In the absence of air resistance, a falling rock gains kinetic energy and loses potential energy, while the total energy of the rock remains constant. in the presence of air resistance, however, the rock eventually reaches terminal velocity. now the kinetic energy is constant, but the potential energy continues to decrease as the rock falls toward the ground. why is some energy missing?

Total energy of a system always remains constant

Explanation:

In this particular case a work against air resistance  have to be done, and this is equal of the decrease in potencial energy.

The principle of energy conservation sates that energy is not created  or lost

but is transformed

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

the temperature in kelvin of 2.0 dm3 of an ideal gas is doubled and its pressure is increased by a factor of four. what is the final volume of the gas