Question

Evaporation of sweat is the human body's cooling mechanism. At body temperature, it takes 2.4 MJ/kg to evaporate water. Marathon runners typically lose about 3 L of sweat each hour.How much energy gets lost to sweating during a 1.7-hour marathon?

Answer 1

This is my work:
total mass of sweat: 3*1.7 = 5.1L = 5.1kg
Heat-loss : Q = mL = 5.1 kg* 2.4*10^6 J/kg = 1.224*10^8 J
and then, energy P = Q/t = 7.2 *10^6 J/s
Am I right?
@ybarrap

Answer 2

oh, no, P = Q/t = 1.224*10^8 J/ 6120s = 20000J/s= 20kJ/s

Answer 3

The only problem I see is that P is power not energy. Q is what you are looking for.

Answer 4

"'How much energy gets lost "


You already found that. You went too far. Energy is a quantity in joules, NOT joules per second... that'd be a rate of energy usage/production.

Answer 5

:) so, it's just Q? how about P in P = Q/t ? what is it?

Answer 6

Power = energy/time

Answer 7

That is power, rate of change of energy per unit time

Answer 8

Thanks you all. :)

Answer 9

That is very easy to confuse. You walk a mile, you run a mile. You use the same amount of energy. But power is different in each case.

Answer 10

because the time used in walking is longer than it is in running, right?

Answer 11

Yes so power will be lower when walking.

Answer 12

Thank you. :)