Work done is force × distance, so one newton-metre (N m) is the same as one joule. Energy is measured in joules. This means 1 J = 1 N m, and 15 N m is 15 J.
The gravitational potential energy gained by lifting an object is E = m × g × h, where m is mass in kilograms, g is gravitational field strength in N/kg and h is the height raised in metres. In this lesson g = 10 N/kg, but always use the value given in the question. A 2 kg object lifted through 5 m gains 2 × 10 × 5 = 100 J, which is equal to the work done by the lifting force.
The kinetic energy of a moving object is E = ½ × m × v², which is half the mass × speed squared. A 4 kg object moving at 3 m/s has ½ × 4 × 32 = 18 J. When an object falls with no air resistance, its gravitational potential energy is transferred to kinetic energy. Whenever energy is transferred by a force, the energy transferred equals the work done: a 30 N force moving a box 5 m transfers 150 J.