Question

Soon after Joe Kittinger began his jump, he was traveling at over 982 kilometers per hour (610 miles per hour). What characteristic of the atmosphere caused him to slow down as he descended (before opening his main chute)? Why did he not hear the sound of air whistling by him as he began to fall?

Short answer

Increasing atmospheric density caused him to slow down, and the thin air did not carry sound efficiently.

Step-by-step solution

Understand the Scenario

Joe Kittinger jumped from a high altitude where the atmosphere is much thinner compared to lower elevations. Consider how atmospheric density changes with altitude and its effects.

Identify the Cause of Slowing Down

As Joe descended, the atmospheric density increased. The denser air at lower altitudes exerted more friction, or drag, on him, which caused him to slow down before he deployed his parachute.

Explain Atmospheric Density and Sound

In the high-altitude, low-density air where Joe started his jump, the thin atmosphere does not carry sound waves effectively. This is why he did not hear the whistling noise that would be expected in denser air closer to the ground.

Key concepts

Atmospheric Density

Atmospheric density refers to how compact air molecules are in a given volume. As altitude increases, atmospheric density decreases. This is because the higher you go, the fewer air molecules there are above you to exert force.

This concept is essential because it affects various phenomena, like air resistance and sound propagation. For Joe Kittinger, as he started his jump from a high altitude, he was in a region of thin air. The air was not dense, meaning fewer air molecules were available to slow him down. However, as he descended, the atmospheric density increased with the decrease in altitude, leading to more air molecules exerting force on him.

In simpler terms, think of atmospheric density as the "thickness" of the air. Thicker (denser) air slows things down more effectively because it provides more resistance.

Air Resistance

Air resistance, also known as drag, is the force that air exerts against a moving object. It acts in the opposite direction to the motion.

The amount of air resistance an object encounters depends on several factors:

• Speed: Faster objects experience more drag.
• Surface area: Larger surfaces facing the direction of motion encounter more resistance.
• Atmospheric density: Denser air exerts more drag.

Joe Kittinger experienced low air resistance early in his fall due to the thin atmosphere at high altitudes. But as he descended, the air became denser, increasing the drag force. This rising air resistance was what gradually slowed him down until he opened his parachute.

Picture dragging a spoon through honey and then water. Honey, being thicker, represents a denser atmosphere that offers more resistance, just as Joe experienced at lower altitudes.

Sound Propagation

Sound travels as waves, requiring a medium like air to propagate. However, the ability of sound to travel depends significantly on the density of the medium. In the thin atmosphere at high altitudes, the air molecules are too sparse to effectively carry sound waves. This is why Joe Kittinger did not hear the rush of air we typically expect in denser conditions.

In lower altitudes, where the air is denser, molecules are closer together, enabling sound waves to transfer from one molecule to another more efficiently. This is why we can easily hear sounds around us at ground level.

For a practical example, think of sound in a vacuum — like outer space — where there are no air molecules at all, and thus no sound can travel. It's the same principle at play at high altitudes, just not as extreme.