Romy watches a weightlifter raise a barbell and wonders who is doing the work here: the lifter, or the barbell?
Both are in the story. The answer depends on which one you are keeping the books for.
Track the energy first. The lifter's force acts upward on the barbell while the barbell moves upward, so energy flows from the lifter to the barbell.
Now pick a system: the thing you keep the energy books for.
Pick the barbell as the system. The lifter's force transfers energy INTO it. Work is done ON the barbell.
Pick the lifter as the system instead. Her force transfers energy OUT of her, to the barbell. Work is done BY the lifter.
Here are the two labels in full.
One transfer earns both labels at once: in the lift, work is done BY the lifter and ON the barbell.
So the question 'who is doing the work?' has a one-word answer: the lifter does the work, and the barbell receives it.
To sort any case, ask where the energy flows relative to your chosen system: inward means work done ON it, outward means work done BY it.
A locomotive tows freight wagons out of a yard. Take the wagons as the system. Is work being done on the wagons or by the wagons?
The locomotive's force transfers energy into the wagons.
Energy flowing into the chosen system means work is done on it.
Work is being done on the wagons.
The same scene, but now take the locomotive as the system. Is work being done on the locomotive or by the locomotive?
The locomotive's force transfers energy out of it, to the wagons.
Energy flowing out of the chosen system means the system does the work.
Work is being done by the locomotive.
A tow truck tows a sedan up a ramp, its force transferring energy to the sedan. Take the sedan as the system. Is work being done on the sedan or by the sedan?
Talia works a rowing machine: her force moves the handle through each stroke, transferring energy to the machine. Take Talia as the system. Is work being done on Talia or by Talia?
Soren says: 'When an escalator carries a passenger upward, work is done by the escalator and on the passenger.' Is Soren correct?
A pickup truck tows a camper trailer along a road, the towbar's force on the trailer pointing forward. Which statement about the energy flow is correct?
Romy winches a rowboat up a boat ramp. First take the rowboat as the system, then take the winch as the system. Which pair of labels is correct?
A pitching machine's launcher arm drives a baseball forward. While the arm is in contact with the ball, which statement is correct?