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zeralda21
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Homework Statement
A bucket of water is raised up out of
a well with a device of the rope, and
pulleys as shown in figure. The bucket is hanging from the
the lower hook and the upper hook is
attached to a fixed cross bar of the well. If
the mass of the bucket and the water together
is m, and the man pulls the rope with
a force F, how large is the span's acceleration
up? The mass of ropes and pulleys
can be neglected, as well as frictional forces.
http://bayimg.com/daOdPAAde
Answer: (3F/m)-g
Homework Equations
F=mg and maybe F=ma
The Attempt at a Solution
Due to gravity there is a force downward. Since the mass of the bucket and the water together is m, the downward force is mg. From figure I see that the device is constructed so that the cable runs three times between the bucket and the hook before I pull it. Therefore the force required for the bucket to move upwards is F=mg/3≥.
From here I am stuck. I know that F=ma and therefore a=F/m. However, if I insert F=mg/3 and m=mg, I get the wrong answer.
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