Direction of Friction Problem HELP

In summary: What about the normal reaction and friction? What is the direction of the frictional force?In summary, the direction of friction depends on the relative slipping motion between two surfaces in contact. In the first case, when a body is rolling on a rough horizontal surface, the friction force is in the same direction as the applied force. In the second case, when a body is rolling down an inclined plane with no external force, the friction force is opposite to the direction of motion. The force of friction always opposes or tends to oppose relative slipping motion and is dependent on the direction of the applied force and the normal reaction.
  • #1
metalInferno
35
0
Direction of Friction Problem .. HELP!

Homework Statement


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 [tex]\theta[/tex] the the force of friction was opposite to the direction of motion . Please explain slowly and step by step as this is very important .

Homework Equations


none

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 .
 
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  • #2
Was the direction of the applied force in situation 1 in the same direction as the velocity of the object?
 
  • #3
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
sure
i get the first one.
 
  • #5
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
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.
 

What is the direction of friction and how does it affect an object?

The direction of friction is the direction in which a force is applied when two surfaces are in contact and one surface is moving or attempting to move against the other. Friction can either slow down or prevent the movement of an object, depending on the direction of the force.

How do you determine the direction of friction in a problem?

The direction of friction can be determined by the relative motion of the two surfaces in contact. If one surface is moving or attempting to move against the other, the direction of friction will be opposite to the direction of motion.

Does the direction of friction always oppose motion?

No, the direction of friction does not always oppose motion. In some cases, such as a car turning around a curve, the direction of friction can actually assist in the motion.

What is the difference between static and kinetic friction?

Static friction acts on an object that is not moving, while kinetic friction acts on an object that is already in motion. Additionally, static friction is typically greater than kinetic friction.

How does the coefficient of friction affect the direction of friction?

The coefficient of friction is a measure of the roughness of the surfaces in contact. A higher coefficient of friction means that there is more friction between the surfaces, and therefore the direction of friction will be stronger and more opposing to motion.

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