Conservation of energy/ efficiency question.

[maccha]

Homework Statement

A climber's gravitational potential energy increases from 14000 J to 21 000 J while climbing a cliff. She expends 18 000 J of energy during this activity. What is the efficiency of this process?

Homework Equations

Efficiency= useful output/ total input

Total energy before = total energy after

The Attempt at a Solution

Well, I've been able to solve all other efficiency/ conservatino of energy questions but for some reason I just don't understand this one. I don't understand what to consider as the input or the useful output energy. Thanks for your help!

 

[LowlyPion]

What was her change in potential energy? Isn't that the useful output?

The work gone into increasing potential energy?

 

[maccha]

Okay well then useful output would be 7 000 J / 14 000 J (input), and efficiency would be 50 % ? But the answer says it is 61 %.

 

[LowlyPion]

No. She expended 18,000 J not 14,000.

 

[maccha]

Oh! Okay. Still doesn't work with the answer though ah.

 

[LowlyPion]

Oh! Okay. Still doesn't work with the answer though ah.

It is 1 - the answer.

I think the answer may be wrong?

 

[maccha]

Yeah we have an exam review booklet which said that was the answer, but I just found the same exam on the internet and it said yours was right, so the booklet answer was wrong. Thanks!

 

[LowlyPion]

That's a relief. Wouldn't make sense otherwise.

Good luck.