Final answer:
The correct statement about a cannonball's motion is that the gravitational force changes its velocity. The horizontal component of its motion is constant, while the vertical component changes due to gravity, resulting in a parabolic trajectory.
Step-by-step explanation:
Relative to the motion of a cannonball, let's explore the statement: 'The gravitational force acting on it changes its velocity'. This statement is the most accurate among the options presented. Once a cannonball is fired, it enters a type of motion called projectile motion. In projectile motion, an object has two components of motion - horizontal and vertical. The horizontal motion is at a constant speed because there are no external forces acting in the horizontal direction (assuming air resistance is negligible). The vertical motion is affected by gravity, which provides a constant acceleration downwards. This means the vertical velocity of the cannonball changes over time due to this acceleration, thus changing the cannonball's overall velocity.
The concept of uniform motion is relevant when discussing such a topic. An object in uniform motion would have a constant velocity, meaning it does not accelerate. However, as the gravitational force does indeed act on the cannonball, changing its vertical velocity, the cannonball does not exhibit uniform motion. Instead, it has a parabolic trajectory characterized by a constant horizontal velocity component and a changing vertical velocity component due to the force of gravity (Newton's law of universal gravitation).
Projectile motion shows that any object under such conditions will fall at the same rate as an object in freefall, independent of its horizontal velocity, which validates that the gravitational force is the single factor affecting the vertical component of the motion.