Romy holds a barbell stationary overhead. Her arms burn with the effort.
Is her force doing work on the barbell? This topic opens by deciding exactly that.
Watch the lift first. Romy raises the barbell from the floor to overhead: her lifting force acts upward on the barbell, and the barbell moves upward.
Her force does work on the barbell.
Now the hold. The same barbell sits stationary overhead. Romy's force still acts upward on it, but the barbell does not move.
No movement means no work: holding a barbell stationary overhead does no work on it, however hard the arms burn.
Effort is not the test. Movement is.
One more zero-work case hides in plain sight. A cart rolls across a level floor: the normal force on the cart acts straight up, and the cart's motion is horizontal, perpendicular to that force.
A force perpendicular to the object's motion does no work on it.
You have already listed the forces acting on an object earlier in the course. To judge work, take the forces one at a time.
An elevator car rises. The cable's force on the car acts upward. Does the cable's force do work on the car?
The car moves, and its motion is upward, the force's own direction, not perpendicular to it.
Yes: the cable's force does work on the car.
A server carries a tray across a level dining room at a steady height. Her supporting force on the tray acts straight up. Does her force do work on the tray?
The tray moves, so the first test passes.
But the tray's motion is horizontal, perpendicular to the upward force.
No: her force does no work on the tray.
A dog team tows a sled forward across snow, and the sled moves forward. The towline's force on the sled points forward. Does the towline's force do work on the sled?
A crane raises a steel beam straight up. The cable's force on the beam acts upward, and the beam moves upward. Does the cable's force do work on the beam?
Talia leans on a parked van with a steady force. The van does not move. Does her force do work on the van?
A hockey puck slides across level ice. The normal force on the puck acts straight up. Does the normal force do work on the puck?
Ugo exerts a steady force on a loaded bookcase, straining hard, but the bookcase does not move. Does his force do work on the bookcase?
Soren says: 'When a forklift raises a pallet, the forklift's upward force does work on the pallet, because the pallet moves in the force's direction.' Is Soren correct?
In which case does the named force do work on the car?
A force acts on a moving suitcase. What decides whether the force does work on the suitcase?
A gondola cabin moves horizontally along a level stretch of its route while the cable's supporting force on it acts straight up. Which feature of the case shows that the supporting force does no work on the cabin?