Direction of static friction

In summary, the static friction points upwards when a ball is rolling upwards on an incline because it opposes the direction of the object's motion at the point of contact. The normal force is always perpendicular to the surface, but in some static equilibrium problems, it may not appear to be due to the direction of the object's motion at the point of contact.
  • #1
Niles
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Homework Statement


1) If I have a ball rolling upwards on an incline, why is the static friction pointing upwards as well (my book does not explain this)?

2) Also, when I deal with a static equilibrium problem, why is it sometimes that the normal force is not perpendicular to the surface? I thought the normal force was always normal?

Thanks in advance.
 
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  • #2
Think about the direction of the object's motion at the point of contact with the surface. The frictional force opposes the direction of the motion. With a rolling object, the overall direction is up the ramp, but at the point of contact with the ramp the wheel is turning toward the bottom of the ramp. Does that make sense?

For your second question do you have an example? The normal force, as the name implies, is always normal to the point of contact.

CS
 
  • #3


1) The direction of static friction is always opposite to the direction of motion. In the case of a ball rolling upwards on an incline, the force of gravity is pulling the ball downwards, causing it to roll upwards. The static friction force acts in the opposite direction to prevent the ball from sliding back down the incline. This is known as the maximum static friction force and it is always parallel to the surface of contact.

2) The normal force is the force exerted by a surface on an object in contact with it, and it is always perpendicular to the surface. In a static equilibrium problem, the normal force may not always be perpendicular to the surface because there may be other forces acting on the object that are not balanced. For example, if an object is resting on a slope, the normal force will have a component parallel to the slope in order to balance the component of the object's weight that is acting down the slope. However, the normal force must still be perpendicular to the surface at the point of contact.
 

1. What is the direction of static friction?

The direction of static friction is always opposite to the direction of motion or the intended motion of an object. It acts to prevent the object from moving.

2. How is the direction of static friction determined?

The direction of static friction is determined by the direction of the external force acting on the object. It always acts in the direction that opposes the motion or intended motion.

3. Does the direction of static friction change?

The direction of static friction can change depending on the external forces acting on the object. If the direction of the external force changes, the direction of static friction will also change to oppose the new direction of motion.

4. Can the direction of static friction be zero?

Yes, the direction of static friction can be zero if there is no external force acting on the object or if the external force is balanced by another force in the opposite direction.

5. How does the direction of static friction affect the motion of an object?

The direction of static friction plays a crucial role in determining the motion of an object. It can either prevent the object from moving or help it to move in a desired direction. It also helps to maintain the stability of an object on a surface.

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