Work & Machines — Chapter quiz · Physical Science: Matter, Forces & Energy, Grade 8

Explain how machines transfer energy, trade force for distance, and improve useful output.

Problem 1

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 2

Why can a ramp reduce force without eliminating the energy needed to lift?
Answer and explanation

The smaller force acts over a longer distance. The load’s height gain still requires the same gravitational energy increase in the ideal model.

The smaller force acts over a longer distance. The load’s height gain still requires the same gravitational energy increase in the ideal model.

Response criteria

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

Problem 3

Independent chapter check: 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.

Problem 4

Illustrative ideal ramps raise the same load by 0.5 m. Ramp A is 1 m long and B 2 m long; measured input forces are 20 and 10 N under the simplified no-loss model. Compare input work and explain the force–distance trade-off. State why actual friction would change the required input.
Answer and explanation

Both have modeled input work 20 J: 20×1 and 10×2. The longer ramp reduces force while increasing travel distance for the same lift; it does not make energy. Actual friction can require greater input work and introduce thermal transfers. The simple along-ramp calculation uses the stated force direction and ideal model.

Both have modeled input work 20 J: 20×1 and 10×2. The longer ramp reduces force while increasing travel distance for the same lift; it does not make energy. Actual friction can require greater input work and introduce thermal transfers. The simple along-ramp calculation uses the stated force direction and ideal model.

Response criteria

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

Problem 5

Why does a two-supporting-segment pulley require more rope movement?
Answer and explanation

Both supporting segments shorten when the load rises. The input rope must supply twice the load’s upward distance in the ideal arrangement.

Both supporting segments shorten when the load rises. The input rope must supply twice the load’s upward distance in the ideal arrangement.

Response criteria

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

Problem 6

Independent chapter check: For an ideal balanced lever, the load exerts 180 N at an arm 0.2 m long. The effort arm is 0.6 m. Find the effort force and explain its distance tradeoff.
Answer and explanation

Effort is 180×0.2/0.6 = 60 N. The effort end travels about three times the load-end distance for the small-motion ideal comparison; the lever does not create energy.

Match turning effects F×arm on opposite sides.

Connect the 3:1 arm ratio to effort travel as well as force.

Effort is 180×0.2/0.6 = 60 N. The effort end travels about three times the load-end distance for the small-motion ideal comparison; the lever does not create energy.

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 7

An illustrative ideal machine takes a 15 N input through 0.8 m and applies a 60 N output through 0.2 m. Compute mechanical advantage and both works. Explain how all results fit one energy account.
Answer and explanation

Mechanical advantage is 60/15=4. Input and output works are each 12 J. The fourfold force is paired with one-quarter the distance, so this ideal model conserves work. Real machines can have losses; a force ratio alone does not establish energy efficiency without displacement or other energy information.

Mechanical advantage is 60/15=4. Input and output works are each 12 J. The fourfold force is paired with one-quarter the distance, so this ideal model conserves work. Real machines can have losses; a force ratio alone does not establish energy efficiency without displacement or other energy information.

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

Does MA=4 mean efficiency is 400%?
Answer and explanation

No. MA is a force ratio; efficiency is an energy ratio. Input and output distances also matter when calculating work.

No. MA is a force ratio; efficiency is an energy ratio. Input and output distances also matter when calculating 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 9

Independent chapter check: A measured machine uses effort 20 N through 0.9 m to lift a 60 N load by 0.2 m. Find actual mechanical advantage and useful efficiency.
Answer and explanation

MA = 60/20 = 3. Input work 18 J; useful output 12 J; efficiency = 12/18×100 ≈ 66.7%.

Keep force ratio separate from work ratio.

Include both travel distances in the energy account.

MA = 60/20 = 3. Input work 18 J; useful output 12 J; efficiency = 12/18×100 ≈ 66.7%.

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

Illustrative machine A receives 250 J and delivers 150 J useful output; B receives 200 J and delivers 150 J. Their task and measurement boundary match. Compare efficiencies and account for each remaining energy amount. Explain why purpose matters to the useful fraction.
Answer and explanation

A is 60% efficient and B 75% for the stated task. The remaining modeled outputs are 100 J and 50 J in other pathways, such as thermal changes or sound. These amounts are not destroyed. An output called waste for lifting might be useful for heating, so usefulness must be defined before comparing fractions.

A is 60% efficient and B 75% for the stated task. The remaining modeled outputs are 100 J and 50 J in other pathways, such as thermal changes or sound. These amounts are not destroyed. An output called waste for lifting might be useful for heating, so usefulness must be defined before comparing fractions.

Response criteria

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

Problem 11

Why is comparing only each design’s best trial misleading?
Answer and explanation

Best trials can reflect random variation or favorable conditions. Comparable repeated results provide a more representative performance comparison.

Best trials can reflect random variation or favorable conditions. Comparable repeated results provide a more representative performance comparison.

Response criteria

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

Problem 12

Independent chapter check: A device receives 240 J, delivering 150 J to its defined useful output and 90 J to other changes. Find efficiency and explain what is wrong with calling the 90 J destroyed.
Answer and explanation

Efficiency = 150/240×100 = 62.5%. The other 90 J remains in other stores/transfers included in a complete account; usefulness is a job definition, not conservation.

Divide useful output by total input.

Account for every supplied contribution with the same energy units.

Efficiency = 150/240×100 = 62.5%. The other 90 J remains in other stores/transfers included in a complete account; usefulness is a job definition, not conservation.

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 13

Illustrative lifting devices each deliver 240 J; A takes 8 s and B 12 s. A receives 400 J and B receives 300 J. Compare useful power and efficiency, then explain why the higher-power device is not automatically more efficient.
Answer and explanation

Useful powers are 30 W for A and 20 W for B. Efficiencies are 240/400=60% and 240/300=80%. A delivers useful energy faster but B directs a larger fraction of input to that output. Power and efficiency answer different questions, and both require the same defined useful output.

Useful powers are 30 W for A and 20 W for B. Efficiencies are 240/400=60% and 240/300=80%. A delivers useful energy faster but B directs a larger fraction of input to that output. Power and efficiency answer different questions, and both require the same defined useful output.

Response criteria

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

Problem 14

Why can rated power alone fail to predict a daily energy bill?
Answer and explanation

The operating duration, actual average power, cycling and energy price are also needed. A rating describes a condition rather than every day’s use.

The operating duration, actual average power, cycling and energy price are also needed. A rating describes a condition rather than every day’s use.

Response criteria

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

Problem 15

Independent chapter check: A device transfers a useful 180 J in 3 s while receiving 240 J in that interval. Find useful average power and efficiency, then explain which changes if the same energy transfers take 6 s.
Answer and explanation

Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.

Use energy/time for power and useful/input for efficiency.

Change only the interval in the second comparison.

Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.

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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