Direction of static friction in rolling motion?

[timnswede]

I don't really have a specific problem, but for example, I was doing a problem where a constant force unwounds a spool of wire (a disk). The force pulls at the top of the disk to the right and the force of static friction is also at the bottom to the right, the same direction as the sphere is moving. But in an example we did in class where a sphere rolls down an incline, the force of static friction was to the left, up the incline and the opposite direction of the way the sphere was rolling. I'm really lost as to how to find out which was static friction is supposed to go, can anyone enlighten me? I wasn't sure where else to post this question so I'm sorry if it is in the wrong place.

 

[Stephen Tashi]

I was doing a problem where a constant force unwounds a spool of wire (a disk). The force pulls at the top of the disk to the right and the force of static friction is also at the bottom to the right, the same direction as the sphere is moving.

For the spool, I think you mean "the cylinder", not "the sphere". What is causing the friction in that problem? My guess will be that the spool is rolling along a horizontal surface and there is friction between the surface and the spool.

But in an example we did in class where a sphere rolls down an incline, the force of static friction was to the left, up the incline and the opposite direction of the way the sphere was rolling.

If you can do arithmetic with negative numbers, you don't actually have to draw vectors in physics diagrams perfectly. If you happen to draw them opposite the way they actually point, you'll get a negative answer for their magnitude. That tells you they point the other way. Of course, it's easier to think about problems if you get the arrows pointed in the right direction to get positive magnitudes.

One approach is to consider what would happen without friction (which may require some thought since we don't encounter such situations in everyday experience). Without friction, a spool on a horizontal surface pulled by a cord unwinding at its top, would just spin "in place". it wouldn't roll along the table. An object going down an inclined plane would slide down instead of rolling. Consider which direction friction must act into produce familiar behavior. Friction would have to move the rolling spool in the direction that the cord is pulling it. Friction on the inclined plane would have to act to make the sphere roll instead of slide.

 

[A.T.]

I'm really lost as to how to find out which was static friction is supposed to go, can anyone enlighten me?

See this post:
https://www.physicsforums.com/threads/investigation-of-a-rotating-cylinder.770027/#post-4847396

 

[timnswede]

For the spool, I think you mean "the cylinder", not "the sphere". What is causing the friction in that problem? My guess will be that the spool is rolling along a horizontal surface and there is friction between the surface and the spool.
If you can do arithmetic with negative numbers, you don't actually have to draw vectors in physics diagrams perfectly. If you happen to draw them opposite the way they actually point, you'll get a negative answer for their magnitude. That tells you they point the other way. Of course, it's easier to think about problems if you get the arrows pointed in the right direction to get positive magnitudes.

One approach is to consider what would happen without friction (which may require some thought since we don't encounter such situations in everyday experience). Without friction, a spool on a horizontal surface pulled by a cord unwinding at its top, would just spin "in place". it wouldn't roll along the table. An object going down an inclined plane would slide down instead of rolling. Consider which direction friction must act into produce familiar behavior. Friction would have to move the rolling spool in the direction that the cord is pulling it. Friction on the inclined plane would have to act to make the sphere roll instead of slide.

Thank you, thinking of what would happen without friction makes a lot of sense. And thank you to you too A.T., that also makes sense.