3 Comments

1. 1. pwbpwb 08:10 PM 9/6/12

   The last line in the "Background" section is incorrect. It reads, "The drag force from the parachute is slightly less than the force of gravity, so the skydiver floats slowly to the ground."
   
   Forces cause changes in velocity (acceleration), however, and not velocity itself. If a skydiver is slowly floating to the ground without speeding up or slowing down, then the drag force exactly cancels the weight. In reality, of course, inconsistencies in the atmosphere and in the parachute cause slight up and down accelerations, experienced as buffeting, because the drag force is sometimes bigger than the weight and sometimes smaller.
   
   More details:
   Bigger parachutes lead to bigger forces, and bigger speeds lead to bigger forces, too. If you are in freefall when you open your parachute, something like the following happens:
   
   1. In freefall you move fast, so initially there is a drag (upward) force much bigger than your weight (downward) force. The drag overcomes the weight, so there is a net upward force that slows you down, at first relatively quickly.
   
   2. Slower speeds lead to smaller drag forces, so as you slow down, the drag force gets smaller, though still larger than your weight. You come to slow down less and less quickly, but you're still falling.
   
   3. Eventually you approach a point where you fall slowly enough that the drag force is equal to your weight. You stop slowing down and continue to fall at terminal velocity.
   
   4. If, at some point, there is an inconsistency in the air density or the parachute shape or something, the drag force can be less than your weight. In that case you speed up a little, because the net force is down.
   
   5. Speeding up increases the drag force, until it pretty much cancels out your weight again.
   
   6. Such inconsistencies can also cause a bigger drag force, which slows you down a little.
   
   7. Slowing down decreases the drag force, until it pretty much cancels out your weight again.
   
   This repeats until you're ready to land.

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2. 2. waterbergs 10:21 AM 9/11/12

   As per pwbpwb above it is rather tragic that purveyors of physics educational material don't get Newton's First law: "An object will stay at rest or continue at constant velocity unless acted on by a resultant force" Constant velocity = no resultant force = gravity equally and oppositely matched by drag. Not rocket science.

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3. 3. Johnay in reply to waterbergs 10:49 PM 9/22/12

   I have no doubt it's used in rocket science, though. :)

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