Physics
Physics
Problem
A ball is thrown straight upward with an initial speed of . Air resistance is negligible. If the ball–Earth pair is chosen as the system, what is the total work done on the system by external forces while the ball rises to its highest point? a) b) c) d)Intuitive explanation
The key insight is recognizing that when you define the system as the ball plus Earth, there are no external forces doing work—gravity becomes an internal force within that system. The only way external forces could do work is if something outside the ball–Earth pair (like air resistance or a hand) exerts a force, but the problem tells us air resistance is negligible and the throw is complete. So even though the ball slows down and loses kinetic energy, that energy is conserved within the system as it converts to gravitational potential energy.
Intuitive explanation
The key insight is recognizing that when you define the system as the ball plus Earth, there are no external forces doing work—gravity becomes an internal force within that system. The only way external forces could do work is if something outside the ball–Earth pair (like air resistance or a hand) exerts a force, but the problem tells us air resistance is negligible and the throw is complete. So even though the ball slows down and loses kinetic energy, that energy is conserved within the system as it converts to gravitational potential energy.