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Question 3: How does the terminal speed of a parachutist before opening a parachute compare to the terminal speed afterwards? Why is there a difference?


  • When a paratrooper jumps out of the plane, two forces act on him/her. One is the force of gravity equal to his weight and the other is air drag. Both these forces act in opposite directions; gravitational force downwards and the drag force upwards. Initially, the gravitational force is greater than the drag force. However, with the passage of time the drag force on the paratrooper increases until it is equal to the force of gravity. Resultantly, when both forces become equal the net force on the paratrooper becomes zero and it is falling with uniform velocity, called terminal velocity.
  • Magnitude of the drag force depends upon two factors; speed of the paratrooper and the exposed or frontal area. When the downward speed of the paratrooper increases, the drag force also increases. Similarly, if the exposed are is larger, the drag force is also larger and vice versa.

Now there are two terminal speeds in a single jump; one before the parachute opens and the other after the parachute opens. The one before opening the parachute is faster and the other when the parachute is opened is slower. The difference is due to the change in the exposed area of the parachute. When the parachute is closed the frontal or exposed area is smaller. As a consequence, the drag force is smaller and the terminal speed is attained after a greater downwards velocity. On the other hand, when the frontal is expanded after the parachute opens, the drag force increases which decreases the downward fall of the an therefore, the terminal velocity also decreases.

So the difference in the two velocities is due the expanded area of the open parachute.

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