Why Does Static Friction Point Upwards on an Incline?

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SUMMARY

The discussion clarifies the behavior of static friction on an incline, specifically addressing why static friction acts upwards when a ball rolls upwards. The frictional force opposes the direction of motion at the contact point, which is directed downwards due to the wheel's rotation. Additionally, the normal force is defined as always being perpendicular to the surface at the point of contact, although its direction may appear non-perpendicular in certain scenarios, depending on the forces acting on the object.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Basic concepts of static friction and its directionality
  • Knowledge of forces acting on objects in static equilibrium
  • Familiarity with inclined plane physics
NEXT STEPS
  • Study the principles of static friction and its applications in inclined planes
  • Learn about the conditions for static equilibrium in physics
  • Explore the concept of normal force in various contexts
  • Review examples of rolling motion and its effects on friction
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Students studying physics, particularly those focusing on mechanics, as well as educators seeking to clarify concepts related to friction and forces on inclined planes.

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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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
 

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