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 RocketsPage 1 of 2 (15 Questions and Answers) Printer Friendly Version

153. How can the total momentum still be zero when two objects are moving rapidly away from each other?
Momentum is a vector quantity, meaning that it has both an amount (a magnitude) and a direction. When two objects are moving rapidly away from one another, they each have momentums but those momentums are in opposite directions. When you add these momentums together to find the total momentum of the two objects, you must consider the directions of those individual momentums. If the two momentums are exactly equal in magnitude but opposite in direction, they will cancel when you add them together and the total momentum will be exactly zero.

154. How does a rocket engine work?
A rocket engine works by ejecting stored material. It pushes on this material to make the material accelerate and the material pushes back on the engine. If the force that the ejected material exerts on the engine is upward and greater than the rocket's weight, the rocket will accelerate upward.

Most rocket engines are chemical engines. They combine stored chemical fuels to produce hot, high-pressure gas. This gas is allowed to expand out of a narrow orifice—the throat of the engine's exhaust nozzle. Gases always accelerate toward lower pressure, so the high-pressure gas moves faster and faster as it rushes out of the nozzle. It reaches sonic velocity (the speed of sound) in the nozzle's throat and continues to move faster and faster as it flows out of the nozzle's widening bell. By the time the gas leave the engine completely, it's traveling several thousand meters per second. A liquid fuel rocket has an exhaust velocity of about 4,500 meters per second or about 3 miles per second. Accelerating the gas to this enormous speed takes a huge force—the engine pushes down hard on the gas. The gas pushes back and propels the rocket upward.

155. How does the use of sticks and fins stabilize rockets?
Sticks and fins both shift a rocket's center of aerodynamic pressure (center of drag) toward the tail of the rocket and behind the rocket's center of mass. As a result, the tail of the rocket normally remains at the rear during flight. The passing air twists the tail of the rocket until it's at the rear of the moving object.

156. How is it that gas moving very rapidly is unable to "communicate" with gas or surfaces in front of it?
When gas is moving slowly through a channel, it can respond to obstacles by flowing around them. For example, when the gas encounters a constriction in the channel, it speeds up to flow quickly through the narrowing and its pressure drops. But when the gas is moving very fast through the channel, it has trouble avoiding obstacles and behaves differently at a constriction. Instead of speeding up to flow smoothly through the narrowing, the gas collides with the walls of the constriction and is pressure rises. It just wasn't able to "sense" the presence of the constriction before it actually hit the constriction. When gas moves faster than the speed of sound in that gas, it can't anticipate changes in its environment and it doesn't follow Bernoulli's equation. That's why the nozzle of a rocket flares outward to handle the supersonic gas that emerges from the nozzle's throat. That high-speed gas experiences a pressure drop as it spreads out into the broad portion of the nozzle. The gas's density drops and its pressure goes down.

157. Since an object orbiting the earth is falling as it orbits, does it gradually get closer to the earth? Would it eventually reenter the earth's atmosphere and fall to the ground?-MG
If the orbiting object doesn't interact with anything but the earth, then the answer is: no, it will continue to orbit forever. That's because, although it is always falling and accelerating toward the earth, its sideways velocity continues to make it miss the earth. It just keeps on missing forever. Moreover, its total energy remains constant—the sum of its kinetic and gravitational potential energies. But if something removes some of its energy, it will gradually shift closer and closer to the earth and will reenter the atmosphere. That reentry occurs for low-lying satellites because they interact with the diffuse atoms in the extreme upper atmosphere. These satellites gradually lose energy and eventually come down in a blaze of frictionally heated material.

679. What is the most explosive and energy releasing combination of chemicals? — RC, Chapman, Australia
A mixture of 1 part hydrogen and 19 parts fluorine by weight is the most energetic possible mixture of chemicals, releasing approximately 13,600 joules of energy per gram. The next most potent mixture is 8 parts oxygen and 1 part hydrogen by weight, releasing approximately 13,400 joules of energy per gram. Because fluorine is such a vigorous oxidizer that tends to cause fires, it isn't practical for rocket propulsion. The hydrogen/oxygen mixture is the basis for the Single Stage to Orbit rockets that are currently being developed. — Thanks to Gary V. Lorenz at NASA for help on this question.

883. Would it be possible for a spacecraft to use electrically powered propulsion? Could it gather atoms and molecules from space and then use an electromagnetic field to push them through a nozzle? — JC, Burnaby, British Columbia
Not only is it possible to use electrically powered propulsion, such systems are already in use on several spacecraft. While they don't scavenge atoms and molecules from space, these ion propulsion engines uses electric forces to accelerate ionized atoms to enormous speeds. As the engine pushes on the ions it accelerates, those ions push back on the engine. The ions rush out into space in one direction and the engine experiences a modest thrust in the opposite direction. While the overall thrust from an ion engine is small, it uses its stored-atom "fuel" very efficiently and can be sustained for a very long time in a solar- or nuclear-powered satellite. Ion engines are used in spacecraft that need small but steady thrust for a long time. Scavenging atoms from space would allow these engines to run for an even longer time, but it's probably not realistic. The atoms in space are typically so rare and so fast-moving that they would be more trouble than they're worth.

938. How do rockets work?
Rockets push stored materials in one direction and experience a thrust force in the opposite direction. They make use of the observation that whenever one object pushes on a second object, the second object exerts an equal but oppositely directed force back on the first object. This statement is the famous "action-reaction" concept that is generally known as Newton's third law. While it seems sensible that when you push on a wall it pushes back on you, this situation is extraordinarily general. For example, if you push a passing car forward, that car will still push backward on you with an equal but oppositely directed force. If you push on your neighbor, your neighbor will push back on you with an equal but oppositely directed force even if your neighbor is asleep! In the case of a rocket, the rocket pushes burning fuel downward and the burning fuel pushes upward on the rocket with an equal but oppositely direct force. If the rocket pushes its fuel downward hard enough, the fuel will push up on the rocket hard enough to overcome the rocket's weight and accelerate it upward into the sky and beyond.

965. Is there any gravitational force between two atoms? — AW, Karachi, Pakistan
Yes, everything in the universe exerts gravitational forces on everything else in the universe. However, those forces are usually so small that they are undetectable. The gravitational forces between two bowling balls are only barely measurable in a laboratory. The gravitational forces between two atoms are so small as to be hopelessly undetectable.

990. Why does the tower of Pisa lean? — CM, Edison, NJ
The tower was built long ago on unstable ground that was unsuitable for supporting such a tall and heavy masonry structure. For an object to remain upright indefinitely, its center of gravity must lie above its base of support and that base of support must be firm at all its edges. The tower's base of support had at least one edge that wasn't firm and that began to sink downward under the weight of the tower. Once this edge sunk a small distance, the tower's center of gravity shifted sideways so that it was above that weak portion of the base of support. This shift in the tower's center of gravity put even more stress on the weak part of the ground and caused additional sinking, additional tipping, and even more shifting of the tower's center of gravity. This process might have toppled the tower over by now were it not for recent efforts to stop the tipping. The base of the tower has been reinforced to prevent further tipping.

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