Things People Know Are True That Aren’t
You see a phenomenon, someone provides an explanation, it seems plausible, you believe it. When you hear something repeated often enough it must be true, right? However, just because people believe something doesn’t mean it’s true. This applies not only to urban legends and old wive’s tales, but to science as well.
Here’s a short list of factoids many people know which are either misleading or flat out wrong:
Meteors burn up from air friction
A speeding object entering Earth’s atmosphere does not burn up from air friction. Meteors enter the atmosphere at many times the speed of sound. This creates a shockwave which compresses the air in front of the object. The heat comes from this air compression. It’s the same principle which makes a diesel engine work without spark plugs, heating the fuel air mixture by compression to a temperature which ignites it. If you don’t believe me, try this.
This is often stated as "hot air rises." This is more misleading than actually wrong by implying heat makes the air go up, which ain’t quite right. Hot air expands becoming less dense. Cooler, denser air pulled down by gravity displaces the hotter, less dense air upwards. The hot air is not rising because it’s hot, it rises because it’s less dense.
Put an ice cube in a glass of tepid water. Which rises, the warm water or the cold ice? In this case it’s the warm water that is denser which displaces the ice up. The temperature isn’t moving it, it’s the density that matters.
Suction pulls liquids up a straw
What’s commonly called suction is not some force of attraction. Air pressure on the liquid pushes it up. A vacuum can’t create a force of attraction. Think about it, what is a vacuum? An area with nothing in it, no air, no matter, no nothing at all. Well then, how is a force created if there’s nothing there to create it? You can’t squeeze blood from a turnip, especially if the turnip doesn’t exist.
There is a limit to how far up a tube you can push a liquid this way, about 34 feet. At that point the weight of the liquid in the straw is too great for the air pressure to push any higher no matter how much suction you have on the upper end. That’s how a barometer works, air pressure pushes mercury up a tube with a vacuum at the top. Low air pressure pushes it less high than high air pressure. Ever hear a meterologist talk about inches of mercury? That’s what they’re referring to.
In fact, if you had a straw that extended all the way out of earth’s atmosphere into the vacuum of space you couldn’t get liquid up the straw any higher. And that’s a whole lot of vacuum, a universe of suction. I suppose you could turn this Straw Into Space into a curiosity, a tourist attraction. That’s a different type of highly unscientific attraction though. How much drawing power that would have is hard to determine.
Pressure differences and air and water flow is what fluid dynamics is about. Not all that easy to explain here. The bottom line is, there is no such thing as a force of suction. The absence of mass and energy cannot be a force, it’s impossible. Less is not more.
A wing’s curved shape creates lift
A common explanation suggests the air flows faster above a wing because it has farther to go over the curved upper wing surface compared to the shorter straight bottom. This lowers the pressure as per the Bernoull principle sucking the wing up. Unfortunately, this defies the laws of physics. Any change of momentum requires a force. Distance is a measurement, it has no physical being and cannot exert a force. There is no force of distance. The air does travel faster over the top, but not because of the greater distance.
Briefly, lift is generated by the angle of attack, the leading edge is above the trailing edge, which combined with thrust creates a vortex which pumps air down, called downwash, propelling the wing up. The accepted explanation of lift is the Lanchester-Prandtl hypothesis of flying. Read either How Planes Can Fly for a brief overview or See How it Flies to get the complete skinny on lift.
The curve in the wing’s upper surface (camber) is not there to create a distance difference, but conforms to the curving airflow to prevent as much turbulence as possible. The curved upper surface prevents loss of lift, but doesn’t generate any lift at all.
Think of it like this, the wing is moving forward into the air ahead which collides with the leading edge and bottom of the wing which imparts a force on the air. The top of the wing is moving away from the air above and behind and so can’t be imparting a force on it.
