Wind energy spins the blades on this wind turbine. The mechanical energy of the moving blades is used to generate electricity. But wind turbines are not suited to all areas of the country, because in some places there’s is not enough wind. Why is an eternal energy source, in this case wind, needed to keep the blades spinning?

As the "wind" pushes the blades and makes them to spin, kinda like an inverse process of how a fan works (where in the fan you impunt energy to make the blades move and generate wind, and in this case the wind makes the blades move to generate electricity), and this movement of the blades creates work that is transformed into electric energy.

Without the wind, you can not generate work in the blades, and then electric energy can not be generated, so you need this source to "push" and impulse torque in the blades.

whatever that guy said up top

Explanation:

a) are muscles that have bands.

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one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.