# Question about work and finding the force of friction

• zstraught
In summary, the conversation discusses finding the force of friction in a problem involving a force diagram with a normal force, weight, and applied force at an angle. There is confusion about whether to use the equation for friction equal to mu times the normal force or Newton's second law. It is clarified that the second choice is not valid and the correct equation should be used. There is also a question about the acceleration, which is determined by finding the net horizontal force and dividing by the mass. It is noted that the normal force is not always equal to the weight in this situation due to the vertical component of the pulling force.
zstraught
Homework Statement
A 2.4*10^2 N force is pulling an 85 kg refrigerator across a horizontal surface. the force acts at an angle of 20.0 above the surface. the coefficient of kinetic friction is .200, and the refrigerator moves a distance of 8.00 m. find (a) the work is done by the pulling force, and (b) the work is done by the kinetic frictional force
Relevant Equations
W=Fd
I drew a force diagram, Normal force up, weight down, force of friction to the left, and Force applied 20 degrees above the positive x axis.
I need clarification to find the force of Friction for part B. Should I set Force of friction equal to mu · normal force or should I use Newton's second law and set the force of friction equal to Fa·cos(20)? They both look like valid choices, but they both give different values of force of friction.

zstraught said:
set the force of friction equal to Fa·cos(20)?

haruspex said:
Would it be 0 since it is moving at a constant velocity?

zstraught said:
Would it be 0 since it is moving at a constant velocity?
Where, in the problem statement, does it say that the velocity is constant? I couldn't find it.

kuruman said:
Where, in the problem statement, does it say that the velocity is constant? I couldn't find it.
I thought " A 2.4*10^2 N force..." means it is a constant velocity. If it isn't, how would I find the acceleration?

zstraught said:
I thought " A 2.4*10^2 N force..." means it is a constant velocity. If it isn't, how would I find the acceleration?
By finding the net horizontal force and dividing by the mass.
As you wrote:
zstraught said:
set Force of friction equal to mu · normal force

zstraught said:
... Should I set Force of friction equal to mu · normal force or should I use Newton's second law and set the force of friction equal to Fa·cos(20)? They both look like valid choices, but they both give different values of force of friction.
The second choice is not valid.
You could pull that refrigerator with huge force and still couldn't increase the amount of kinetic friction resisting your effort.

That coefficient of kinetic friction being 0.20 means, in practical terms, that once you get that refrigerator moving and continue pulling it across that horizontal surface, the friction force that resists your pulling remains relatively constant at a value of 20% the weight of the refrigerator.

http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#kin

Lnewqban said:
##\dots## the friction force that resists your pulling remains relatively constant at a value of 20% the weight of the refrigerator.
Not quite. The pulling force has a vertical component, therefore the normal force is not equal to the weight.

Lnewqban
kuruman said:
Not quite. The pulling force has a vertical component, therefore the normal force is not equal to the weight.
Exactly!
Thank you, kuruman.
In this case, the pulling-up angle of 20° reduces the normal and friction forces some, as well as the the horizontal component of the pulling force, which is the one doing the positive work in question.

Last edited:

## 1. What is the force of friction?

The force of friction is a resistive force that acts between two surfaces in contact with each other. It opposes the motion of an object and is caused by the roughness of the surfaces and the intermolecular forces between them.

## 2. How is the force of friction calculated?

The force of friction can be calculated using the formula F = μN, where F is the force of friction, μ is the coefficient of friction, and N is the normal force between the two surfaces.

## 3. What factors affect the force of friction?

The force of friction is affected by the roughness of the surfaces, the weight of the object, and the type of material the surfaces are made of. It also depends on the normal force, which is the force exerted by one surface on the other.

## 4. How does the force of friction affect an object's motion?

The force of friction acts in the opposite direction of an object's motion, so it can slow down or stop the object's motion. It also causes objects to heat up due to the energy lost in overcoming friction.

## 5. How can the force of friction be reduced?

The force of friction can be reduced by using lubricants, such as oil or grease, between surfaces. Smoother surfaces can also decrease the force of friction. Additionally, reducing the weight or the normal force can also decrease friction.

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