What force is necessary for a person to hold a 50 kg mass over their head?

An object resting on a table weighs 100 n. with what force is the object pushing on the table? with what force is the table pushing on the object? explain how you got your answer.

Aweight lifter raises a 1600 n barbell to a height of 2.0 meters. how much work was done? w = fd a) 30 joules b) 3000 joules c) 320 joules d) 3200 joules

Would it be correct to say the doppler effect is the apparent change in the speed of a wave due to the motion of the source? be sure to fully explain your position using at least 3 complete content related sentences.