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Old Apr 13th 2019, 12:14 AM   #1
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how do i find force of friction without μ or acceleration

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?
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Old Apr 13th 2019, 12:29 AM   #2
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You cannot find the coefficients of friction for a desk ... only for the desk and a particular object ... different objects will give different coefficients on the same desk ....

So you have a "car" ..( a block of wood?) , put it on the desk and find what horizontal force is needed at the moment when movement starts ....call this Fstatic ...

You can measure the weight of the object call this F(in Earth's gravity)

These are the two numbers you need to find the static coefficient of friction. I'm sure you can do the rest...
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Old Apr 13th 2019, 01:48 AM   #3
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Oz touched on the difference between static and kinetic friction
have a quick look at <Wikipedia : Coefficient of Friction>
Friction is a function of the two materials in contact, the area of contact, and the force pushing them together.
It can be determined experimentally by simply measuring the force required to drag one object across the surface of the other.

Note that the wheel reduces friction by reducing the area of contact.
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Old Apr 13th 2019, 03:52 AM   #4
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Originally Posted by Woody View Post
Note that the wheel reduces friction by reducing the area of contact.
Well ... with a wheeled car there is no sliding of the surface of the wheel over the surface of the table .. The static friction of the table has not been overcome when the car moves (unless breaks are applied and the wheels are locked) ...

Without applied breaks , what has been overcome when the car starts to move is the static friction at the axle ...another story .
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