student drops a rock from rest at a distance h above the ground such that the rock hits the ground at time t0. At what distance above the ground should the rock be dropped such that it hits the ground at a time 2t0 after it is released from rest?

Independent variable is something that is changed in a experiment. 

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other

1) what is light? light is an electromagnetic wave visible to the human eye.  2) what is a reflection? the rebounding of a light ray when it hits a mirror.3) what is the incident ray? contacts the surface of an object.4) what is the reflected ray? bounces off a reflective surface.5) what is the normal ray? perpendicular to the surface of a mirror.6) what is the angle of incidence? an angle formed by the incident ray and the normal.7) what is the angle of reflection? an angle formed by the reflected ray and the normal.8) what is refraction? (bending.)9) what is a converging lens? bring together light rays that pass through.10) what is a diverging lens? spreads out light passing through.11) focal point? the real point where the refracted rays meet when the incident rays run parallel.