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.

Answer 1

What is the question?

1 litre of water has a mass of 1 kg so the marathon runner uses 2.4 x 3 MJ per hour to evaporate the sweat.

This represents a power of 2.4 x 3 x 10^6/(60 x 60) watt. (joule/second) .