# Skydiving basic physics

1. Jun 23, 2014

### BrainMan

(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?

2. Jun 23, 2014

### Nathanael

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.

3. Jun 24, 2014

### BrainMan

If the pennies have less resistance to acceleration shouldn't they fall faster due to the acceleration due to gravity?

4. Jun 24, 2014

### ehild

I think those pennies were thrown upward.

ehild

5. Jun 24, 2014

### BrainMan

No he just opened his hand and the pennies flew upward.

6. Jun 24, 2014

### CWatters

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.

Last edited by a moderator: May 6, 2017
7. Jun 24, 2014

### BrainMan

Yeah that was the show I saw it on. So is the terminal velocity of the penny just less than the diver?

Last edited by a moderator: May 6, 2017
8. Jun 24, 2014

### Orodruin

Staff Emeritus
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...

9. Jun 24, 2014

### CWatters

I had visions of the skydiver landing and then being hit by the coins he "dropped" earlier.

10. Jun 25, 2014

### BrainMan

OK thanks guys!