12. A helicopter of mass 2900 kg takes off vertically and ascends to a height of 45 m above the ground. A constant lift force of 3.2 x 104 N acts on it. Calculate: a. i. the work done on the helicopter by the lift force ii. the change in the gravitational potential energy of the helicopter. b. Explain why the answers in a. i. and ii. are different. C. Determine the final vertical speed of the helicopter.