Sunday, November 04, 2007

FOR EVERY ACTION, THERE IS AN EQUAL AND OPPOSITE REACTION? "NEWTON"......said otherwise

CORRECTED: For every action, there is not an equal and opposite reaction.(William J. Beaty)

Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means is this:
For every "acting upon", there must be an equal "acting upon" in the opposite direction.
Or in modern terms...
For every FORCE applied, there must be an equal FORCE in the opposite direction.
So while it's true that a skateboard does fly backwards when the rider steps off it, these motions of "action" and "reaction" are not what Newton was investigating. Newton was actually referring to the fact that when you push on something, it pushes back upon you equally, even if it does not move. When a bowling ball pushes down on the Earth, the Earth pushes up on the bowling ball by the same amount. That is a good illustration of Newton's third Law. Newton's Third Law can be rewritten to say:
For every force there is an equal and opposite force.

Or "you cannot touch without being touched."

Or even simpler: Forces always exist in pairs.

INFRARED LIGHT : A FORM OF HEAT ?

CORRECTED: NO, INFRARED LIGHT IS NOT A KIND OF HEAT   (William J. Beaty)

Infrared light is invisible light. When any type of light is absorbed by an object, that object will be heated. The infrared light from an electric heater feels hot because the light is EXTREMELY BRIGHT LIGHT. Just because human eyes cannot see the light which causes the heating does not mean that the light is made of some mysterious entity called "heat radiation." When bright light shines on an absorptive surface, that surface heats up.

And this is no benign misconception. Those who fall under its sway may also come to believe that *visible* light cannot heat surfaces (after all, visible light is not "heat radiation.") Misguided science students may wrongly believe that warm objects emit no microwaves (since only IR light is "heat radiation"), even though hot objects actually do emit microwaves. Or they may believe that the glow of red hot objects is somehow different than the infrared glow of cooler objects. Or they may believe that IR light is a form of "heat," and is therefore fundamentally different than any other type of electromagnetic radiation.

In his book "Clouds in a Glass of Beer," Physicist C. Bohren points out that this "heat" misconception may have been started long ago, when early physicists believed in the existence of three separate types of radiation: heat radiation, light, and actinic radiation. Eventually they discovered that all three were actually the same stuff: light. "Heat radiation" and "actinic radiation" are simply invisible light of various frequencies. Today we say "UV light" rather than "actinic radiation." Yet the obsolete term "heat radiation" still lingers. Since human beings can only see certain frequencies of light, it's easy to see how this sort of confusion got started. Invisible light seems bizarre and mysterious when compared to visible light. But "invisibility" is caused by the human eye, and is not a property carried by the light. If humans could see all the light in the infrared spectrum, we would say things like this: "of COURSE the electric heater makes things hot at a distance, it is intensely BRIGHT, and bright light can heat up any surface which absorbs it."

PS, if you're interested in physical science misconceptions, Bohren's Book is an excellent resource. He's like me, and complains about several specific misconceptions which keep his students from understanding science.