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Old Feb 11th 2009, 05:49 PM   #1
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Join Date: Jan 2009
Location: Longview, TX
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Friction Problem

I am having trouble setting up this problem in my physics HW.
I have tried many different things with the force with friction formula.
$\displaystyle F_fr=u_k*F_n$
I have tried and I don't know how to set up this problem correctly.

The problem is

Three masses, M1= 3.3 kg, M2= 4.1 kg, and M3= 7.1 kg are
made of the same material. They are on a horizontal surface
and connected by massless strings. The system accelerates
to the right at 2.43 m/sdue to an external force F pulling on M3.
Given that T1 is 17.44 N.

Calculate the coefficient of
kinetic friction μk?

Calculate the magnitude of the force F?

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Old Feb 12th 2009, 12:56 AM   #2
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I can't see which masses T1 pulls upon, but suppose it pulls to the right on m1. We are given that "the system" accelerates to the right at 2.43 meters per second, which means (I assume) that m1 has this acceleration. If T1 was the only force acting on m1, it would accelerate faster than 2.43. . So the magnitude of the frictional force on m1 must be what keeps the law force = (mass) (acceleration) true. Let mu be the coefficient of friction. (-mu m1 g + 17.44) = m1 (2.43) Similarly Suppose T1 pulls .to the left on m2. Then ( -17.44 - mu m2 g + T2) = m2 (2.43) where T2 is the tension on the next spring. For m3, we have (-T2 - mu m3 g + F) = m3 (2.43).

Thats 3 equations in the unknowns T2,F and mu. Perhaps you can solve for them, pretending the masses are known. I don't know of the masses remain in the answer or cancel out.
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