a cord runs two massless, frictionless pulleys. A canister with mass m=20kg hang

Question

a cord runs two massless, frictionless pulleys. A canister with mass m=20kg hangs from one pulley, and you exert a force F on the tree end of the cord.  What must be the magnitude of F if you ae to lift the canister at a constant speed?  To lift the canister by 2.0 cm, how far must you pull the free end of the cord?  During that lift, what is the work done on the canister by your force ( vi the cord) and the gravitatonal force? ( hint- when a cord loops around a pulley as shown, it pulls on the pulley with a net force that is twice the tension in the cord.)

Explanation / Answer

2F = mgF    = mg / 2    = 20 * 9.8 / 2    = 98 N
(b)
Distance to pull free cord is
2 * lift = 4 cm
(c)
work = mgh    = 20 * 9.8 *0.04    = 7.84 Joules
(d)
work done by grav force = - m g h = -7.84 J

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