Skydiving basic physics

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(Problem) I was watching a show and it showed a skydiver drop some pennies as he was falling. The pennies flew straight up into the air. I was wondering why this happened. Shouldn't the pennies have stayed next to the diver since they would have the same acceleration due to gravity? I know that wind resistance plays a part but the pennies should have less resistance because they have less surface area so they should fall down? I also thought that the pennies are actually staying in the same place and the diver was falling but shouldn't the pennies want to maintain the same speed because of Newton's first law? What is really going on in this example?
 

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  • #2
Nathanael
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The pennies do indeed have less air resistance acting on them, but, air resistance is a force (not an acceleration) so the pennies'/diver's masses come into play.

I think of "mass" as "a measure of resistance to acceleration"

So although the pennies have a smaller upward force on them from air resistance, they have a much lower resistance to acceleration, meaning that the smaller force results in a larger effect on the acceleration of the pennies.
 
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  • #3
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The pennies do indeed have less air resistance acting on them, but, air resistance is a force (not an acceleration) so the pennies'/diver's masses come into play.

I think of "mass" as "a measure of resistance to acceleration"

So although the pennies have a smaller upward force on them from air resistance, they have a much lower resistance to acceleration, meaning that the smaller force results in a larger effect on the acceleration of the pennies.

If the pennies have less resistance to acceleration shouldn't they fall faster due to the acceleration due to gravity?
 
  • #4
ehild
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(Problem) I was watching a show and it showed a skydiver drop some pennies as he was falling. The pennies flew straight up into the air. I was wondering why this happened. Shouldn't the pennies have stayed next to the diver since they would have the same acceleration due to gravity? I know that wind resistance plays a part but the pennies should have less resistance because they have less surface area so they should fall down? I also thought that the pennies are actually staying in the same place and the diver was falling but shouldn't the pennies want to maintain the same speed because of Newton's first law? What is really going on in this example?
I think those pennies were thrown upward.

ehild
 
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  • #5
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I think those pennies were thrown upward.



ehild

No he just opened his hand and the pennies flew upward.
 
  • #6
CWatters
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Now here is a surprise....

Mythbuster determined that the terminal velocity of a penny is (only) about 65mph...

http://en.wikipedia.org/wiki/MythBusters_(2003_season [Broken])

Firing a penny at terminal velocity (65 miles per hour (105 km/h)) into concrete and asphalt disks and a ballistics gel head with a human skull failed to result in any penetrations, likely because the speed is too low and a penny's mass too small.
Where as the terminal velocity of a skydiver is faster at 122mph...

http://en.wikipedia.org/wiki/Terminal_velocity

..the terminal velocity of a skydiver in a belly-to-earth (i.e., face down) free-fall position is about 195 km/h (122 mph or 54 m/s).
So that would appear to explain it.
 
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Now here is a surprise....

Mythbuster determined that the terminal velocity of a penny is (only) about 65mph...

http://en.wikipedia.org/wiki/MythBusters_(2003_season [Broken])



Where as the terminal velocity of a skydiver is faster at 122mph...

http://en.wikipedia.org/wiki/Terminal_velocity



So that would appear to explain it.

Yeah that was the show I saw it on. So is the terminal velocity of the penny just less than the diver?
 
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  • #8
Orodruin
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Yes. Unlike in high-school (and some university) classes, air resistance is definitely not negligible ;)
I still would like to avoid getting a terminal velocity penny in my head though...
 
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  • #9
CWatters
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I had visions of the skydiver landing and then being hit by the coins he "dropped" earlier.
 
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  • #10
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OK thanks guys!
 

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