Work: Transferring Energy with a Force — Practice · Grade 8 Science

Chapter: Work & Machines

What this practice covers

By the end of Work: Transferring Energy with a Force you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: work, joule, displacement, model, evidence, system boundary, assumption.

Problem 1

A 5 N force moves a cart 4 m in the same direction. Find its work.
Answer and explanation

20 J

5 × 4 = 20.

Problem 2

You push a wall that remains still. Mechanical work on the wall is…
  1. zero
  2. equal to your mass
  3. always 100 J
  4. infinite
Answer and explanation

zero

The wall has no displacement.

Problem 3

Why can you feel tired while holding a heavy bag still although no mechanical work is done on the stationary bag?
Answer and explanation

Your body uses chemical energy, but the bag has no displacement. The objects and energy transfers being considered differ.

Problem 4

A horizontal 15 N force moves a box 4 m in the same direction. Find the work; compare with holding it still. Explain the reasoning in your own words.
Answer and explanation

60 J while moving; 0 J on the stationary box by the supporting force.

Use W=Fd=15×4=60 J.

With zero displacement, mechanical work on the stationary box is zero.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 5

A 12 N aligned force moves a toy 3 m. Find work. Show the reasoning, not just the conclusion.
Hints

A force without displacement transfers no mechanical work to the object in this simple model.

Answer and explanation

W=12×3=36 J.

Problem 6

A box moves horizontally while its weight acts vertically. What work does weight do over that horizontal displacement?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Zero in this ideal case, because force and displacement are perpendicular.

Problem 7

Would the same horizontal displacement give work by a vertical force?
Answer and explanation

Not in this simple perpendicular-force case. Only the component along displacement contributes to that mechanical work.

Not in this simple perpendicular-force case. Only the component along displacement contributes to that mechanical work.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 8

A supplied object moves 3 m horizontally while a 4 N force acts in that direction. In another case a person holds the object still with an upward force. Find the work in the first case and explain the ideal mechanical work in the second. Clarify the everyday meaning of effort.
Answer and explanation

The first force transfers 4×3=12 J through the aligned displacement. For the chosen held object, no displacement means zero ideal mechanical work by the holding force. The person may still use metabolic energy to maintain muscle activity. Everyday effort and the technical force–displacement quantity answer different questions.

The first force transfers 4×3=12 J through the aligned displacement. For the chosen held object, no displacement means zero ideal mechanical work by the holding force. The person may still use metabolic energy to maintain muscle activity. Everyday effort and the technical force–displacement quantity answer different questions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Opposite forces make different contributions to workOpposite forces make different contributions to workmoving boxDisplacement: 3 m rightPush: 10 NResistance: 4 NPush work: +30 J. Resistance work: −12 J. Net work: +18 J.Forces and displacement are distinct arrows with distinct units.
For the stated constant aligned forces over 3 m rightward displacement, the push does +30 J of work and resistance does −12 J on the box. Their net contribution is +18 J. A red force arrow is not the same quantity as the blue displacement arrow. The diagram omits vertical interactions that do no work in this horizontal idealization.
For this unit’s illustrated case, if resistance becomes 6 N over the same 3 m, what happens to its work and to the net work?
Answer and explanation

Resistance work becomes −18 J; the push remains +30 J, so net work is +12 J. Work signs depend on the force direction relative to displacement.

Positive work transfers energy into the chosen object through that force; negative work transfers energy out through that force in this account. Add contributions only after their directions and common displacement are clear.

Resistance work becomes −18 J; the push remains +30 J, so net work is +12 J. Work signs depend on the force direction relative to displacement.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 10

A box moves 4 m right under a constant 12 N rightward push and 3 N leftward resistance. Find each work contribution and net work.
Answer and explanation

Push +48 J; resistance −12 J; net +36 J in the stated aligned-force comparison.

Multiply each force by the common displacement and include direction.

Add the work contributions rather than adding force and energy units.

Push +48 J; resistance −12 J; net +36 J in the stated aligned-force comparison.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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