A roller-coaster car stops above a vertical drop. Suddenly it falls under gravity. The drop is $60$metres high and at the bottom of the drop the car travels at $125 \mathrm{km}/\mathrm{h}$. The acceleration experienced by the people in the car is $10 \mathrm{m} {\mathrm{s}}^{-2}$. The mass of the car and its passengers is $1210 \mathrm{kg}$. Calculate the force exerted on the car and its passengers. 

 

A crane is used to lift a steel girder to the top of a high building.

A motor is fitted to the crane. This motor accelerates the girder at $0.3 \mathrm{m} {\mathrm{s}}^{-2}$. Calculate the force which the crane applies to the girder to produce this acceleration.

Srikanth Kidambi is an excellent Indian badminton player who represented India in Rio Olympics. The diagram shows him in between a match.

The shuttlecock weighs very little. When you drop it from a height of a few metres, it accelerates at first but soon reaches a steady speed.

Explain, why the shuttlecock accelerates at first, and why the shuttlecock reaches a steady speed.