The correct answer is:
A) 1000 J
In fact, energy cannot be destroyed nor created: this means that the total energy of the person must remain constant.
The initial energy of the person is U=8000 J, in form of gravitational potential energy. When he jumps and dives into the pool, this energy is converted into kinetic energy (K, 7000 J) and partially into thermal energy, Et. The final energy of the person, sum of kinetic energy and thermal energy, must be equal to the initial energy:
[tex]U=K+E_t[/tex]
from which we can find the thermal energy:
[tex]E_t = U-K=8000 J - 7000 J = 1000 J[/tex]
