Designing & Revising a Collision Protector — Practice · Grade 8 Science

Chapter: Motion

What this practice covers

By the end of Designing & Revising a Collision Protector you should be able to:

12 questions, written answer, each with a worked explanation.

Vocabulary used: criterion, constraint, stakeholder, operational definition, iteration, model, evidence, system boundary, assumption.

Problem 1

Why define the dent measure before seeing results?
Answer and explanation

A fixed rule allows consistent judgments and avoids changing success criteria to favor a preferred design.

A fixed rule allows consistent judgments and avoids changing success criteria to favor a preferred design.

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

Does a larger contact face automatically mean a smaller total force?
Answer and explanation

No. It can lower pressure for the same force by spreading contact area, but total force depends on the stopping interaction.

No. It can lower pressure for the same force by spreading contact area, but total force depends on the stopping interaction.

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

Why is B’s single 10 N reading an incomplete comparison?
Answer and explanation

It omits its other readings and variability. Evaluate all planned trials along with constraints and damage observations.

It omits its other readings and variability. Evaluate all planned trials along with constraints and damage observations.

Response criteria

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

Problem 4

Does combining useful features prove the new protector is better?
Answer and explanation

No. Features can interact and violate limits. The revised design requires new tests under the same evaluation rules.

No. Features can interact and violate limits. The revised design requires new tests under the same evaluation rules.

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

Illustrative designs have A: mean peak 13 N, thickness 4 cm; B: mean peak 9 N, thickness 7 cm. Maximum permitted thickness is 5 cm. Choose a qualifying starting design and propose an evidence-based revision.
Answer and explanation

A qualifies under the thickness rule; B does not despite its lower mean peak. Start with A and investigate whether a more compressible arrangement within 5 cm lowers the peak. Repeat matched tests, record all results and evaluate permanent deformation as well as force. Lower mean peak alone is not universal protection.

A qualifies under the thickness rule; B does not despite its lower mean peak. Start with A and investigate whether a more compressible arrangement within 5 cm lowers the peak. Repeat matched tests, record all results and evaluate permanent deformation as well as force. Lower mean peak alone is not universal protection.

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

A revised case protects one trial but fails two. Can its successful trial justify calling it reliable?
Answer and explanation

No. All planned trials matter. Report the failures, investigate their causes, and revise the design or the limited claim rather than selecting only the success.

No. All planned trials matter. Report the failures, investigate their causes, and revise the design or the limited claim rather than selecting only the success.

Response criteria

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

Problem 7

Evaluate and correct: The design with the lowest one-trial reading is always best.
Answer and explanation

Judge repeated evidence, variability, damage measures and hard constraints, rather than one selected reading.

Judge repeated evidence, variability, damage measures and hard constraints, rather than one selected reading.

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

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

Combine A’s stable arrangement with B’s more compressible layer if the size and mass limits allow it. Predict reduced typical peak while retaining consistency, then collect new data. The combination is a proposal until tested. No. Features can interact and violate limits. The revised design requires new tests under the same evaluation rules.

Combine A’s stable arrangement with B’s more compressible layer if the size and mass limits allow it. Predict reduced typical peak while retaining consistency, then collect new data. The combination is a proposal until tested. No. Features can interact and violate limits. The revised design requires new tests under the same evaluation rules.

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

Protector C gives repeated peaks 11, 17 and 14 N within the allowed thickness. Calculate mean and range and explain why the single lowest trial is insufficient.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

Mean is 14 N and range 6 N. The 11 N minimum omits the other trials and understates variability. Compare all planned results with criteria, constraints and damage observations before choosing or revising.

Mean is 14 N and range 6 N. The 11 N minimum omits the other trials and understates variability. Compare all planned results with criteria, constraints and damage observations before choosing or revising.

Problem 10

A revised case protects one trial but fails two. Can its successful trial justify calling it reliable? Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

No. All planned trials matter. Report the failures, investigate their causes, and revise the design or the limited claim rather than selecting only the success.

No. All planned trials matter. Report the failures, investigate their causes, and revise the design or the limited claim rather than selecting only the success.

Problem 11

A cushioning feature needs a mechanism and a testA cushioning feature needs a mechanism and a testFlat pad: little travelRings: room to compressSame test loadA predicted mechanism: geometry changes deformation and stopping travel.A drawing predicts a feature; repeated measurements decide whether it helps.
The comparison proposes that a compressible ring arrangement provides more deformation travel than a very thin pad under a matched test load. Geometry is shown schematically, and no measured force reduction is supplied by this drawing. A protector must still be checked for stability, maximum compression, size and repeat performance; adding material without a mechanism is not enough.
For this unit’s illustrated case, which measurement could distinguish “more travel” from “a prettier design”?
Answer and explanation

Measure deformation or stopping distance/time under matched loads and then evaluate peak-force/damage criteria using an appropriate safe test or supplied records.

A mechanism explains how the feature changes the interaction. OpenSciEd’s ring sketches below give concrete shapes to compare, but an unloaded drawing cannot establish their performance. Predict, test, revise and retest.

Measure deformation or stopping distance/time under matched loads and then evaluate peak-force/damage criteria using an appropriate safe test or supplied records.

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 12

A design must stay below 30 N in every trial. Its supplied results are 22, 24 and 35 N. Does a mean of 27 N establish that it meets the criterion? Explain a proper next step.
Answer and explanation

No. The 35 N trial fails the every-trial limit even though the mean is 27 N. Preserve the result, investigate variation, revise a mechanism and retest under matched conditions.

Read the criterion as every trial, not the mean.

Use all records before recommending a tested revision.

No. The 35 N trial fails the every-trial limit even though the mean is 27 N. Preserve the result, investigate variation, revise a mechanism and retest under matched conditions.

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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