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

• EF17xx
In summary: The maximum possible force of static friction is equal to the weight component down the ramp, but it will only be as large as necessary to cancel out that component and keep the object at rest on the incline. In summary, the force of static friction on an object on a ramp is greater than the weight component down the ramp, but it will only be as large as necessary to keep the object at rest on the incline.
EF17xx
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..

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.

## 1. What is static friction on a ramp?

Static friction on a ramp is the force that prevents an object from sliding down the ramp when it is at rest.

## 2. How is static friction on a ramp calculated?

Static friction on a ramp is calculated by multiplying the coefficient of static friction between the ramp and the object by the normal force acting on the object.

## 3. What factors affect static friction on a ramp?

The factors that affect static friction on a ramp include the type of surface the ramp and object are made of, the weight of the object, and the angle of the ramp.

## 4. How does the angle of the ramp affect static friction?

The angle of the ramp affects static friction by increasing it as the angle increases. This is because as the angle increases, the normal force acting on the object also increases, resulting in a greater force of static friction.

## 5. Can static friction on a ramp be greater than the force of gravity?

Yes, it is possible for static friction on a ramp to be greater than the force of gravity if the angle of the ramp is steep enough. This is because the normal force acting on the object will be greater than the force of gravity, resulting in a greater force of static friction.

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