Impulse & Collision Evidence — Practice · Grade 12 Science

Chapter: Work, Energy & Momentum

What this practice covers

By the end of Impulse & Collision Evidence you should be able to:

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

Vocabulary used: impulse, external impulse.

Problem 1

A 3 kg object slows from 5 m/s to 0. Find signed impulse.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

-15 N·s

3(0−5)=−15.

Problem 2

Does padding conserve an object’s momentum?
Answer and explanation

No; it changes momentum over a longer time and can reduce average force.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 3

A 2 kg object changes velocity −3 to+1 m/s. Find signed impulse.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

8 N·s

2(1−(−3))=8.

Problem 4

Why define the system for collision conservation?
Answer and explanation

External forces can transfer momentum into or out of the selected system.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 5

An object has momentum change 12 N·s over 0.30 s. Find average net force magnitude. Show the reasoning, not just the conclusion.
Hints

A reversal requires signed velocities, not just a speed difference.

Answer and explanation

12/0.30=40 N.

Problem 6

Why can padding reduce average collision force without changing final momentum?
Hints

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

Answer and explanation

It extends the time over which the same impulse is delivered; the force–time area remains the momentum change.

Problem 7

A longer stop can reduce average force for a fixed changeA longer stop can reduce average force for a fixed changeForce magnitude (N)010020030000.010.020.030.04Time (s)Short pulse:3 N·sLong pulse:3 N·s
Calculated equal-impulse pulses, not medical or crash-test evidence.
Interpret the supplied visual. Why must rebound be compared when testing two cushions?
Answer and explanation

Rebound changes final velocity and therefore Δp, so the “same momentum change” assumption may no longer hold.

For a fixed momentum change, increasing the stopping interval reduces average net force because Favg=Δp/Δt. The two calculated triangles have equal 3 N·s area but different durations and peaks. Real cushioning can also change rebound, contact geometry and pulse shape, so a longer interval alone does not guarantee a specific peak or injury outcome. Compare actual force histories under controlled conditions rather than treating a simple average-force model as a complete safety prediction.

Rebound changes final velocity and therefore Δp, so the “same momentum change” assumption may no longer hold.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

A 0.50 kg object slows from 6 m/s to 0 over 0.15 s. Find impulse and average net force. If it rebounds at−2 m/s instead, find the new impulse.
Answer and explanation

Stopping impulse=0.50(0−6)=−3 N·s and Favg=−20 N. With rebound, impulse=0.50(−2−6)=−4 N·s, so the fixed-impulse comparison no longer applies.

Stopping impulse=0.50(0−6)=−3 N·s and Favg=−20 N.

With rebound, impulse=0.50(−2−6)=−4 N·s, so the fixed-impulse comparison no longer applies.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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