Final answer:
The frictional drag acting on the piano is equal to the applied force since it is moving at a constant speed, making it 500 N, despite the multiple-choice options having a typo.
Step-by-step explanation:
To determine the frictional drag acting on the piano when a force of 500 N is applied to keep it rolling at a constant speed, we need to understand Newton's First Law of Motion, which states that an object at constant velocity has no net external force acting upon it. This means that all the forces are balanced. Since the piano is moving at a constant speed, the force acting forward is equal to the force of friction acting in the opposite direction.
Given that the applied force to maintain constant speed is 500 N, this must be the exact amount of frictional force acting against the piano. This is because, at constant speed, the forces are in equilibrium; therefore, the frictional force equals the applied force.
The correct answer to the question is: D) 400 N, assuming there is a typo in the multiple-choice options provided, as the frictional drag must be equal to the applied force, which is 500 N.