# Finding Mechanical Energy without knowing Efficiency

1. Mar 10, 2013

### jennyskywalker

1. The problem statement, all variables and given/known data
A 65 kg man goes for a 72 minute hike up a mountain that is 400.0 metres in vertical height. The energy consumption rate for this climb is 10.1 kcal/min. What is the Mechanical Energy (useful work energy) expended for this hike? (in kcal)

2. Relevant equations
ME= (Efficiency x Eout)/100

3. The attempt at a solution
10.1 x 72 = 727.2 (Therefore Eout=727.2) But I don't know the efficiency! So how do I figure this out?

2. Mar 10, 2013

### SteamKing

Staff Emeritus
How much energy must the man expend to lift his mass up the mountain?

3. Mar 10, 2013

### jennyskywalker

727.2 kcal? But that is the total energy expended, including heat energy. ME=(efficiency x 727.2)/100. That's all I know!

4. Mar 10, 2013

### jennyskywalker

I asked on another forum, and they said ME= mgh, but according to my teacher, PE (potential energy)=mgh, and I'm looking for Mechanical energy!

5. Mar 10, 2013

### SteamKing

Staff Emeritus
energy is energy.

The man expended so many kcal thru his metabolism while walking up the hill. That you have calculated.

However, in moving his mass up the hill, a certain portion of this energy was consumed in moving his mass up the hill against gravity. This energy is equal to the change in gravitational potential from the bottom of the hill to the top.