is used mainly in submarines that remain underwater

Your correct main answer is a submarine

Explanation:

Submarines are usually used for underwater warfare, and naval submarines are equipped with torpedoes, missiles, and high-powered nuclear weapons. The use of these along with their advanced tracking system helps to target ships and boats from below as well as other enemies. They can also work on targets that are on land.

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Sonar is used mainly in submarines that remain underwater

Explanation:

Sound travels through water a lot faster than through air. Sonar is used in submarines because having a window in the depths of the ocean would not be practical because of the amount of light down there and the pressure down there is a lot more than at sea level.

If I was given more context I'm sure it could give a more helpful answer.

the amount of current in the coil

the force on the thing and the distance the force moves the thing