An object is moving at constant speed in a straight line. Which of the following would cause it to move differently? A. It is acted on by balanced forces. B. It is acted on by unbalanced forces. C. It is pushed by two new, exactly opposing forces. D. It ceases to be subject to any force.

B.

According to Newton's first law, it states that an object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an external unbalanced force. For an object to be moving at a constant speed in a straight line means that no force is opposing it's state of motion. This means that we are considering a frictionless scenario to begin, meaning the object will slide on forever unless an unbalance force is acted on it. The only way to interrupt this state of constant motion is if the object is acted on by an unbalanced force as stated by Newton's first law.

The other choices are incorrect because being acted upon by balanced force means no net force, or in essence, no force. The same idea goes for exactly opposing forces (it's just another way of saying balanced force). Lastly, if an object is already in motion and ceases to be subject to any force, it would still move on forever and not cause any changes in motion.