Why is it necessary to use at least one electromagnet in a motor instead of using only permanent magnets?

Because as you might already know electromagnets use a conductive metal and electricity to make the said electromagnet. So what do you get if you use the electromagnet and move it through coiled wire. The magnetism makes a electrical current thus making your motor.

I hope that this was helpful for you. :3  

Sorry i don’t know what to do

When someone eats food