December 1, 2012 Issue

Physics To Go 131 - Supersonic Skydive

image credit: STS-41B, NASA; image source; larger image

Astronomy Picture of the Day: To Fly Free in Space

This is astronaut Bruce McCandless, orbiting along with the Space Shuttle in 1984 as he tests his rocket pack. When he stepped outside to begin his spacewalk, why didn't he fall back to Earth? He stayed in orbit because before and after he stepped outside, McCandless and the Shuttle had the same velocity. The force of Earth's gravity bent his path into the same orbit as the shuttle; that's because the acceleration of gravity does not depend on the mass of the object being accelerated. To learn more about McCandless' spacewalk, see Astronomy Picture of the Day: To Fly Free in Space and Footloose.

Now look at the image at the top right of Felix Baumgartner beginning his supersonic skydive. When he stepped out, his balloon had been lifting him slowly in the thin stratospheric air, so his initial velocity was quite small. Gravity then pulled him back to Earth.

### Physics at Home

Feather Drop

Felix Baumgartner was nearly in free fall as he stepped out of his capsule at an altitude of 39 kilometers (24 miles). At this altitude, the atmosphere is so thin that a falling object experiences hardly any air resistance, so it accelerates rapidly to very high speed (Baumgartner's skydive exceeded the speed of sound).

Watch the video at Feather Drop to see how the air resistance on a feather depends on the density of the air it is falling through. First you'll see the feather fall through air at atmospheric pressure, and then through air at a very low pressure (a very good vacuum).

### From Physics Research

image © Red Bull GmbH; image source; larger image

Mission to the Edge of Space

The figure at the bottom of the photo is Felix Baumgartner, shortly after he jumped from an altitude of 39 kilometers (24 miles). There, the thin atmosphere produces very little air resistance, so he was almost in free fall (in free fall, the only force acting on an object is gravity). He accelerated rapidly, broke the sound barrier, and went on to about Mach 1.2 (20% above the speed of sound).

For more detail, visit this Mission webpage. To learn more about the Stratos project, see Mission to the Edge of Space.

### Worth a Look

Planetary and Satellite Motion

Check out this video to learn about satellite orbits.

For further reading, see Planetary and Satellite Motion.

### Recent Submissions

 Nov 29 Feather Drop Nov 29 Planetary and Satellite Motion Nov 28 Backstage Science Nov 28 Astronomy Picture of the Day: To Fly Free in Space