Frequently Made Errors in Mechanics - Friction - Comments

[haruspex]

haruspex submitted a new PF Insights post

Frequently Made Errors in Mechanics - Friction

Continue reading the Original PF Insights Post.

 

[robphy]

With regards to the work done by forces,
I often hear blanket statements like
"friction always does negative work" and "normal force always does zero work".

 

[haruspex]

With regards to the work done by forces,
I often hear blanket statements like
"friction always does negative work" and "normal force always does zero work".

Yes.. the one about the normal force belongs in the "Forces" post. Unfortunately, my permission to edit it seems to have been taken away again. I'm forming the impression that a blog system (where an item is published and becomes cast in stone, only modifiable by a comment chain) is not suitable for what I thought this was about. It doesn't make a for usable reference text.

 

[AdityaDev]

If a disk slides on a rough surface, then friction will make the disk roll right? Because its the only force that can create a torque. Am I correct?

 

[jbriggs444]

If a disk slides on a rough surface, then friction will make the disk roll right? Because its the only force that can create a torque. Am I correct?

A wrench can produce torque without friction. So can an electric motor. Or tidal gravity. Or a windmill [where, to be clear, it can be the "lift" that is producing torque and the "drag" is not essential].

 

[AdityaDev]

A wrench can produce torque without friction. So can an electric motor. Or tidal gravity. Or a windmill [where, to be clear, it can be the "lift" that is producing torque and the "drag" is not essential].

I was talking about one particular case. Also, If the wrench was frictionless, It will simply slide instead of rotating the object right?

 

[jbriggs444]

I was talking about one particular case. Also, If the wrench was frictionless, It will simply slide instead of rotating the object right?

Most wrenches (box end, open end, adjustable, socket, etc) push on the faces of hexagonal and square nuts and bolts at a position offset from the center of the face. Friction is unimportant to their use.

Edit: Hard to hold them without friction, of course. Though not completely impossible.

 

[AdityaDev]

Understood. If the bolt was circular, then friction will be important right?
Also, for a ring, if a force is applied tangentially at its topmost point, then no friction is required to make it roll. And this true only for a ring.
Force equation:
$$F=ma$$
Torque eqation:
$$FR-fR=MR^2\alpha$$
$$F-f=MR\alpha$$
And for pure rolling, $R\alpha=a$
So $$F-f=Ma$$
Since F=Ma, f=0.