Final answer:
A fired cannonball in space will continue in a straight line at constant speed due to lack of external force, illustrating Newton's first law of motion. For a LightSail-1 type craft, continual photon bombardment leads to continual acceleration.
Step-by-step explanation:
The statement that best describes what would happen to a cannonball that is fired in space is: It will continue moving in a straight line at constant speed. This behavior is due to Newton's first law of motion, which states that an object in motion will stay in motion at a constant velocity, and an object at rest will stay at rest, unless acted upon by an external force. In the vacuum of space, there is no air resistance or other significant force to stop or decelerate the cannonball, so it would continue to move at the same speed in the direction it was fired.
The effect of continual photon bombardment on the motion of a craft similar to LightSail-1, if set into interplanetary launch, would be continual acceleration of the craft. This is because photons exert a small amount of force when they strike a surface, and in the vacuum of space, this force can cause an object like LightSail-1 to accelerate due to the momentum transfer from the photons.