# How do i find the friction force without μ or acceleration?

## Homework Statement:

I need to find the static and kinetic coefficient of a desk for a physics lab. we have pulleys, string, a scale, a "car" and a few other things. How would I find the force of friction?

## Relevant Equations:

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no idea

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haruspex
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Friction involves two surfaces. What are they? Or do you mean rolling resistance?
The "scale" is as in a weighing scale?
What other things?

Friction involves two surfaces. What are they? Or do you mean rolling resistance?
The "scale" is as in a weighing scale?
What other things?
I guess the desk and the floor.
scale is for weighing yes. things that can be used for weight (like paper, beads etc.), measuring tape and i think thats it but we are allowed to bring anything else we want.

haruspex
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I guess the desk and the floor.
scale is for weighing yes. things that can be used for weight (like paper, beads etc.), measuring tape and i think thats it but we are allowed to bring anything else we want.
Which of all the items you listed can directly measure a force?

Which of all the items you listed can directly measure a force?
nothing? there's a way for sure i just cant think of anything. does anything come to mind?

neilparker62
Homework Helper
If your 'scale' can measure tension in a string, just attach it to object(s) and pull - take reading at the point where the object(s) just begin to move. Alternatively increase slope of surface till object just begins to slide - then $μ_s=tan(θ)$ if I remember correctly. I can't quite see how to do kinetic friction but I'd guess you will need some kind of 'ticker timer' experiment whereby the object(s) are pulled along the table surface by a falling weight of known value. You should be able to calculate coefficient of kinetic friction if you can obtain a value for the acceleration of falling mass and/or mass sliding on the table.

haruspex
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If your 'scale' can measure tension in a string, just attach it to object(s) and pull
Please do not provide answers so early. Try nudging a little first. The student must be encouraged to think.

If your 'scale' can measure tension in a string, just attach it to object(s) and pull - take reading at the point where the object(s) just begin to move. Alternatively increase slope of surface till object just begins to slide - then $μ_s=tan(θ)$ if I remember correctly. I can't quite see how to do kinetic friction but I'd guess you will need some kind of 'ticker timer' experiment whereby the object(s) are pulled along the table surface by a falling weight of known value. You should be able to calculate coefficient of kinetic friction if you can obtain a value for the acceleration of falling mass and/or mass sliding on the table.
by scale i mean bathroom scale

Please do not provide answers so early. Try nudging a little first. The student must be encouraged to think.
how would my bathroom scale measure the tension in string

haruspex
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by scale i mean bathroom scale
Doesn't that measure force?

haruspex
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how would my bathroom scale measure the tension in string
It can measure the force provided by the weight of a mass. How could you use that weight?

Doesn't that measure force?
force of gravity. i need force of friction
It can measure the force provided by the weight of a mass. How could you use that weight?
weight is equal to force of gravity...
i could get the mass from the weight...
thats all i can think of

haruspex
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force of gravity. i need force of friction

weight is equal to force of gravity...
i could get the mass from the weight...
thats all i can think of
How could you use the weight of the mass?

haruspex
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What do you need the desk to do if you are going to find the maximum static frictional force it has from the floor?

What do you need the desk to do if you are going to find the maximum static frictional force it has from the floor?
i would need to get the desk to start moving

How could you use the weight of the mass?
would the weight which equals to force of gravity be equal to force Newton which is the same as force tension?

haruspex
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would the weight which equals to force of gravity be equal to force Newton which is the same as force tension?
If you use weight in the right way, yes.
Gravitational force is horizontal. What direction do you need the force on the desk? How can you arrange that?

If you use weight in the right way, yes.
Gravitational force is horizontal. What direction do you need the force on the desk? How can you arrange that? it would look like that (ignore the masses labelled)

haruspex