How Everything Works   Page 11 of 160 (1595 Questions and Answers) Printer Friendly Version

101. If you don't want your tires to blow up from too much thermal energy, then why aren't tires white (to absorb less sunlight and thus receive less thermal energy)?
That's an interesting question. It's probably difficult to manufacture white tires and they'd probably look terrible after they'd been driven a while. The old white-wall tires where difficult to keep clean. Because the pressure in a tire varies with its temperature, your tires will probably go over their optimum pressure on a hot sunny day. But driving them on the road also heats them, probably more than sunlight does. Because their temperatures increase during hard use, the tires are evidently capable of handling pressures much higher than their normal fill pressures.

102. Many large boats seem to taper down toward the water line. If their hulls follow this trend, their centers of mass will be high above their centers of buoyancy, making the boats unstable (like standing in a canoe). How do these things stay upright?
You're right that the boats must keep their centers of gravity lower than their centers of buoyancy. A boat with its center of gravity above its center of buoyancy will flip over, just as an upright broomstick will flip over if you support it only from below. But because a boat with a narrow tapered hull will go deeper into the water than one with a wide flat hull, the boat with the tapered hull may actually have a lower center of gravity than the boat with the wide flat hull. For example, imagine adding a long thin vertical plate to the bottom of a canoe, effectively converting the canoe's wide flat hull into a thin tapered hull. That canoe will be much more stable than before. So the shape of a boat's hull isn't as important as where the boat's weight located is relative to its center of buoyancy (the effective location of the buoyant force on the boat).

103. What is the buoyant force?
When you displace a volume of air and replace that volume with something else, the air around the volume still pushes on it as before. If that volume had remained air, then it would have just floated there, suspended by a force from the surrounding air. Now that the volume has been replaced by something else, it still experiences the same suspending force. That suspending force is the buoyant force. It's actually created by a slight imbalance in the pressures around the volume. The pressure at the bottom of the volume is slightly higher than on top, so the air exerts a net upward on the volume. This pressure imbalance is in turn created by gravity and the fact that the air near the ground must support the air above it against the force of gravity.

104. Why do high altitude places have different cooking temperatures than sea-level places?
The air pressure is lower at high altitudes than it is near sea level because there is less atmosphere overhead to support. This decreased air pressure affects the way water boils. Molecules can always evaporate from the surface of a pot of water, even when that water is cold, but above a certain temperature, water molecules can begin to evaporate from the interior of the water as steam, the gaseous form of water, in a process we call boiling. The temperature at which boiling occurs depends on the ambient air pressure because it can only proceed when there is enough pressure inside the steam bubbles to make them grow larger. At high altitudes, the lower air pressure makes it easier for these steam bubbles to form and grow, so they occur at lower temperatures. That's why water boils at a lower temperature at high altitudes. Once water reaches its boiling temperature, any heat that you add to it tends to cause the water molecules to boil away rather than to make the water hotter—so it's hard to heat water-containing food hotter than the boiling temperature of water. Since food needs high temperatures to cook, they don't cook as easily at high altitude where the low boiling temperature of water tends to keep the food from getting very hot.

105. Why does a balloon collapse without air inside it while a vacuum bell jar does not?
A balloon's skin is too weak to support the air around it. If you don't put any gas inside the balloon, the atmosphere around it will push it inward and it will collapse. But a vacuum bell jar is made of an extremely strong plastic that easily supports the weight of the atmosphere on top of it. Even with nothing inside it, it remains full size.

106. Why don't soap bubbles float forever (I believe they fall when not influenced by wind currents) if you have the "same" air both inside and outside the bubble?
The average density of an air-filled soap bubble is just slightly higher than that of the surrounding air. That's because the soap film itself is denser than air and because the air inside the bubble is very slightly compressed, thus having a slightly higher density than the surrounding air. Because the bubble's average density is slightly higher than that of the surrounding air, the bubble will slowly sink in still air and will eventually reach the ground.

107. Can air have gravitational potential energy?
Yes. However, you often don't notice this because as you lower a volume of air downward, you displace a similar volume of air upward. Thus you can't just raise or lower air to observe changes in its gravitational potential energy. You'd have less trouble if you compressed the air tightly together, perhaps turning it into a liquid, and then raised or lowered it. It's gravitational potential energy would then be much more noticeable.

108. How does water move toward your mouth through a straight straw if you don't suck on the straw?
If the straw is horizontal and the water wasn't moving to begin with, it won't move toward your mouth unless you suck. To make the water accelerate, it must experience net force and the two ways to achieve that net force are (1) to create a pressure imbalance on the water's ends and (2) to have the water's weight accelerate it. In a horizontal force with no pressure imbalance on it, there is no net force on the water and it doesn't accelerate.

109. I was wondering about the change in pipe sizes within a house. In many cases, water pipes coming to a house are very large, only to drop to small pipes when they reach the house. Does this mean that the water from the water company is slow velocity, high pressure, and houses turn this water into fast velocity, low pressure?
Yes, but the effect is not so extreme. As the water from the water company enters the narrower pipes in your house, it does have to speed up slightly and its pressure does drop slightly. But its pressure is still well above atmospheric pressure. However, the fact that the water must move faster through the narrower pipes in your house means that this water loses energy relatively quickly in your house. And the more water you draw through your house's plumbing, the larger the fraction of its energy it loses. That's why drawing a huge amount of water out of one faucet will diminish the flow through another faucet—increasing the flow by opening that first faucet wastes the energy of the water reaching the second faucet and it flows out more slowly.

110. In a siphon, what makes water flow from one container to the other without a pump?
The water is propelled by a pressure imbalance. When the water level in one container is higher than that in the other container, the pressures at the two ends of the siphon aren't equal. There is more pressure on the high water side than on the low water side. As a result, the water accelerates toward the low water side and the water levels gradually become equal.

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