Mechanical Advantage — Practice · Grade 8 Science

Chapter: Work & Machines

What this practice covers

By the end of Mechanical Advantage you should be able to:

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

Vocabulary used: input force, output force, mechanical advantage, model, evidence, system boundary, assumption.

Problem 1

A machine provides 45 N output for 15 N input. Find actual mechanical advantage.
Answer and explanation

3

45 ÷ 15 = 3.

Problem 2

An ideal ramp is 3 m long and 1 m high. Find ideal mechanical advantage.
Answer and explanation

3

Length ÷ height = 3 ÷ 1.

Problem 3

Why might a real ramp need more force than an ideal calculation predicts?
Answer and explanation

Friction opposes movement and transfers some energy to thermal energy.

Problem 4

A machine lifts a 120 N load with 40 N effort. Find actual mechanical advantage. Explain the reasoning in your own words.
Answer and explanation

Mechanical advantage is 3; this does not specify efficiency without distances.

Mechanical advantage=output force/input force.

120/40=3; the force units cancel.

Response criteria

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

Problem 5

A device delivers 96 N using 24 N input. Calculate actual mechanical advantage. Show the reasoning, not just the conclusion.
Hints

It describes force multiplication, not energy creation.

Answer and explanation

96/24=4.

Problem 6

Two devices have MA=3, but one wastes more energy. Must their efficiencies be equal?
Hints

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

Answer and explanation

No. MA alone lacks the input and output displacement or work needed to determine efficiency.

Problem 7

Does force ratio 4 mean energy efficiency 400%?
Answer and explanation

No. Force is gained by trading displacement. The ideal energy efficiency here is 100%, with equal 12 J input and output work.

No. Force is gained by trading displacement. The ideal energy efficiency here is 100%, with equal 12 J input and output 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

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 9

A ratio and an absolute force answer different questionsA ratio and an absolute force answer different questionsMachine AEffort 25 N; output 100 NMA = 100/25 = 4Machine BEffort 40 N; output 120 NMA = 120/40 = 3A has greater force advantage; B supplies a greater absolute output force.Keep the test loads and task comparable before ranking performance.
In the supplied records, A multiplies effort force by four while B multiplies it by three. B’s output force is nonetheless larger because the input force also differs. The records distinguish ratio from total output and do not establish one machine as universally better. A fair test of the same task needs matched requirements and measurements of travel and energy.
For this unit’s illustrated case, which has greater mechanical advantage, and which has larger output force in the supplied records?
Answer and explanation

A has MA 4, greater than B’s MA 3. B has output 120 N, larger than A’s 100 N. These are different comparisons.

Actual mechanical advantage is measured output force divided by measured effort force. A high ratio cannot by itself establish high efficiency, faster work or suitability for a particular job.

A has MA 4, greater than B’s MA 3. B has output 120 N, larger than A’s 100 N. These are different comparisons.

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 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.

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