An electromagnetic wave is a kind of a wave which, needs a medium to travel through

only travels in a vacuum

travels in presence of medium or even in a vacuum

Gunpowder residue is most likely to show up where on a shooters hands

The two required components for a generator are

The period of a pendulum varies directly as the square root of the length of the pendulum and inversely as the square root of the acceleration due to gravity. find the period when the length is 144 cm and the acceleration due to gravity is 980 cm per second squared, if the period is 7pi seconds when the length is 289 cm and the acceleration due to gravity is 980 cm per second squared.