Collisions, Deformation & Changes in Motion — Practice · Grade 8 Science

Chapter: Motion

What this practice covers

By the end of Collisions, Deformation & Changes in Motion you should be able to:

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

Vocabulary used: collision, deformation, elastic deformation, permanent deformation, peak force, model, evidence, system boundary, assumption.

Problem 1

Why is an afterward photograph insufficient to detect temporary deformation?
Answer and explanation

An object can deform during contact and then recover. The after-image does not show the complete time sequence.

An object can deform during contact and then recover. The after-image does not show the complete time sequence.

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 greater acceleration of the lighter cart imply a larger collision force on it?
Answer and explanation

No. Equal interaction forces can cause different acceleration because masses differ, and net-force calculations must account for other forces.

No. Equal interaction forces can cause different acceleration because masses differ, and net-force calculations must account for other forces.

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

Can speed alone predict every dent?
Answer and explanation

No. It influences motion energy, but material response, geometry and how the object stops also affect damage.

No. It influences motion energy, but material response, geometry and how the object stops also affect damage.

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

What do these peak records support?
Answer and explanation

Lower measured peak force for this padded setup under the stated matched conditions. They do not certify every pad or establish identical average forces or injury protection.

Lower measured peak force for this padded setup under the stated matched conditions. They do not certify every pad or establish identical average forces or injury 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 5

Illustrative matched-cart tests show bumper thickness 15 mm before, 8 mm during and 15 mm afterward; peak force is 12 N rather than 24 N for a rigid stop. Explain the temporary deformation and compare peak values without declaring all collisions safe.
Answer and explanation

The bumper undergoes elastic deformation in this simplified record because it regains its thickness. The padded peak is half the rigid-stop peak under matched conditions. Deformation can allow a longer stopping interval, but the readings do not prove that every object is protected or give the entire force–time history. Energy is redistributed rather than erased.

The bumper undergoes elastic deformation in this simplified record because it regains its thickness. The padded peak is half the rigid-stop peak under matched conditions. Deformation can allow a longer stopping interval, but the readings do not prove that every object is protected or give the entire force–time history. Energy is redistributed rather than erased.

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

Two collisions have the same initial kinetic energy but different damage. Is energy conservation violated?
Answer and explanation

No. Different materials and stopping conditions can distribute the energy differently. Conservation does not require the same peak force, deformation or damage.

No. Different materials and stopping conditions can distribute the energy differently. Conservation does not require the same peak force, deformation or damage.

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: A recovered bumper never changed shape.
Answer and explanation

It may have changed temporarily during contact; use a before–during–after record.

It may have changed temporarily during contact; use a before–during–after record.

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

Illustrative tests for matched carts at the same speed show peak readings of 18, 19, 20 N without padding and 9, 10, 11 N with padding. Check repeated trials, measuring method and pad thickness before generalizing to other objects. Lower measured peak force for this padded setup under the stated matched conditions. They do not certify every pad or establish identical average forces or injury protection.

Illustrative tests for matched carts at the same speed show peak readings of 18, 19, 20 N without padding and 9, 10, 11 N with padding. Check repeated trials, measuring method and pad thickness before generalizing to other objects. Lower measured peak force for this padded setup under the stated matched conditions. They do not certify every pad or establish identical average forces or injury 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 9

A bumper is 12 mm thick before contact, 7 mm during and 11 mm after. Explain what the record supports about temporary and remaining deformation and what it cannot establish about force.
Hints

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

Answer and explanation

It compresses 5 mm temporarily and retains a 1 mm thickness change after the recorded interaction. It does not fully recover in that record. Thickness alone does not establish a peak force or permanent behavior over all later times; a suitable force/time record and longer observation are needed.

It compresses 5 mm temporarily and retains a 1 mm thickness change after the recorded interaction. It does not fully recover in that record. Thickness alone does not establish a peak force or permanent behavior over all later times; a suitable force/time record and longer observation are needed.

Problem 10

Two collisions have the same initial kinetic energy but different damage. Is energy conservation violated? Explain a condition that would change this conclusion.
Hints

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

Answer and explanation

No. Different materials and stopping conditions can distribute the energy differently. Conservation does not require the same peak force, deformation or damage.

No. Different materials and stopping conditions can distribute the energy differently. Conservation does not require the same peak force, deformation or damage.

Problem 11

Separate motion before contact from forces during contactSeparate motion before contact from forces during contactBefore contactA: 1 kgB: 1 kgAt restDuring contactA: 1 kgB: 1 kgAfter ideal elastic collisionA: 1 kgB: 1 kgAt restRed: paired contact forces
These equal-mass carts illustrate an ideal one-dimensional elastic collision: the initially moving cart can stop while the initially stationary cart moves. Interaction-force arrows apply during contact, while velocity arrows describe motion before or after. The snapshots omit detailed deformation and other forces and do not claim that all real collisions exchange velocities or conserve mechanical kinetic energy.
For this unit’s illustrated case, why should the contact-force arrows not be carried into the separated after-collision snapshot?
Answer and explanation

The contact interaction has ended once the carts separate. A remaining velocity is not evidence of a continuing contact push; other real interactions must be named separately.

A timeline is important because different interactions exist at different stages. An elastic collision is an ideal case where the system’s kinetic energy is retained after separation. A damaged or thermally dissipative collision can have a different outcome.

The contact interaction has ended once the carts separate. A remaining velocity is not evidence of a continuing contact push; other real interactions must be named separately.

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

Two carts receive equal opposite 12 N contact forces in a stated horizontal interval. Their masses are 3 and 1.5 kg. Compare accelerations and explain whether equal force means equal velocity change over the same duration.
Answer and explanation

Accelerations are 4 and 8 m/s² in opposite directions. Over the same duration the lighter cart’s velocity change has twice the magnitude, despite the equal force pair.

Use each receiver’s mass in Fnet/m under the supplied net-horizontal-force assumption.

Multiply acceleration by the common interval to compare velocity changes.

Accelerations are 4 and 8 m/s² in opposite directions. Over the same duration the lighter cart’s velocity change has twice the magnitude, despite the equal force pair.

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.

Browse all courses