One fall, two system choices
Changing the system changes the energy account, not the fall.
Gravity is external to the ball. Its work increases the ball’s kinetic energy.
No drag · uniform g · Earth’s motion neglected
Energy accounts at the same instant
Ball only
Gravity is externalΔKball = Wgravity
The ball's energy increases by the work done by gravity.
Ball + Earth
Gravity is internalΔK + ΔUg = 0
Gravitational energy belongs to the Earth–ball interaction.
The ball is released from rest. Switching systems pauses at the current instant without resetting the fall. The kinetic-energy values are the same in both accounts. The larger system includes gravitational potential energy, with its zero at the bottom of the fall. Earth’s kinetic energy is neglected. The animation ends just before contact; it does not model an impact.
Space release/pause/play · R restart · H hide controls · T light/dark · F11 full screen · Switch systems to pause and compare.