What is the work input to lift a box of 810 with a 50% mechanical efficiency?

Work input = 1620

Explanation:

Given the following data;

Work output = 810

Efficiency = 50%

To find the work input;

Mathematically, efficiency is given by the formula;

Substituting into the formula, we have;

Cross-multiplying, we have;

Work input = 1620

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.