Homework Help: Direction of Friction Problem HELP

1. Dec 20, 2007

metalInferno

Direction of Friction Problem .. HELP!!!!

1. The problem statement, all variables and given/known data
i am a student studying very basic rotational motion . i recently came across these problem while i was reading my book .
1. in 1 question , when a body was rolling (with external force) on a rough horizontal surface , the force of friction was in the same direction as the applied force .

2. a body was rolling down an inclined plane (with no external force) with angle of inclination $$\theta$$ the the force of friction was opposite to the direction of motion .

Please explain slowly and step by step as this is very important .

2. Relevant equations
none

3. The attempt at a solution
all i noe is that when a body rotates the friction force is supposed to be opposite and should somehow increase the angular acceleration . but if in a case when it is in the same direction as the motion it should decrease the angular acceleration . please feel free to point out if i m wrong .

Last edited: Dec 20, 2007
2. Dec 20, 2007

uber

Was the direction of the applied force in situation 1 in the same direction as the velocity of the object?

3. Dec 20, 2007

Shooting Star

The frictional force always opposes or tends to oppose relative slipping motion of two surfaces in contact. When a sphere is rolling without slipping, it is the friction which makes the rolling possible. Let's see why.

For example, imagine trying to push a sphere on frictionless ice, say. The sphere will simply slide on the ice, according to Newton's law, and won't have a tendency to roll.

Now suppose, you are pushing the sphere on a surface which has friction. Then, at first, the force of friction will prevent or try to prevent this sliding, which means that the force of friction will act in the opposite direction to which you are pushing. So, the bottom most point will not move, because friction is opposing it. So the body will tend to roll, and not slide. But if the sphere is rolling, then there is a tendency for the bottom most point on the sphere to go backwards, which friction should oppose. So, at the bottom most point, friction is actually acting in the direction of rolling motion as long as the sphere is rolling without any slipping between the sphere and the surface.

If you push it very hard, then the body will partly roll and partly slide on the surface.

This takes care of your first question. Would you like to try your hand at the second?

4. Dec 20, 2007

metalInferno

sure
i get the first one.

5. Dec 20, 2007

metalInferno

now i'll be more precise ;
in the first case ,the force was tangential whereas in the second case the force was through the centre of mass(as in the diagram) . hope it helps u 2 understand the question.

6. Dec 20, 2007

Shooting Star

If you read my post once more, you'll see that I've never mentioned that the force was tangential. It's good for any force acting at any point on the sphere.

The logic in the 2nd case is exactly the same. Compare it with the case I've discussed. The force that we talked about has become the component of its weight along the plane acting downward.