A delivery worker lifts a heavy parcel from the ground and places it onto a shelf. The parcel is then pushed horizontally along the shelf at a constant speed until it reaches the edge, where it falls off and lands on the floor below. Explain the energy transfers and transformations that occur from the moment the worker begins to lift the parcel to the moment it hits the floor.

Edexcel A-Level Physics (9PH0) — 2.3 Energy and work done · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The worker does positive work on the parcel while lifting it, increasing its gravitational potential energy (PE).\nWhen the parcel is pushed across the shelf at constant speed, the worker does work against friction; this energy is transferred to thermal energy in the parcel and shelf.\nAt the edge the parcel’s PE begins to decrease as it falls.\nDuring the fall the decreasing PE is converted into kinetic energy (KE), so the parcel speeds up.\nWhen the parcel strikes the floor, its KE is transferred to thermal energy (and a small amount of sound) in the parcel and floor, so the parcel comes to rest.

Examiner tips

  • Use the word ‘work’ and ‘energy store’ as in the mark scheme.
  • Show the sequence of transfers: work → PE → thermal, then PE → KE → thermal/sound.
  • Mention constant speed to justify friction work.
  • Keep the answer concise and use UK spelling.

Common mistakes

  • Confusing work done by the worker with work done by gravity.
  • Omitting the frictional work when the parcel is pushed.
  • Failing to state that kinetic energy is converted to thermal energy on impact.

Mark scheme (5 marks)

  1. Work is done by the worker on the parcel as it is lifted, transferring energy to the parcel's gravitational potential energy store.
  2. When the parcel is pushed horizontally at constant speed, work is done against friction, transferring energy to the thermal energy stores of the parcel and shelf.
  3. The gravitational potential energy store of the parcel decreases as it falls from the shelf.
  4. The gravitational potential energy store is transferred to the kinetic energy store of the parcel as it falls (it speeds up).
  5. When the parcel hits the floor, the kinetic energy store is transferred to the thermal energy stores (and sound) of the parcel and floor.

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