A person walks to the top of the hill

[lolbob07]

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

Homework Statement

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?

Homework Equations

Not sure what to write here??

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

 

[billy_joule]

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

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

 

[lolbob07]

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

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

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

 

[CWatters]

Where does your figure of 1477350 come from?

 

[lolbob07]

Where does your figure of 1477350 come from?

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

 

[billy_joule]

I agree:

75 x 9.8 x 240 = 176400

Is it referring to how much energy the person uses per metre of walking?

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.

 

[lolbob07]

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.

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?

 

[billy_joule]

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?

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?

 

[lolbob07]

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?

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

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

 

[billy_joule]

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

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

Yes.

The persons effort (energy expended) is the input.

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

 

[lolbob07]

Yes.

The persons effort (energy expended) is the input.

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

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