# Friction causes a torque - then why do we sum it as a regular force?

• Differentiate it
In summary, friction causes a torque which is balanced out by the normal force. In most physics problems, we only add the applied force and friction, without considering torque. When dealing with both translational and rotational motion, we add translational forces to F=ma and torques to τ=Iα. Torque is not a force and is not typically included in a free body diagram, but may be specified as an applied torque in some problems.
Differentiate it
Homework Statement
Friction causes a torque which is balanced out by the normal force. In most physics problems, we simply sum the applied force and friction, without considering anything about torque. I'm kind of new to Rigid Body Dynamics, so it'd be helpful if someone also explained how translational forces and torque are added in a system where there is both translational and rotational motion and/or sent links to some online resources. Thanks!
Relevant Equations
F=ma

Differentiate it said:
Homework Statement:: Friction causes a torque which is balanced out by the normal force. In most physics problems, we simply sum the applied force and friction, without considering anything about torque. I'm kind of new to Rigid Body Dynamics, so it'd be helpful if someone also explained how translational forces and torque are added in a system where there is both translational and rotational motion and/or sent links to some online resources. Thanks!
Relevant Equations:: F=ma

View attachment 313399
Torque is not a force so we do not include it on a FBD. We also don't sum it along with forces. (Check the units... the units for torque and force are different.)

I don't know what you mean by "Friction causes a torque which is balanced out by the normal force." A torque can only be balanced by another torque, not a force, and the torque due to the friction force is not canceled by the torque due to a normal force.

-Dan

Hall, Lnewqban and Differentiate it
topsquark said:
Torque is not a force so we do not include it on a FBD. We also don't sum it along with forces. (Check the units... the units for torque and force are different.)

I don't know what you mean by "Friction causes a torque which is balanced out by the normal force." A torque can only be balanced by another torque, not a force, and the torque due to the friction force is not canceled by the torque due to a normal force.

-Dan
Right right, I'm just stupid, no probs no probs i get it

Differentiate it said:
Friction causes a torque which is balanced out by the normal force.
The torque caused by a force is always in respect of some chosen axis. In the diagram, using an axis at ground level, F applies a torque, Ff doesn’t.
The exception is when you have a pair of equal and opposite forces along parallel lines of application. In that case the net torque is the same no matter what axis you choose.
What you can say is that if the system is static then the torque due to the F, Ff pair balances the torque due to the N, mg pair.
Differentiate it said:
how translational forces and torque are added in a system
You add the forces to write Fnet=ma, and add the torques to write τnet=Iα.
topsquark said:
Torque is not a force so we do not include it on a FBD.
It's quite standard to include torques in an FBD where appropriate. In some posed questions, applied torques are specified as such, rather than as pairs of equal and opposite forces.

Lnewqban
Differentiate it said:
Right right, I'm just stupid, no probs no probs i get it

-Dan

gmax137, hmmm27, Lnewqban and 3 others

## 1. What is the relationship between friction and torque?

Friction is a force that resists the motion of an object. When this force acts at a distance from the axis of rotation, it creates a torque, or a rotational force, that can cause an object to rotate.

## 2. Why do we consider friction as a regular force when calculating torque?

Friction is a regular force because it acts in the same direction as the motion of the object. This means that it does not cause any change in the direction of the object's motion, but rather slows it down or prevents it from sliding.

## 3. How does friction affect the magnitude of torque?

The magnitude of the torque caused by friction depends on the coefficient of friction, the normal force, and the distance between the point of contact and the axis of rotation. The greater the coefficient of friction and the normal force, the greater the magnitude of the torque.

## 4. Can friction cause a torque in the absence of other forces?

Yes, friction can cause a torque even in the absence of other forces. This is because friction is a force that acts at a distance from the axis of rotation, creating a torque that can cause an object to rotate.

## 5. How does the direction of friction affect the direction of torque?

The direction of friction and torque are always opposite to each other. This means that if friction is acting in a clockwise direction, the torque will be in a counterclockwise direction, and vice versa.

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