# Homework Help: Static Friction

1. Jan 14, 2008

### Gear300

When an object, such as a wheel, rolls across a surface, static friction allows it to move. When taking into account rotational motion accompanied by translational motion across such a surface, what effect does static friction have on it other than allowing the object to move? Furthermore...what are the situations for which static friction can accelerate an object?

Last edited: Jan 14, 2008
2. Jan 14, 2008

When we look at rolling, we can say that the object (wheel/ball/whatever) is rotating around the point of contact with the floor, P. P is also moving forward with the translational speed of the rolling object. Static friction is what allows P to move forward rather than the object simply spinning in place.
Static friction accelerating an object? When we have a large block on a frictionless floor, and a smaller block on top of it. They are both moving together; static friction has not yet been overcome. We apply a force to the system to accelerate it until we break static friction. The sudden change in frictional force causes the large block to accelerate. But, I don't know whether this counts expressly as static friction accelerating the body.

3. Jan 14, 2008

### Gear300

I see...so static friction simply allows for the rotation to occur at point P. That implies that it does not take part in accelerating the object in any direction, right? Although, I heard somewhere that inclined planes are an exception to this; how so?

4. Jan 15, 2008

With an inclined plane, you have the same situation, but now you have a gravitational force acting on the wheel, which counteracts the static friction force that facilitates smooth rolling. If the force exceeds the constant static friction point at P, the wheel will slip. But rolling downhill is the same as rolling with a constant applied force. As long as this force does not exceed that of static friction, the wheel will not slip.

5. Jan 15, 2008

### Gear300

Alright, thanks