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Home Minds On Newton's Laws Mission NL11 Air Resistance Help Help 4

Mission NL11 Air Resistance

The Question

A skydiver steps off a plane at an altitude of 10 000 feet. As the skydiver falls, her speed steadily increases. This causes the air resistance force to ...

Minds On Time

The first blank of this question might be the easiest of the two blanks. Air resistance depends directly upon speed. As the skydiver falls, she speeds up. This increasing speed leads to a greater force of air resistance. The second blank of this question is the more difficult blank. The acceleration of the skydiver, like the acceleration of any object is calculated as the Fnet/ mass ratio. The skydiver's mass will not change as she falls. Any change in the acceleration - increase or decrease - would cause the corresponding change in the net force. The net force is the vector sumof the two forces. To determine if the acceleration increases, decreases or remains the same, think about the net force. Draw a free-body diagram, labeling the two forces acting upon the falling skydiver. Then think about how the two forces change over time (if at all) and how that change would result in a change in the net force and the corresponding acceleration.

Don't be Fooled

This question is an often missed question because of the common confusion of speed with acceleration. Speed refers to how fast the skydiver is moving; her speed is obviously increasing. But don't be fooled! An increasing speed does not necessarily mean an increasing acceleration. The acceleration of the skydiver refers to the rate at which she changes her speed. A skydiver could be speeding up but changing the rate at which she speeds up. Suppose that the skydiver has a speedometer attached to her that allows us to read the speed (and see it increase) but also allows us to observe the rate at which the speed increases. The needle could continually be moving towards higher and higher speeds; this would mean the speed is increasing. But the needle would start to move rapidly at first and more gradually near the end of the dive; this would mean the acceleration is decreasing. There would still be an acceleration (an increase in speed), but the rate of this acceleration would be diminishing over time - a decreasing acceleration.

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