane without slipping, the frictional force is what causes it to rotate.

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Steve Miller il 23 Apr 2023
There are many ways to define friction. The Simscape Multibody Contact Force library, and many other force laws, make the friction force depend on the relative velocity because that is a very simple friction law to implement and understand. It does not require any state transition logic when speed is 0. This is referred to as a continuous friction law.
This type of friction will permit modeling of rolling on a surface.
• For the case of a rigid ball in a pure rolling state, no friction is required at the contact point.
• A ball on a ramp which is not yet rolling will only roll if there is friction
• A continuous friction law will permit some slip (relative velocity) between the ball and the ramp at the start of the test so that the ball will start rolling.
• As the ball approaches pure rolling, that friction force will reduce. Steady state will eventually be reached, and there will be some small amount of slip. That amount of slip can usually be adjusted in the friction force law.
--Steve
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