Question

a 2 kg ball is dropped from a height of 20m. what is its kinectic energy when it is 16m off the ground?

Answer 1

You could calculate the velocity, since you know the acceleration due to gravity but you'll have to figure out how long it has been falling to get v. That's not too hard. But an easier way might be to recall that the total energy U+K is constant. So initially all of the energy was potential energy, U. At the very end right before it hits the ground all of the energy is kinetic, K. At all points in between its a combination of the two. Get the formula for gravitational potential energy and calculate it at 20m and at 16m. The difference of these two is the amount that has been converted to kinetic energy so far.