How Everything Works
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Page 16 of 160 (1595 Questions and Answers)

151. If air in a turbofan engine bypasses the jet engine after going through the turbofan, why does the jet engine even exist in the system?
The turbofan engine has a giant fan at its inlet, with a much smaller turbojet engine behind it. That turbojet engine is what provides the mechanical work needed to turn the giant fan. About 5 to 10% of the air passing through the fan then passes into and through the turbojet behind it. The turbojet uses this air for its operations: compressing it, burning fuel in it, and then extracting most of that hot air's energy as rotational work. This rotational work is used to power the giant fan.

152. In a high bypass ratio turbofan engine, does the fan turn at the same RPM as the power turbine section — or is it geared down to run slower?
A turbofan engine uses a small turbojet engine to turn a giant fan and it is this fan that provides most of the engine's propulsion. The question asks whether the fan is turned directly by the turbojet engine or whether gears are use to allow the larger fan to spin more slowly than the smaller turbojet. This is an interesting question, particularly since many of the parts inside a jet engine are spinning almost as fast as they can tolerate without ripping themselves apart.

A turbofan engine contains two separate rotating assemblies or "spools," each of which is powered by hot exhaust gases flowing out of the combustion chamber through some turbine discs and each of which spins some compressor disc that push air toward the combustion chamber. The shorter of the two spools is hollow and the lower spool passes through its center.

The shorter spool, which spins at about 12,000 rpm, derives its power from high speed gas flowing through its turbine blades just after the combustion chamber and it powers a high pressure compressor just in front of the combustion chamber. The longer spool, which spins at about 4,000 rpm, derives its power from low-pressure gas flowing out of the high-pressure turbine and it powers both a low-pressure compressor in front of the high-pressure compressor and the actual turbofan blades. Overall, there is a rapidly turning hollow spool right around the combustion chamber and a more slowly spinning solid spool that extends both in front of and behind the high-speed spool. It's the low speed spool that spins the turbofan itself.

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.

158. How is it that the torques cancel when you turn a bicycle?
During a turn, you lean the bicycle into the turn. For example, when you turn left, you lean the top of the bicycle toward the left. The result is that you (and the bicycle) experience two torques. First, the support force from the ground tries to rotate you one direction—it tries to make your head go left and your feet go right. Second, friction from the ground, which is making you and the bicycle accelerate toward the left as part of the turn, tries to rotate you in the opposite direction—it tries to make your head go right and your feet go left. These two torques will cancel one another if you are leaning just the right amount. As a result, the bicycle doesn't undergo angular acceleration and you don't tip over.

159. Is there a difference in the types of handles of a bike? On some bikes, there is the (upright) handlebar and on some the (drop) handlebar. Is there a purpose?
The shape of the handlebar determines your riding position. The upright position is generally more comfortable but, by sitting you upright, it increases the pressure drag you experience. Drop handlebars lower your body and make you more aerodynamic, but that position isn't as comfortable.

160. Please explain the advantage of air tires with less mass.
The rim of the wheel travels at a different speed from the rest of the bicycle. The top of the wheel heads forward faster than the bicycle, while the bottom of the wheel heads forward more slowly than the bicycle. But because kinetic energy is proportional to the square of speed, the increase in the top of the wheel's energy caused by its increased speed more than makes up for the decrease in the bottom of the wheel's energy caused by its reduced speed. The overall result is that the wheel rim has twice as much kinetic energy as it would have if it were simply sliding forward without turning. This fact is important because it means that you want as little mass in the wheel rim as possible. Every kilogram there counts double when you are trying to start up from rest. By putting air inside the tire, rather than rubber, you reduce the mass at the wheel rim and make the bicycle easier to start.
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