# Confused with friction directions

1. Sep 12, 2008

### Hippoman

If I have a cylinder rolling down a slope, the friction would point towards UP the slope, right?
So if instead the cylinder was _pulled_ up the slope by a force, wouldn't the friction direction be down? I have a problem because I have two separate lecture notes which state different answers and I don't know which of them is right. One says the friction would point up (which I think is wrong) and the other one says down.

Also, it's confusing because physics books say that friction always opposites movement. But if the cylinder is rolling on a plane, say, right, then the rotating movement would be clockwise. Why isn't the friction then opposing that rotating movement so that the friction force would be pointing right also? I mean, the lowest point of the cylinder is always moving left.

Secondly, if you put a rolling cylinder that's in the air (also rotating clockwise for example) to a plane and it starts to _slide_ at first, which way would the friction be at that case?

I've been searching the net and physics books for hours and I can't make these questions clear to myself, please help!

2. Sep 12, 2008

### Staff: Mentor

Correct on both counts (assuming the cylinder rolls without slipping).
Better to think of friction as opposing slipping between surfaces.
If the cylinder is rolling along a horizontal plane without accelerating, then there's no static friction at all (ignore rolling friction and other complications). Also, with respect to the surface, assuming rolling without friction, the contact point of the cylinder always has zero speed.
Which way would the surfaces tend to slip against each other? Friction opposes it. (Also, compare this with a bit of common experience. Which way will the cylinder start moving? It's friction that makes it move.)

3. Sep 12, 2008

### edziura

These are excellent questions.

"Also, it's confusing because physics books say that friction always opposites movement. But if the cylinder is rolling on a plane, say, right, then the rotating movement would be clockwise. Why isn't the friction then opposing that rotating movement so that the friction force would be pointing right also? I mean, the lowest point of the cylinder is always moving left."

Consider these:

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4. Sep 12, 2008

### Staff: Mentor

edziura's second attachment illustrates "rolling" friction. I suggest that you ignore such complications until after you've nailed down the direction of static friction.

5. Sep 12, 2008

### Hippoman

Thanks Doc Al and edziura! Helped a lot.

About that answer for the second problem, if we continue with common experience. What will make the cylinder stop in the real world if the friction is the force that makes it move in the same direction? Is it the rolling friction in that case?