No Infinite Energy?

Classical Mechanics Level 2

A perpetual motion machine is a machine that can do work indefinitely without an energy source.

Which law(s) of physics make a perpetual motion machine impossible?

Newton's laws of motion Equivalence principle Conservation of energy Laws of thermodynamics

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Ajala Singh by Ajala Singh , 21, India

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3 solutions

Krishna Karthik
Nov 18, 2018

Don’t ignore the second law of thermodynamics when you’re doing this problem. It also states why a perpetual motion machine doesn’t work.

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Ajala Singh
Nov 21, 2016

The first law of thermodynamics states that is the change in internal energy of the system, is equal to the sum of the change in heat supplied to the system, and the change in work done to the system.

The second law of thermodynamics states that the entropy of an isolated system or any cyclic process never decreases; it will either increase or remain the same.

As such, these imply that perpetual motion machines cannot do work indefinitely without an external energy source.

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The output of any machine cannot be larger than the input energy (we usually achieve about 30%)

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While that is a true statement, which of the laws of physics result in that conclusion?

Calvin Lin Staff - 4 years, 6 months ago

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