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how do I explain perpetual motion to my 8 years old sister?
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?
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.
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.
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.
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