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Page 10 of 160 (1595 Questions and Answers)

91. If the fictitious force you experience on a loop-the-loop isn't greater than your weight, will you fall?
Yes. If you go over a loop-the-loop too slowly, so that you don't accelerate downward quickly enough, you will leave the track and fall. That's why some roller coasters strap you in carefully before taking you upside-down slowly. Without the supports, you would fall out of the car.

92. If you feel fictitious force upward on a loop the loop, how can that fictitious force make objects fall upward? Is fictitious force fictional or real?
As you travel over the top of the loop the loop, you observe the world from an inverted perspective. The sky is below you and the ground is above you. If you were to take a coin out of your pocket and release it, you would see it fall toward your seat. From that observation, and the feeling of being pressed into your seat, you might think that gravity is suddenly pulling you toward the sky. It isn't. Gravity is still pulling you toward the ground, but you are in a car that is accelerating rapidly toward the ground. As a result, the car is having to push you toward the ground with a force on the seat of your pants. You feel pressed into your seat because the car is pushing you downward hard. When you release the coin, it seems to fall toward the sky, but it's really just falling more slowly than you are. With the car pushing you downward, you're accelerating toward the ground faster than the coin and you overtake it on the way down. It drifts toward the seat of the car because the car seat accelerates toward it. As you can see, the only forces around are the force of gravity and support forces from the car. There is no outward or upward force here. The fictitious force is truly fictional; a way of talking about the strange pull you feel toward the outside of the loop.

93. When a ball swings in a horizontal circle at the end of a string, what's the force on the ball pulling it straight? What's the force pulling it out?
Let's neglect gravity, which isn't important in this horizontal motion problem. When a ball swings in a circle at the end of a string, there is only one force on it and that force is inward (toward the center of the circle). We call such a force a centripetal force, meaning toward the center. There are many kinds of centripetal forces and the string's force is one of them. As for the ball's tendency to travel in a straight line, that's just the ball's inertia. With no forces acting on it, it will obey Newton's first law and travel in a straight line. There is no real force pulling the ball outward. But a person riding on the ball will feel pulled outward. We call this feeling a fictitious force. Fictitious forces always appear in the direction opposite an acceleration. In this case (an object traveling in a circle) the outward fictitious force is called centrifugal "force." But remember that it's not a real force; it's just the object's inertia trying to make it go in a straight line.

94. When you spin an object around a fixed point, a sling for example, does the object at the end build up energy that causes it to shoot out quickly when released?
Yes. As you whip the object around on a string, you are doing work on it. You do this by making subtle movements with your hand, exerting forces that aren't exactly toward the center of the circle. When you do this, the object begins to travel faster and faster, so its kinetic energy increases. Traveling in a circle doesn't change this kinetic energy because kinetic energy is proportional to speed squared, and doesn't depend on direction. Finally, when you let go of the string, the object stops circling and begins to travel in a straight line. It carries with it all the kinetic energy you gave it by whipping it about.

95. Why is the outward force in a loop-the-loop a "fictitious" force? Why isn't it a "real" force?
A real force causes acceleration. If the outward "fictitious" force on a circling object were "real," that object wouldn't circle. It would accelerate outward. When you swing an object around on a string, you feel the object pulling outward on the string. But it isn't itself being pulled outward by anything. What you're feeling is the object's inertia trying to make it travel straight. The inward force you're exerting on it isn't opposing some real force, it's causing the object to accelerate inward.

96. Have you heard about the Egyptian Lighthouse (one of the seven wonders of the ancient worlds) that was found in the sea? Well, people are proposing lifting the 70-ton sections of the lighthouse with balloons. Can a balloon do this? What special aspect of the balloon will lift 70-ton bricks? What kind of balloon would be used?
I would guess that they intend to use balloons in the water. The blocks are at the bottom of the sea, so they must be lifted up to a boat. A balloon experiences an upward net force that is equal to the difference between the upward buoyant force on it and its downward weight. If the balloon displaces air, then the buoyant force on it is rather small and it would have to be extraordinarily big to displace enough air to lift a 70-ton block. But if it displaces water, then the buoyant force on it is much greater. To displace 70 tons, it would only have to displace about 65 cubic meters of water. That's not hard at all. The balloon could be made of heavy reinforced canvas and still work just fine underwater.

97. How can a balloon support the air around it (pressure wise) and still rise?
If you could have filled a balloon with nothing at all, it would float very nicely because it would have had no weight and the only force on it would have been the buoyant force upward. But an empty balloon will be crushed by the surrounding air, which will push inward on its surface with enormous forces. To keep the balloon from crushing, you must fill it with gas. Since this gas will weight the balloon down, you should choose the lightest gases around: hot air or helium. In the case of hot air, a relatively small number of air molecules create the pressure needed to keep the balloon inflated. With helium atoms, lots of helium atoms are needed to create the pressure but helium atoms are very light and their total weight is less than that of an equal volume of air. Thus the upward force on the helium filled balloon is more than its weight and floats upward.

98. How do clouds exist? If oxygen molecules, which must weight less than water vapor are drawn toward earth, then why aren't the clouds?
First, water molecules are lighter than the average air molecule so a balloon filled with water vapor would actually float in air. But that isn't what you're asking. Clouds exist because when water condenses from vapor to liquid, it often forms extremely tiny water droplets. These droplets are so small that they experience lots of air resistance as they try to move about. They begin to fall but quickly reach terminal velocity at perhaps a millimeter per second. The water droplets drift downward so slowly that they hardly move. That's what's happening in a cloud. The water droplets are trying to fall, but air resistance is slowing their descents.

99. How do submarines sink if they have air inside?
The net force on the submarine depends on its average density, not on the density of one of its constituents. If the average density of the submarine is less than that of water, it will float upward in water. If the average density of the submarine is more than that of water, it will sink downward in water. To determine the submarine's average density, you need to divide its overall mass by its overall volume. While the submarine does contain air, a low-density material, it also contains steel, a high-density material. The submarine's average density turns out to be very nearly that of water. Fine adjustments to its density, made by pumping water in and out of ballast tanks, determines whether the submarines floats or sinks.

100. How does altitude affect the presence of molecules?
As you travel upward, the air around you has less and less pressure. That's because it's supporting less and less weight above it. As long as the temperature isn't changing much, this decrease in pressure is caused by a decrease in density: the air molecules are become less tightly packed together. The result is that at high altitude, each breath you take delivers fewer air molecules into your lungs. Actually, air usually gets colder at higher altitudes, a change which keeps the air's density from decreasing so rapidly. The higher you go, the colder the air gets and the more molecules you need in each liter of air to maintain its pressure.
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