Sky-diving :

A Case of Terminal Velocity

1. 1. When a skydiver jumps out of the plane, the only force acting on him is the pull due to gravity (his weight) so he accelerates downwards at 10 m/s.
2. 2. As he start to fall, the air resistance opposing his motion begins to builds up. The net force downwards is reduced. He still accelerates, but at rate lower than 10 m/s.

3. 3. Air resistance increases with speed. Eventually the air resistance is equal to the weight of the skydiver. the forces are balanced. The skydiver falls at a steady speed (terminal velocity) of about 200 km/h.
4. 4. The skydivers now opens his parachute. The large surface areas of the parachute gives rise to high air resistance acting upwards which is greater than the weight of the skydiver. The net force is upwards and the skydiver decelarates - his speed reduces.
5. 5. The skydiver continues to slow down until the air resistance acting upwards equal the weight of the skydiver. By this time, the skydiver reaches a new terminal velocity of around 10 m/s. The skydiver now drifts down to the ground with this speed.
6. 6. Note : When the skydiver is travelling at his terminal velocity, the net force acting him is zero irrespective of whether the parachute is closed or open. When there is an unbalanced force acting on him, he speeds up or slows down.

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