# Circular motion : direction of frictional force

1. Dec 19, 2008

### fluidistic

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.

2. Dec 19, 2008

### Hootenanny

Staff Emeritus
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!).

Last edited: Dec 19, 2008
3. Dec 19, 2008

### Staff: Mentor

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.

4. Dec 19, 2008

### fluidistic

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?

5. Dec 19, 2008

### fluidistic

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 :
, wow, that was well said. Now I fully understand. Thank you.

6. Dec 19, 2008

### Hootenanny

Staff Emeritus
As Doc Al mentioned, I was a little confused by your question since a particle is simply a point and therefore there cannot be any static friction if the particle is moving.

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