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

[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

 

[topsquark]

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

 

[Differentiate it]

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

 

[haruspex]

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, F_f 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, F_f pair balances the torque due to the N, mg pair.

how translational forces and torque are added in a system

You add the forces to write F_net=ma, and add the torques to write τ_net=Iα.

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.

 

[topsquark]

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

If you were stupid you wouldn't have asked us to help you clarify your question!

-Dan