I don't know for sure but i think that it is because the force him pushing her pushed him back as well
Hope i helped
Answer:

Explanation:
Using the first law of thermodynamics:

Where
is the change in the internal energy of the system, in this case
,
is the heat tranferred, and
is the work,
with a negative sign since the work is done by the system.
From the previous equation we solve for heat, because it is the unknown variable in this problem

And replacing the known values:



The negative sign shows us that the heat is tranferred from the system into the surroundings.
Looks like you simply substitute the length of the femur