how much work is preformed to lift 20 N object 3m? what is the PE after it is lifted? if it is dropped what is its KE and velocity just before it hits the ground?
A 20N object requires a 20N force to lift it, and when that force moves 3m against gravity, the work done is: 20 * 3 = 60 J. That is the PE of the object after it has been lifted. It is also the KE of the object just before it hits the ground. If it hits with speed v, and m is the mass of the object, then: m = 20 / 9.81 kg. (1/2)mv^2 = 60 v = sqrt(120 / m) = sqrt(120 * 9.81 / 20) = 58.86 m/s.
