Physics Forces Question (Static Friction)

[glam407]

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Hi Guys,

I was wondering if someone could explain

a) Why is Static Friction always higher than Kinetic Friction?

B) If two blocks were on a board, and the board were raised at one end -- why is the angle of the board with respect to the horizon necessary to determine static friction force?
( and why isn't the mass, the distance the block slid, etc...not important?

Thank you so much.

 

[ClemsonFirst]

Hi Guys,

I was wondering if someone could explain

a) Why is Static Friction always higher than Kinetic Friction?

B) If two blocks were on a board, and the board were raised at one end -- why is the angle of the board with respect to the horizon necessary to determine static friction force?
( and why isn't the mass, the distance the block slid, etc...not important?

Thank you so much.

A) static friction is higher because it is a stronger resistance to movement than kinetic. It takes more force to start an object sliding (static) than to keep it sliding (kinetic).

B) It is necessary because you have to know how much the force of gravity is pulling the block down the slope. Static friction will oppose this force to the same amount to prevent moving. However, it should be important to know the mass of the block to know how much force gravity is pulling with (for normal force)

Static friction is the friction of a non-moving object, so it will never be relevant how far the block slid for static friction.

 

[BerkReviewTeach]

A) static friction is higher because it is a stronger resistance to movement than kinetic. It takes more force to start an object sliding (static) than to keep it sliding (kinetic).

B) It is necessary because you have to know how much the force of gravity is pulling the block down the slope. Static friction will oppose this force to the same amount to prevent moving. However, it should be important to know the mass of the block to know how much force gravity is pulling with (for normal force)

Static friction is the friction of a non-moving object, so it will never be relevant how far the block slid for static friction.

Totally agree on your explanation to A. It's pretty much the way it is, meaning once something breaks free on a uniform surface, it'll keep going.

On B, you might want to rethink the importance of mass. The thing about static friction is that it is a reactionary force, so it varies. When an object lies on a flat surface, there is a normal force cancelling out the weight, so there is no net force. Hence, there is no force due to static friction. As the plane is raised to an increasing angle, the magnitude of the normal force is reduced. In order to remain stationary, the static friction must increase enough to offset the decrease in normal force. This occurs until you reach the maximum possible static friction, mus x N.

At the point it's on the verge of breaking free, we have mg sin(theta) = musmg cos(theta). The mg cancels out which is why you don't need the mass. The equation reduces to sin (theta) = mus cos(theta), so mus = sin(theta)/cos(theta).

Conceptually, if you need a steeper angle to get the block to break free, then the maximum frictional force must be larger, which fits with mus = sin(theta)/cos(theta). Also, mus is unitless, which also fits with the equation.

So in a question you might see something like the following:

Addition of glue to the surface on which the block resides will have what impact on the experiement?
A) It will decrease the threshold angle at which the block breaks free and slides.
B) It will decrease the magnitude of the normal force at all angles above zero.
C) It will increase the value of cos(theta) in the mus calculation.
D) It will increase the tan(theta) for the threshold angle at which the block breaks free and slides.

 

[neurodoc]

For an object resting on an inclined plane, there are electrostatic forces that need to be overcome before the object starts to slide. Set up a kinematic diagram. You have mg pointing downward, the angle the plane makes with the horizontal, and you describe vector forces with a downward (mg) component and components normal to the plane and along the plane. When the plane is inclined sufficiently to overcome the electrostatic forces that hold the object in position, the object begins to slide down the plane under gravity.

 

[circulus vitios]

Hi Guys,

I was wondering if someone could explain

a) Why is Static Friction always higher than Kinetic Friction?

B) If two blocks were on a board, and the board were raised at one end -- why is the angle of the board with respect to the horizon necessary to determine static friction force?
( and why isn't the mass, the distance the block slid, etc...not important?

Thank you so much.

A. Because it makes sense? Imagine a world where static friction wasn't always greater than kinetic friction.