#1
Best of Luck to ya..

A mass m1 on a horizontal shelf is attached by a thin string that passes over a frictionless peg to a 2.2 kg mass m2 that hangs over the side of the shelf 1.5 m above the ground (Figure 5-62). The system is released from rest at t = 0 and the 2.2 kg mass strikes the ground at t = 0.84 s. The system is now placed in its initial position and a 1.2 kg mass is placed on top of the block of mass m1. Released from rest, the 2.2 kg mass now strikes the ground 1.3 seconds later.
Determine the mass m1.
Determine the coefficient of kinetic friction between m1 and the shelf.

Some useful information..
Fnet=m2*g-m1*g*u=(m1+m2)*a
Where m2 is the mass of the falling block, m1 is the block on the flat surface whose friction pulls against m2's fall. u is coefficient of friction. g is gravity, and a is accel.
I think you get accel with 1.5=a/2*0.84^2 = 4.2517
You probably need to use systems of equations, but I just can't get the right answer.
#3
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#4
We have a physics thread I think, or a science thread. You'll get more luck in there.
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