# How great is the friction force?

#### uwmphysics

I was wondering if someone could help explain this. I dont understand how to calculate the friction force...

So heres the question I'm working on

You are applying a 400-newton force to a freezer full of chocolate chip ice cream in an attempt to move it across the basement. It will not budge. The weight of the freezer (including ice cream) is 1000 N. The friction force exerted by the floor on the freezer is
a. 400 N
b. greater than 400 N but less than 1000 N
c. 1000 N
d. greater than 1000 N

thanks

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#### nrqed

Homework Helper
Gold Member
uwmphysics said:
I was wondering if someone could help explain this. I dont understand how to calculate the friction force...

So heres the question I'm working on

You are applying a 400-newton force to a freezer full of chocolate chip ice cream in an attempt to move it across the basement. It will not budge. The weight of the freezer (including ice cream) is 1000 N. The friction force exerted by the floor on the freezer is
a. 400 N
b. greater than 400 N but less than 1000 N
c. 1000 N
d. greater than 1000 N

thanks
You should start with a free body diagram. What are the forces acting on the freezer? In what direction are they acting? Using the fact that the net force is zero (the acceleration is zero since the freezer is at rest), the answer should then be clear.

#### uwmphysics

thank you...good idea. I made the diagram, and it would have an unbalenced force without friction, so i think friction has to be 400 N.

do you think I'm right?

#### nazzard

Gold Member
Hello uwmphysics,

you are lucky, instead of calculating the movement of blocks you got a freezer full of chocolate chip ice cream

I assume the applied force is meant to point horizontally at the centre of mass of the freezer (no tilting).

In which direction does the friction force point and where is it's origin? Remember the freezer does not budge while applying 400N.

Regards,

nazzard

#### nazzard

Gold Member
uwmphysics said:
thank you...good idea. I made the diagram, and it would have an unbalenced force without friction, so i think friction has to be 400 N.

do you think I'm right?
Ha, slow forum for me today. Your answer is correct.

#### nrqed

Homework Helper
Gold Member
uwmphysics said:
thank you...good idea. I made the diagram, and it would have an unbalenced force without friction, so i think friction has to be 400 N.

do you think I'm right?
Yes. Good job. (I am assuming that the person is pushing *horizontally*, otherwise the answer would be different.

The free body diagram should have 4 forces: a normal force acting upward (due to the floor pushing on the freezer), the weight of the freezer (i.e. the force of gravity, acting downward), the force applied by the person, 400 N to the right (let's say) and a static friction force to the left. Since the net force is zero, the friction force must have a magnitude equal to the force applied by the person.

It is *always* a very good idea to draw a free body diagram!!

Patrick

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