How high must a 20kg box be lifted to give it 784j of a potential energy

What are atoms of the same element with varying number of neutrons

This is for my 8th grade science class, i really don’t understand. can someone me? , lol.

Light traveling in water, nwater = 1.33, strikes a plastic block at an angle of incidence of 51.4°; part of the beam is reflected and part is refracted. if the index of refraction of the plastic is 2.0, what is the angle made by the reflected and refracted beams?