What force must be applied to the end of the rope in order to evenly lift a load weighing 10 kg with the help of a stationary block, if the efficiency of this mechanism is 80%?
m = 10 kg.
g = 10 N / kg.
Efficiency = 80%.
The stationary block is one of the simple mechanisms that changes the direction of action of the force. For it, we write down the efficiency: efficiency = Ap * 100% / Az. Where Ap and Az are useful and expended work when lifting a load.
Ap = m * g * h.
Az = F * l.
When lifting a load with a fixed block, the lifting height h of the load is equal to the length of the rope that fell l: h = l.
Efficiency = m * g * h * 100% / F * h = m * g * 100% / F.
The formula for determining the force, with a uniform lifting of the load, will look like: F = m * g * 100% / efficiency.
F = 10 kg * 10 N / kg * 100% / 80% = 12.5 N.
Answer: for uniform lifting of a load using a stationary block, it is necessary to apply a force F = 12.5 N.
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