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This question is causing great confusion in my head.

Which of the following is an example of potential rather than kinetic energy? A) the muscle contractions of a person mowing grass

B) water rushing over Niagara Falls

C) light flashes emitted by a firefly

D) a molecule of glucose

E) the flight of an insect foraging for food

Answer: D

A) Muscle contractions is caused by the use of potential energy in ATP used by actin and myosin.

B) This is a classic example of what potential energy means.

C) Bioluminescence is a process caused by the use of chemical (potential) energy.

D) Yes, the molecule of glucose does contain potential energy. BUT, it is not potential energy itself...

E) Flight is kinetic energy. However, you can still boil this down to ATP in actin and myosin.

How do I approach this problem? My teacher said this is the type of problem she would give us. But this is so ambiguous. Am I lost?

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    $\begingroup$ If you find one of the answers solving your problem accept it. Doesn't matter which! $\endgroup$
    – One Face
    Jan 27 '15 at 12:44
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I think you are right and many examples are ambiguous. Even D) is ambiguous as a molecule is always in motion due to entropy except, perhaps, at absolute zero (0 Kelvin). But normally, it does contain kinetic energy.

However, as you already identified the ambiguity, look at the best, or in this case least incorrect answer; The only option without any physical movement explicitly mentioned in the question description is D). All the others mention 'emit', 'flight', 'rushing down', etc.

Also ask your teacher :)

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  • $\begingroup$ Is emitting really a form of kinetic energy? Maybe they were referring to "light energy" and not the potential energy that was used to make the light. $\endgroup$
    – yolo123
    Jan 26 '15 at 3:14
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    $\begingroup$ A moving photon has, however small, kinetic energy $\endgroup$
    – AliceD
    Jan 27 '15 at 3:15
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Going by the answer, the key to interpret this question seems to be the absence of kinetic energy.

Total Energy (mechanical) = Potential + Kinetic (let's ignore the pressure component as this question does not involve it).

So the key to answer this question would be a system where the full energy is represented by the Potential energy.

In a contracting muscle - let's consider gross dynamics - the muscle is in motion (the myofibrils are moving) ~ Kinetic Energy

Water moving in a falls - Definitely kinetic energy (In fact this system is one where the potential energy is constantly decreasing while the kinetic energy is increasing constantly)

Bioluminescence - Light Energy which is neither kinetic nor potential (as I understand physics, so correct me if I am wrong)

Glucose as a source of energy - In its unutilised state (only during which it is called glucose) is purely potential energy. When glucose is talked in terms of potential energy, the movement of glucose inside a medium is not what is being talked about.

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    $\begingroup$ Yes, well broken down answer +1 $\endgroup$
    – AliceD
    Jan 27 '15 at 12:45
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You'll find the answer glucose best fits the definition of potential energy, as in it's standard state the glucose has all of it's energy stored in it's bonds (1). Every other answer is some sort of expenditure of energy (ATP in the case of firefly luciferase reaction, see (2)).

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