# Homework Help: A person walks to the top of the hill

1. Aug 18, 2014

### lolbob07

Just want to make sure If I have solved this question correctly.

1. The problem statement, all variables and given/known data
A 75 kg person walks to the top of a 240 m high hill expending 5.6 x 10^5 J of energy/
a. What was the potential energy of the person at the top of the hill?
b. What is the efficiency of the person in walking up the hill?

2. Relevant equations
Not sure what to write here??

3. The attempt at a solution

Solution to a.
GPE = mgh. M = 75, G = 9.8 m/s, H = 240. ∴ 75 x 9.8 x 240 = 176400 J

Solution to b.
Efficiency = Energy out / Energy in x 100%. ∴ 176400 / 560000 x 100% = 31.5

Is this correct? I am not quite sure...
Thanks

Last edited: Aug 18, 2014
2. Aug 18, 2014

### billy_joule

Does an efficiency greater than 100% seem right to you?

It implies a human can create energy by walking up hills...

3. Aug 18, 2014

### lolbob07

No it does not, which is the reason I asked.
Is it referring to how much energy the person uses per metre of walking?

4. Aug 18, 2014

### CWatters

Where does your figure of 1477350 come from?

5. Aug 18, 2014

### lolbob07

Hahahha! Well thanks! I think that solves it :) Just changed it

6. Aug 18, 2014

### billy_joule

I agree:

75 x 9.8 x 240 = 176400

It tells you about the losses in a system.

e.g. petrol motors are around 20-30% efficient; that is to say the mechanical energy output (at the crankshaft) is only 20-30% of the chemical energy (petrol) input, the rest is lost as heat, sound etc

You need to identify what the input and output in your system is.

Last edited: Aug 18, 2014
7. Aug 18, 2014

### lolbob07

Not sure if this is correct. But, is 176400 the input and 383600 is the output (560000 - 176400)? Does the 383600 count as wasted energy?

8. Aug 18, 2014

### billy_joule

No on both counts.

Where is the energy used to go up the hill coming from?

What is the final (output) energy once the top of the hill has been reached?

9. Aug 18, 2014

### lolbob07

No, sorry I can't understand how to identify the input and output.

Is the final output energy the potential energy at the top?

10. Aug 18, 2014

### billy_joule

Yes.

The persons effort (energy expended) is the input.

I see you've edited your OP, it looks correct now.

11. Aug 18, 2014

### lolbob07

lol. I thought you noticed that before. Sorry! It was just a numbers error :/
Thanks for the help.