Register to reply

Circular motion : direction of frictional force

by fluidistic
Tags: circular, direction, force, frictional, motion
Share this thread:
fluidistic
#1
Dec19-08, 09:06 AM
PF Gold
fluidistic's Avatar
P: 3,225
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.
Phys.Org News Partner Physics news on Phys.org
New complex oxides could advance memory devices
Nature's designs inspire research into new light-based technologies
UCI team is first to capture motion of single molecule in real time
Hootenanny
#2
Dec19-08, 09:12 AM
Emeritus
Sci Advisor
PF Gold
Hootenanny's Avatar
P: 9,772
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!).
Doc Al
#3
Dec19-08, 09:16 AM
Mentor
Doc Al's Avatar
P: 41,570
Quote Quote by fluidistic View Post
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.

fluidistic
#4
Dec19-08, 09:17 AM
PF Gold
fluidistic's Avatar
P: 3,225
Circular motion : direction of frictional force

Quote Quote by Hootenanny View Post
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?
fluidistic
#5
Dec19-08, 09:20 AM
PF Gold
fluidistic's Avatar
P: 3,225
Quote Quote by Doc Al View Post
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.
Hootenanny
#6
Dec19-08, 09:20 AM
Emeritus
Sci Advisor
PF Gold
Hootenanny's Avatar
P: 9,772
Quote Quote by fluidistic View Post
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.


Register to reply

Related Discussions
Force and direction of motion Classical Physics 2
Direction of the frictional force Introductory Physics Homework 2
Direction of frictional force Help! Introductory Physics Homework 5
Circular Motion question. Need direction Introductory Physics Homework 1
Frictional force direction Introductory Physics Homework 2