With the pulley system shown at the left
it is found that lifting the weight a distance of 0.1m requires an average force of 7.5N. What is the efficiency of this pulley system?With the pulley system shown at the left
it is found that lifting the weight a distance of 0.1m requires an average force of 7.5N. What is the efficiency of this pulley system?In order to lift the weight 0.1m, the end of the rope must move twice that far --- 0.2m.
Eff = (work output)/(energy input) = (1J)/(1.5J)
= (1/1.5) = 0.66 = 66%.
A rusty pulley system is used to lift an engine out of an automobile. The engine weighs 4000 Newtons and is lifted a distance of 0.1 meters when you pull the chain a distance of one meter with a force of 500 Newtons. What is the efficiency of this pulley system?
Work out = 4000N × 0.1m = 400Nm = 400J
Work in = 500N × 1m = 500Nm = 500J
Efficiency = work out/work in = 400J/500J = 0.8
The efficiency is 80%.
Suppose that a rusty pulley system lifts an object 0.1 meters for every meter you pull its input chain. If this system is 50% efficient, what is the largest weight this system can lift with a force of 500N on the input chain?
First figure what it could lift if it were 100% efficient. The mechanical advantage is 1m/0.1m = 10, so it could then lift 10 × 500N or 5000N.
Next, notice that you only get out half of the work that you put in. Since the distances are the same, that means you only get half the output force or 2500N.
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