Brook T.

asked • 03/01/18

Physics Math

You are helping your friend move a new refrigerator into his kitchen. You apply a horizontal force of 265 N in the positive x direction to try and move the 59 kg refrigerator. The coefficient of static friction is 0.75

A.) How much static frictional force does the floor exert on the refrigerator? Give magnitude (in N)?
 
B.) What maximum force (in N) do you need to apply before the refrigerator starts to move?

Arturo O.

Student,
 
For part (B), you should have asked "What MINIMUM force do you need to apply before the refrigerator starts to move?"  There is no upper limit to the force that can make the refrigerator move, but there is a lower limit.
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03/02/18

1 Expert Answer

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Philip P. answered • 03/01/18

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The maximum force of static friction is:
 
f = μmg  newtons
  • μ = coeffficient of static friction = 0.75
  • m = mass of the object = 59 kg
  • g = acceleration due to gravity = 9.8 m/s2
 
Plug in the numbers and compute the answer.  You will see that f > the 265 newtons you are applying, so the fridge isn't going anywhere.  The minimum applied force (F) needed to move the refrigerator is F ≥ f.
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Brook T.

This explains the answer to part B, but how do you figure out the first part?
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03/02/18

Philip P.

The maximum amount of frictional force exerted by the floor on the refrigerator is just the force of static friction, f = μmg.  The way friction works is that the force it exerts is exactly equal and opposite to your applied force, up to the maximum frictional force.  So if the max frictional force is 400 N and you push with 300 N, the opposing frictional force will be 300 N.  If you push with 350 N, it will oppose you with 350 N.  If you push with 450 N, it can only oppose you with 400 N, its max value.  So to move the fridge, you must apply a force more than the max frictional force.
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03/02/18

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