Answer:
a
because the mechanical wave is when it goes over and over again
The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field.
Friction occurs when the surfaces and heat of two surfaces grind against each other.
I think these are the answers
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