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survivorboi · Jul 28, 2009, 01:25 PM

The other day I kind of went too far on physic matters with my 8 years old sister and go into perpetual motion. I said that perpetual motion machines can't exist because it would violate the 1st and 3rd law of thermodynamics.

Why not? She asked.

Then I figure out that even I don't really understand all the 100% energy efficient thing. I know that you need energy to fight air resistance and friction, but why can't perpetual machines exist? And how do I explain it to my sis?

Thanks a tank! :D:D:D:D:D:D

JudyKayTee · Jul 28, 2009, 01:51 PM

Why there aren't any Perpetual Motion Machines

survivorboi · Jul 28, 2009, 05:19 PM

Huh?? What do you mean JudyKayTee? Well, because it violates the 1st, 2nd, and 3rd thermodynamics law? Right?

Fr_Chuck · Jul 28, 2009, 05:47 PM

First of course it is because we believe they can't exist. You know people used to say airplanes could not exist, rockets could not, TV, and so on.

It is often the people who ask why not that will change the world.

You don't need to explain it to a 8 year old

ballengerb1 · Jul 28, 2009, 05:59 PM

Perpetual motion does exist, just not where we live. In space, without friction or gavity, objects in motion stay in motion.

asking · Jul 28, 2009, 06:11 PM

No. Actually, even in space, perpetual motion machines cannot exist. First of all there IS some friction, because space is not an absolute vacuum. Second, there are other ways in which energy is lost besides friction. In any process where energy is transformed--say from a chemical battery to electricity to the movements of a mechanical bear or from nuclear fusion in the sun to the radiation of light-- disorder increases. The increasing disorder (called "entropy") is where the inefficiency comes from.

Imagine, climbing 10 flights of stairs. As you climb, your body heats up, due to inefficiencies in transforming the energy in sugar (glucose) into muscle contractions. In order to understand the details of these inefficiencies, you have to know more about physics and chemistry.

Don't worry, survivorboi, you got it right!

Edit: PS. If you don't think it takes energy to overcome entropy (increasing disorder), try not cleaning your house and see how it looks at the end of 12 months.

ballengerb1 · Jul 28, 2009, 06:14 PM

Title of the post was "how do I explain perpetual motion to my 8 years old sister" and I never mentioned a machine. He somehow got into machines where trying to explain perpetual motion.

asking · Jul 28, 2009, 06:17 PM

It's the same.

KISS · Jul 28, 2009, 06:21 PM

Perpetual motion requires friction to be zero. Friction can be bearing or air resistance.

Suppose that a perpetual motion machine has to rotate. Once the rotation starts, friction on the bearing keeps consuming energy until it stops.

Now one that's harder. Take a ball on a string and move it, enventually it comes to a stop. Why? Gravity, air resistance and some friction in the string where it's attached.

No matter how slight, if not equal to zero, you cannot have perpetual motion.

The pendulum in a cukoo clock is a bit different.

ballengerb1 · Jul 28, 2009, 06:27 PM

How do we explain this? YouTube - The OC MPMM - Alsetalokin's Video [2008/01/04]

jenniepepsi · Jul 28, 2009, 06:32 PM

Um... yes perpetual motion machines exist...

survivorboi · Jul 28, 2009, 07:36 PM

Originally Posted by ballengerb1

Perpetual motion does exist, just not where we live. In space, without friction or gavity, objects in motion stay in motion.

Hey asking, what makes you say that? Laws are made to be broken right?

survivorboi · Jul 28, 2009, 07:42 PM

Originally Posted by asking

No. Actually, even in space, perpetual motion machines cannot exist. First of all there IS some friction, because space is not an absolute vacuum. Second, there are other ways in which energy is lost besides friction. In any process where energy is transformed--say from a chemical battery to electricity to the movements of a mechanical bear or from nuclear fusion in the sun to the radiation of light-- disorder increases. The increasing disorder (called "entropy") is where the inefficiency comes from.

Imagine, climbing 10 flights of stairs. As you climb, your body heats up, due to inefficiencies in transforming the energy in sugar (glucose) into muscle contractions. In order to understand the details of these inefficiencies, you have to know more about physics and chemistry.

Don't worry, survivorboi, you got it right!

Edit: PS. If you don't think it takes energy to overcome entropy (increasing disorder), try not cleaning your house and see how it looks at the end of 12 months.

Man, you gave me an example I can really understand! So, disorder increases when energy isn't involved?