Gravity pulls you toward the center of the Earth
The gravitational pull of a planet is not towards the center of the mass strictly speaking. The mass doesn’t gang up, pool its gravity and all pull in the same direction. A person on the surface is pulled in every direction there is mass. There’s just a lot more mass directly down than in every other direction. If you were in a hollow cavity at the center of a planet, the gravity would be pulling you equally in every direction outward, you would float.
Considering what else goes on inside a planet like Earth, what with a superheated core and molten rock sloshing about, I don’t think it’s worth the trip to see if it works firsthand.
There’s actually no such thing, it’s an effect and not a force. It comes from a combination of momentum plus a centripetal force deflecting the moving object so its path curves. Together this results in a centrifugal effect, a counter-force directly against the centripetal force. The Moon orbits the Earth in a circle because it has momentum and a centripetal force, gravity. You can have momentum without a centripetal force or a centripetal force (like gravity) without momentum, but you can’t have a centrifugal effect without both.
This might seem like a distinction without a difference, but it is meaningful in science circles. Forces are causes, effects are results.
Saltwater is full of salt
Salt is sodium-cloride crystals. Saltwater doesn’t have these crystals floating in it. The sodium and clorine are broken apart and attached to water molecules. When you evaporate the water the sodium and clorine recrystalize into salt.
Water spins down the drain counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere
Hurricanes rotate counter-clockwise in the northern hemisphere and cyclones rotate clockwise in the southern hemisphere. This is the result of what’s called the Coriolis effect. As the Earth revolves on its axis, the further away from the equator you get, the slower the surface is moving. Hurricanes are miles and miles across so the difference in the surface speeds between north and south is considerable, this is what imparts the spin.
On the other hand, the speed difference between the northern and southern parts of your bathtub is practically nothing, unless you have a really enormous tub. The Coriolis effect in it is also practically nothing. The imperfect shape of the tub and currents in the water from filling or bathing activity have more impact to determine the direction water spins down the drain than does the Coriolis effect.
The Gulf Stream warms Europe’s climate
Well, it does, but not much. The major cause of the mild climate of Europe is the Rocky Mountains, strangely enough. Topographically forced atmospheric waves over the Rocky Mountains and the conservation of angular momentum circulates the atmosphere south over North America and north over the Atlantic warming Europe. The Gulf Stream warms Europe only about 3 degrees, this other phenomena warms it 15 to 20 degrees.
The Greenhouse effect
I won’t tell you there is no such thing, but it is something of a bad name because it doesn’t work anything like a greenhouse. A greenhouse traps heat by preventing conduction and convection of heated air. The so-called greenhouse effect doesn’t do any of that, but rather effects radiative heat gain and loss. Not even close to the same thing.
Leaning a bike makes it turn without steering
Maybe you know this debate. Some say you can turn a bike simply by leaning because of gyroscopic forces from the spinning wheels. Others say it’s because when leaning, the bike tires ride on their sides which in effect makes them cones. Both sides in this debate are wrong.
The main problem with people’s ideas about the gyroscopic and cone effects on turning bikes is what might control a single wheel does not work on two wheels connected with a frame. There are forces on a bike which don’t exist on a single wheel, and all must be accounted for to understand how it works.
A bike turns by tracking, the effect created by having a steerable front wheel connected by a frame to a trailing wheel. These tracking forces are more powerful than the gyroscopic torque or the effect of conical wheels. You can steer a bike riding no-handed by leaning because the bike’s design geometry makes it track. This takes a bit of explanation, which I’ve done in The Wheels That Don’t Turn.
Nature abhors a vacuum
OK, I admit I’m tossing this one in for fun because strictly speaking, nature doesn’t abhor anything, nature has no emotions or desires or anything like that. This expression is like the old Greek idea of impetus, things wanting to be at rest or seeking the lowest level or whatever.
Though if you think about it, most of the universe is a vacuum. Or is it? Every star emits particles of light. They must be all over the place. And isn’t the universe chock full of plasma and whatnot? Whatever the case may be, nature doesn’t feel one way or the other about it.
© Terry Colon, 2008