Circular motion : direction of frictional force

In summary: Kinetic friction is the only type of friction that exists in the world, and it always acts in the opposite direction of the motion. This is why a circular motion is possible with static friction - the force of kinetic friction balances the force of static friction.
  • #1
fluidistic
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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.
 
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  • #2
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:
  • #3
fluidistic said:
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.
 
  • #4
Hootenanny said:
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?
 
  • #5
Doc Al said:
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.
 
  • #6
fluidistic said:
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?
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.
 

1. What is the direction of frictional force in circular motion?

The direction of frictional force in circular motion is always opposite to the direction of motion of the object. This means that if the object is moving clockwise, the frictional force will act in a counterclockwise direction, and vice versa.

2. Does the direction of frictional force change in circular motion?

Yes, the direction of frictional force can change in circular motion. As the direction of motion of the object changes, the direction of frictional force also changes to always act in the opposite direction.

3. How does the direction of frictional force affect the speed of an object in circular motion?

The direction of frictional force acts as a resistive force, slowing down the object's speed in circular motion. This force is proportional to the coefficient of friction, the normal force, and the object's velocity.

4. Can the direction of frictional force be different for different points on the object in circular motion?

Yes, the direction of frictional force can vary for different points on an object in circular motion. This is because the direction of motion and the normal force can also vary at different points on the object.

5. How can the direction of frictional force be determined in circular motion?

The direction of frictional force can be determined by using the right-hand rule. This rule states that if you point your right thumb in the direction of motion, your fingers will curl in the direction of frictional force. Alternatively, you can also use the equation Ff = μN, where Ff is the frictional force, μ is the coefficient of friction, and N is the normal force.

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