Kilogram(kg)
It's not the SI unit of mass in the metric system however.
The answer is latent heat. The specific latent heat of vaporisation, L_v, of a substance is the energy input required for each kilogram to be converted from liquid to gas by evaporation. The 'specific' means per kilogram, so more generally latent heat of vaporisation is the energy taken in during the process for a given mass.
Here we are not vaporising the substance. We are in fact condensing it, the reverse process. All this means is the latent heat is released as electrostatic potential decreases in the water, as opposed to being absorbed. I hope this helps you :)
Work done by a given force is given by
here on sled two forces will do work
1. Applied force by Max
2. Frictional force due to ground
Now by force diagram of sled we can see the angle of force and displacement
work done by Max = 
Now similarly work done by frictional force
Now total work done on sled
