A fireman aims his hose at an angle of 30 degrees. What is the expected shape of the water's trajectory?

The expected shape of the water's trajectory is a parabola due to the influence of gravity acting on the water as it is propelled from the hose. When a projectile, such as water being sprayed from a hose, is launched at an angle, its motion can be described using the principles of projectile motion.

In the absence of air resistance, the horizontal and vertical motions are independent of each other. The horizontal motion progresses uniformly, while the vertical motion is subject to the acceleration due to gravity, which pulls the water downward. As a result, when you combine these two motions, the path traced out by the water forms a parabolic shape.

This parabolic trajectory occurs regardless of the angle of projection, so long as the spray is not vertical or purely horizontal. The angle of 30 degrees specifically provides an optimal arc that allows the water to reach a greater distance before falling to the ground. Thus, the correct answer reflects the fundamental characteristics of projectile motion in physics.