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Old Apr 21st 2009, 05:19 AM   #1
MEC
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Pulley/Cable Problem

Can someone show a step by step solution of this problem? Thanks!

The pulley/cable system shown in the attachment is connected to a mass, m, which is accelerating downward at an acceleration of 8 ft/s2, despite the force in the cable, F = 100 lbs. What is the mass, m, of the object?

a = 8 ft/s2
F = 100 lbs
m = ?
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Pulley/Cable Problem-1.pdf  
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Old Apr 22nd 2009, 04:49 AM   #2
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As usual cant open your thumbnail so working blind.

Accln due to gravity is 32 ft/sec^2. Since it is lower here obviously the force opposes the grav force.

The net force on the mass is therefore f given by

f = m x 8 = m x 32 - 100 . Solving, m = 4.166 lbs.
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Old Apr 22nd 2009, 11:20 AM   #3
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Since force F is applied to the cable, the tention through out the cable will be F (assumong cable and pulleys massless). Now the force upward due to cable will be 6F( six cables support the mass) and force down ward is mg. Since the mass is accellerating down ward hence net force on it(mg - 6F) will be down ward. Using Newton's second law, we get

mg - 6F = ma. Put the values of g and a ( = 8 ft/s^2) and find m.
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