# Unbelievable physics

Gale
Physics are often really difficult to understand or even believe when you're first learning them.

In one of my classes we did an experiment where you have two balls on some springy thingy. What happens is that one ball drops straight down and the other is given an initial horizontal velocity by the spring. Since they're launched from the same height and neither have and initial vertical velocity, they land at the same time. My teacher didn't plan on doing the experiment, but when we went over a problem in class that brought up a similar idea, we all had such a terrible time believing that it was possible that both balls would land at the same time.

The classic gravity experiment with a rock and a feather is similar. When you neglect air resistance, they both will land at the same time when dropped. Thats a tough one to swallow when you first learn it. And it seems there's a lot of things like that in physics.

My question is why is that? Is it something we're taught as kids that makes it hard for us to believe these things later on? Is it some human nature thing? It seems just about everyone has a hard time with these sort of concepts. Reason?

Staff Emeritus
Gold Member
For the example you gave, your everyday experience with air resistance makes it very hard for you to understand how physics works without it. You're just totally accustomed to seeing dandelion seeds blowing in the breeze and bowling balls hurting people's toes.

Physics tries to explain the simplest cases on a phenomenon first. In the case of gravity, we get rid of air resistance so that we can experiment with the gravitational force in isolation. It just happens that for normal objects, air resistance is not at all negligible, so you feel like someone's lying to you when they say that the dandelion seed and bowling ball actually fall with the same acceleration.

For some physical systems, you automatically know what's going to happen. For example, if you have a block of metal at 100 degrees and another identical block at 50 degrees, you know immediately what will happen when you put them together -- they'll equalize at 75 degrees. There is no complicating effect like air resistance in that phenomenon, so your intuition about it is exactly correct.

Keep in mind that it gets even worse as you go higher into physical understanding... quantum mechanics governs the behavior of the very small, and it's very different from what you're accustomed to. Nothing in the macroscopic world works the way things do on the microscopic world. It means you have to trust your model and your experiment, even when you really have no experience with the domain from your own five senses. It's like exploring an alien landscape. Weird, but (apparently) true.

- Warren

Often it goes against what's called "common sense". Einstein once said something to the effect that common sense is merely the collection of preudices gathered in one's lifetime. The truth is, the universe's workings don't give a damn about our common sense. If throughout our lives, or even our evolutionary history, we encoutered environments where there was no air resistance, it wouldn't seem hard to us, or if we regularly traveled at relativistic velocities, SR would seem common sense to us.

If you think that some classical physics stuff is hard to get, just wait until quantum physics. Waves are particles and particles are waves at the same time...a particle/wave can be in two places or more at once, all probability. You toss an electron at a barrier with not nearly enough kinetic energy to get through and it does. That's what makes it so interesting! Its like "whoa! that can do that?!. It just takes a lot of math to describe it, and because so many people lack math beyond precalc at the most, they tend to think physicists are wrong.

One of the great strengths of the human brain is its subconcious processing ability. It allows you to make sense of what you see without thinking through every little detail. Most people are familiar with a few ways that this gets tricked. There are a few optical illusions that take advantage of this. You may have seen the "which line is longer?" puzzle:

>-----<
<----->

which probably doesn't work when typed. (they are the same length, but I guess the use of separated '-'gives it away)

But anything that contradicts a lifetime's worth of observations will send you for a bit of a loop. This is because your subconcious has done some thinking for you, and prepared you for what to expect.

Another neat thing to try - Cross you fingers and close your eyes. Have a friend rub a pencil near your fingertips where they cross. It feels like two things are touching you. You're not used to touching things like this, so your subconcious processors tell you it is two things.

Njorl

Fuego
a lot of things in physics aren't exactly intuitive. our 'natural' ideas of how the world works are based on what we see in everyday life. we see rocks falling faster than feathers, so we assume rocks fall faster than feathers because they're heavier. air resistance is such a common thing, its always there, that we don't even think about it. the genious of galileo and Newton was they could think about these situations without basing their thoughts on everyday experiences.

quartodeciman
If a projectile has to shoot out to the side, we think it has to suddenly decide to start falling at a later time, rather like cartoon character Wiley Coyote running off a cliff. Ancient illustrations of catapulted objects show them going out straight and hitting the target. The actual operators would need to know better than that to succeed, but the misimpression is apt to be strong.

