Understanding Static Friction on a Ramp: Why is the Object Stationary?

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Discussion Overview

The discussion revolves around the concept of static friction acting on an object on an inclined plane. Participants explore the conditions under which the object remains stationary, examining the relationship between static friction and the component of gravitational force acting down the ramp.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the relationship between static friction and the weight component down the ramp, questioning how an object can remain stationary if static friction is greater than the weight component.
  • Another participant suggests that the confusion arises from an unclear problem statement and emphasizes the importance of calculating the maximum possible friction force using the normal force and friction coefficient.
  • A different participant clarifies that the force of static friction is not always equal to its maximum value and that it adjusts to balance the forces acting on the object, specifically the component of weight down the ramp.
  • One participant asserts that the force of static friction will equal the component of weight down the ramp at equilibrium, leading to the conclusion that the object is at rest.
  • A later reply confirms the assertion that the magnitude of static friction will match the gravitational component down the ramp, reinforcing the idea of equilibrium.

Areas of Agreement / Disagreement

Participants generally agree on the principle that static friction adjusts to balance the forces acting on the object, but there is some disagreement regarding the interpretation of static friction's maximum value and its implications for the object's motion.

Contextual Notes

There is a reliance on the definitions of static friction and the conditions for equilibrium, which may not be fully articulated in the discussion. The calculations and assumptions made by participants are not explicitly detailed, leaving some aspects unresolved.

EF17xx
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Hi ,

The Static Friction force of an object on a ramp(inclined plane) making an angle with the horizontal is greater than the weight component down the ramp. My textbook says that this means that the object on the ramp is stationary as the frictional force is greater than the weight component force down the ramp. However for an object to be still or in equilibrium shouldn't the forces add up to 0. In this case if the frictional force is greater, the object should technically be moving up the ramp and intuitively this is obviously wrong so could someone please clarify this for me ? as I am a bit confused..
 
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The confusion here is due to an unclear problem statement. You calculate the maximum possible friction force from the normal force and friction coefficient. You use the FBD to find the actual friction force.
 
EF17xx said:
The Static Friction force of an object on a ramp(inclined plane) making an angle with the horizontal is greater than the weight component down the ramp.
How do you know this? I suspect you think that the force of static friction is calculated from fs = μsN (N = normal force). This is not the correct way to find the force of static friction. As @jrmichler suggests, you should draw the FBD and figure out what it is.

The right side of the equation fs = μsN is the largest value that the force of static friction can have. This means that static friction can be no larger than that value. If the component of the weight down the ramp is less than that maximum value, then the object will be at rest on the incline.
 
Ok so the force of static friction will not actually be the largest value and will only be big enough to cancel out the component down the ramp at the points on the ramp that it can but as we know that the force of static friction can be larger than the component down the ramp at that specific point as I calculated both the max force of static friction and the size of the component down the ramp they will be cancelling each other out. But the magnitude of the force of static friction will simply be equal to the magnitude of the force component down the ramp and therefore the object is at rest. Is this correct?
 
EF17xx said:
But the magnitude of the force of static friction will simply be equal to the magnitude of the force component down the ramp and therefore the object is at rest. Is this correct?
That is absolutely correct.
 

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