Final answer:
A moving rocket sled with no horizontal forces acting upon it will continue moving at a constant velocity in accordance with Newton's first law of motion.
Step-by-step explanation:
When considering what happens to the motion of a moving rocket sled when there are no horizontal forces acting on it, it is essential to apply the principles of Newton's laws of motion. With zero horizontal forces, the sled will not experience any horizontal acceleration. According to Newton's first law, also known as the law of inertia, an object in motion will continue to move at a constant velocity unless acted upon by a net external force. Therefore, in the absence of horizontal forces, the answer is (b) the sled continues moving at a constant velocity. When there are no horizontal forces acting on a moving rocket sled, and assuming there are no other external forces like friction, the sled will continue moving at a constant velocity. According to Newton's first law of motion, an object in motion will stay in motion with a constant velocity unless acted upon by a net external force. In this case, the absence of horizontal forces means there is no net force to change the sled's velocity. Therefore, the correct answer is: b) It continues moving at a constant velocity.