Static friction force

In summary, the question asks for the expression that describes the frictional force on a stationary box when a force is applied at an angle above the horizontal. The two possible equations are F=umg or F=uFcos(theta), with the latter being more accurate since the forces must balance out in both the vertical and horizontal components. The normal force equation is only relevant for the maximum static frictional force, where it is the force at which the box moves due to overcoming static friction.
  • #1
physgirl
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0

Homework Statement



On a box (of mass "m"), force "F" is applied at "theta" degree angle above the horizontal. The box has static friction coefficient of "u." When this force is applied, and the box remains stationary, what expression describes the frictional force?

Homework Equations



F=uN??
(N is normal force)

The Attempt at a Solution



I came down to 2 choices:
F=umg
or
F=uFcos(theta)

I initially thought the former, because I keep remember being drilled the equation F=uN, and about how it's all about the normal force when it comes to friction.

But then when I thought about it, forces must balance out... so if there's no net acceleration, applied force (i.e. Fcos(theta)) MUST balance out frictional force, no?

And now I'm just confusing myself.
 
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  • #2
Right, yes the forces must balance out.

When you split the force into vertical and horizontal components what do you get?
 
  • #3
right, so if the box isn't moving (yet), then for vertical, we just have normal force = gravity force (i think?), which is just mg=mg... then for horizontal, we have applied force (Fcos(theta))=frictional opposing force (u*F*cos(theta))...

i think i get the confusion. so the normal force equation is only relevant for *maximum* static frictional force equation, where that's the force at which the box moves b/c it overcomes static friction. yes?
 
  • #4
yes but you see, your normal reaction is not just N=mg since the vertical component of the force Fy is present and depending how the force is applied, the normal reaction will change.
 

1. What is static friction force?

Static friction force is a force that exists between two surfaces in contact with each other, preventing them from sliding against each other when one of the surfaces is in motion.

2. How is static friction force different from kinetic friction force?

Static friction force is the force that exists between two surfaces when one is stationary, while kinetic friction force is the force that exists when two surfaces are in motion relative to each other. Static friction force is typically greater than kinetic friction force.

3. What factors affect the magnitude of static friction force?

The magnitude of static friction force depends on the type of surfaces in contact, the normal force between the two surfaces, and the roughness of the surfaces.

4. Can static friction force be greater than the applied force?

Yes, static friction force can be greater than the applied force if the surfaces in contact have a high coefficient of friction and the applied force is not enough to overcome the static friction force.

5. How do you calculate static friction force?

The formula for calculating static friction force is Fs = μsN, where Fs is the static friction force, μs is the coefficient of static friction, and N is the normal force between the two surfaces in contact.

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