The impact of a falling feather (or better, a light falling ball) is much less than the impact of a heavy falling ball. Horizontally, we usually attribute stopping impact at a wall to the speed of an object. The main mistake there is that speed-squared is a better estimator than just the speed. In either case we are apt to explain the greater impact of the heavier object to its greater speed, and ignore the difference in mass. Just turn that picture from horizontal to vertical.

My favorite example is looking at ones self in a full-length mirror. Left and right sides are obviously reversed in the mirror image, but top and bottom are not. Why?

A beautiful and freaky little experiment was done by G. B. Airy in the nineteenth century. Maybe your teacher will do it. Place a tiny intense light bulb at one end of a demonstration table. Mount a microscope lens at the other end of the table, pointed directly at the bulb. Now set a small non-reflecting black bead or disk directly in the path between the light and the lens, held up by a frame and ultrafine wires. The position is important. Turn off the room lights. The light going to the lens is eclipsed by the small obstacle and no light is expected to be seen through the lens. Unbelievably, the lens shows a tiny, bright point of light centered on the bead. No, there is no hole drilled through the bead or disk; it is solid. After the class has examined everything and found it to be on the up-and-up, teacher launches into a big talk about wavelets, Huygens wave constructions and Fresnel zones (almost) cancelling each other.

Gale
quartodeciman, good post. We're going to have to do that experiment sometime, sounds wicked neat. I especially like the comment about the coyote... silly as it seems, i think that might actually have something to do with it. We just grow up with little things like that, so it changes how we think of things. Changes our common sense.
We'd even been doing pleanty of stuff with particle motion in two dimensions and stuff... which was supposed to help us realize how obvious it was that both balls would hit at once since their verticle componets of their velocity were equal, but nope, we were still so confused. It was funny, my teacher couldn't stop laughing.

Tom D
Each atom is equally attracted to gravitational force.
(Again in a vacuum ) an individual atom ( at same temperature as the normal object ) would drop at same rate as any collection of atoms into parts, or any collection of parts into any whole.
The best way I read of in understanding that is this:
If you had two bricks ( two parts ), would you expect them to fall any faster if you tied a thread of cotton between them ( you now have a larger whole instead of two smaller, lighter independent parts ) ?
Repeat the experiment using shorter cotton threads until the bricks are touching.
Your common sense says the cotton thread can not affect the falling bricks' speed, and that the two combined into one fall at same rate as two separate parts.

Mentor
Originally posted by Gale17
My question is why is that? Is it something we're taught as kids that makes it hard for us to believe these things later on? Is it some human nature thing? It seems just about everyone has a hard time with these sort of concepts. Reason?
Its part of human nature to look for reasons for things and in the absence of good data, you simply make up a reason that fits with the limited data. Often that reason is wrong.

By the way, I did both of those experiments of yours, Gale. We had a little apparatus that fired and dropped an object at the same time for the projectile motion one and a plexiglas tube and vacuum pump for the rock and feather one.

quartodeciman
Thanks, Gale. I made a mistake. It was Arago's experiment, not Airy's. I often confuse those two guys.

The last time I saw that experiment demonstrated, it was in a solid enclosure. You could look in and see the light spot, but you could not take it apart and check the alignment. That wasn't very impressive. The following setup is more like it.

Arago experiment and Poisson spot --->
http://www.physics.brown.edu/Studies/Demo/optics/demo/6c2010.htm

A committee judged the wave theory presentation of Fresnel for a prize. They were mostly dubious of wave theory, but Poisson, one of them, suggested this incredible experiment. Arago performed it and reported back that it worked.

Siméon Denis Poisson (1781 -1840) bio --->
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Poisson.html
Dominique François Jean Arago (1786 - 1853) bio --->
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Arago.html

*search down the following page for 'grand prix' and read forward*
Augustin Jean Fresnel (1788 - 1827) bio --->
http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Fresnel.html

The light at a position behind the obstacle (but not too close) has been lovingly called the "Poisson spot" ever since.