# Circular motion : direction of frictional force

by fluidistic
Tags: circular, direction, force, frictional, motion
 PF Patron P: 3,143 Hi PF, I have a question. Say a particle describes a circular motion over a table. We have that the modulus of the centripetal force must equal the one of the static friction force, right? And according to Newton's second law the frictional force must be parallel to the radius pointing at the particle, but in the opposite direction. However I thought that the frictional force always point in the opposite direction of motion. In the case of a circular motion the centripetal acceleration always point through the center of the path while the motion is circular. Hence my question is : in what direction does point the frictional force in the case of a circular motion? (My guess is that it points in the opposite direction of the center of the path, while my intuition would say it's tangent to the circular path). Thank you.
 PF Patron Sci Advisor Emeritus P: 9,789 How is the particle being constrained to move in a circle? Edit: Another point to make is that kinetic friction always acts in the opposite direction to motion, but this is not the case for static friction (since there is no motion!).
Mentor
P: 40,278
 Quote by fluidistic Hence my question is : in what direction does point the frictional force in the case of a circular motion?
I assume you are thinking of an object like a car that can roll, not a particle. Is friction the only force acting on the object? Is the object undergoing uniform circular motion? (Constant speed.) If so, then friction must provide the centripetal force and must act towards the center of the circle.

Friction acts to prevent slipping between surfaces. Without friction to keep it going in a circle, the object would slide outwards. Friction prevents that.

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P: 3,143

## Circular motion : direction of frictional force

 Quote by Hootenanny How is the particle being contained to move in a circle?
I don't understand well the question. The particle moves in a circular motion because of the frictional force between the table and the particle itself. This force is responsible for the centripetal force, hence the circular motion of the particle. Or am I wrong?
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P: 3,143
 Quote by Doc Al I assume you are thinking of an object like a car that can roll, not a particle. Is friction the only force acting on the object? Is the object undergoing uniform circular motion? (Constant speed.) If so, then friction must provide the centripetal force and must act towards the center of the circle. Friction acts to prevent slipping between surfaces. Without friction to keep it going in a circle, the object would slide outwards. Friction prevents that.
Ah ok, I get it. The answer was conform to my guess and Newton's second law, but in counter of my intuition. Thanks.
EDIT :
 Edit: Another point to make is that kinetic friction always acts in the opposite direction to motion, but this is not the case for static friction (since there is no motion!).
, wow, that was well said. Now I fully understand. Thank you.
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