Answer:
D. The escaping water exerts an equal and opposite force on the hose.
Explanation:
To be able to solve this question we must know that the firefighter is in balance, that is, he feels some forces when using the hose, but these forces do not make him move.
Let's analyze each of the answers:
A. The hose material is very elastic.
The forces are felt at the hose outlet and independent of the rigidity of the hose, the force is a function of the outlet speed and the outlet area of the hose
B. Since the hose is at rest, it tends to stay at rest.
The hose is in balance not at rest, and is in balance due to the force exerted by the fireman.
C. The force exerted on the water equals the mass of the water times its acceleration.
This is the definition of Newton's second law, and it is useful for moving bodies. But for this case it does not apply since there is no movement, the fireman grabs the hose in such a way that there is a balance.
D. The escaping water exerts an equal and opposite force on the hose.
This is the answer, and corresponds to Newton's third law, the fundamental principle of Statics studies. This law tells us that every force applied on a body corresponds to an equal and opposite force in the opposite direction. Just like when we try to push a wall, the wall exerts an equal force and in the opposite direction. The balance would be interrupted in this case when the speed of water outflow is so great that the fireman's strength is not enough to maintain balance.
