A load weighing 150 kg is lifted using a movable block, the efficiency of which is 75%.

A load weighing 150 kg is lifted using a movable block, the efficiency of which is 75%. What is the mass of a block if friction in the block can be neglected?

To find out the mass of the indicated movable block, consider the formula: η = Apol / A = mg * g * h / (F * S) = mg * g / ((mg + mbl) * g) = mg / (mg + mbl), whence we express: mg + mbl = mg / η and mbl = mg / η – mg = (1 / η – 1) * mg.

Data: η – unit efficiency (η = 75% = 0.75); mg is the mass of the load being lifted (mg = 150 kg).

Let’s calculate: mbl = (1 / η – 1) * mg = (1 / 0.75 – 1) * 150 = 50 kg.

Answer: The mass of the indicated movable block must be 50 kg.