Final answer:
Gauge pressure reads zero for water emerged from a hose into the atmosphere because it is defined relative to atmospheric pressure and reads zero when it equals the atmospheric pressure (option a). One still feels a force due to the kinetic energy of the moving water being transmitted when it hits an obstacle.
Step-by-step explanation:
The question poses why water that has emerged from a hose into the atmosphere has a gauge pressure of zero. The correct option here is a) Atmospheric pressure offsets gauge pressure.
Gauge pressure is defined as the pressure relative to atmospheric pressure. It is a common feature for most pressure gauges to ignore the atmospheric pressure, thus reading zero at this level. Gauge pressure is considered positive when it is above atmospheric pressure and negative when below it. Therefore, once the water leaves the hose, the gauge pressure drops to zero because it has become equal to the atmospheric pressure.
Despite the gauge pressure being zero, when you place your hand in front of the water stream, you feel a force. This is due to the kinetic energy of the moving water being converted into work on your hand. The water in motion has a certain amount of energy, thanks to its velocity and mass. Once it encounters an obstacle, like your hand, that energy is transferred. This is why you feel a pressure even though the gauge pressure is zero—it is the energy transfer that you are experiencing, not an actual pressure above the atmospheric pressure.