DSE Physics | Momentum and Collisions Tutor Prep Guide

DSE Physics | Momentum and Collisions Tutor Prep Guide | Conservation and Impulse

2026-11-01

Tutor lesson-prep guides · Secondary Physics

As soon as a collision appears, students reach for energy conservation and write that kinetic energy is unchanged, so the answer is wrong. What they have not separated is this: momentum is conserved in every collision, kinetic energy only in elastic ones. One example, two laws, kept apart on the page.

Objectives — Students can: (1) write the before-and-after momentum equation and solve for the final velocity; (2) use impulse Ft = delta p to find average force; (3) decide whether a collision is elastic and compute the kinetic energy lost; (4) handle one-dimensional motion in opposite directions with signs.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Vectors and signs	right is positive	reverse motion takes a minus sign
Newton second law	F = ma	impulse follows from it
Kinetic energy	half m v squared	the squared term is the usual slip

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Define momentum	p = mv, unit kg m s-1	0.40 x 3.0 = 1.2
8-20 min	Conservation	before equals after	final speed 1.2 m/s
20-32 min	Impulse	Ft = delta p	average force 14 N
32-44 min	Energy check	compare half m v squared	lost 1.08 J
44-54 min	Everyday use	airbags, soft mats	longer time, smaller force
54-60 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: First move: draw two frames and write only momentum

Before: 0.40 kg at 3.0 m/s to the right carries 1.20 kg m s-1; 0.60 kg is at rest, momentum zero. Total 1.2 kg m s-1. After: they stick together, mass 1.00 kg.
Conservation: (0.40 + 0.60) x v = 1.2, so v = 1.2 m/s. Ask the student: why can the masses simply be added after the collision? Student task: make the 0.60 kg block approach at 1.0 m/s instead and recompute.

Scene 2: Impulse: the same momentum change over a longer time is a smaller force

The 0.60 kg block goes from rest to 1.2 m/s, a change of 0.72 kg m s-1. With a contact time of 0.05 s the average force is 0.72 divided by 0.05, which is 14.4 N.
An airbag stretches the contact time to 0.50 s and the force falls to 1.44 N: same change, ten times less force. Ask the student: why bend the knees on landing? Student task: work out the average force when the contact time is 0.20 s.

Scene 3: Energy check: conservation of momentum is not conservation of energy

Kinetic energy before is half x 0.40 x 3.0 squared = 1.80 J; after it is half x 1.00 x 1.2 squared = 0.72 J. The 1.08 J lost is 60 per cent of the original, going into sound and heat.
A collision where the bodies stick is always inelastic; use energy conservation only when the question says elastic. Ask the student: where did the energy go? Student task: write one line saying when kinetic energy conservation may be used.

04 Question types and worked steps

Type	Example	Steps	Answer
Final speed	two masses and initial speeds	one momentum equation	1.2 m/s
Average force	contact time given	F = delta p divided by t	14 N
Elastic test	compare kinetic energy	equal only if elastic	60 per cent lost
Head-on collision	one speed negative	fix the positive direction	carry the minus sign

Sanity check (teach this): Total momentum must match: 1.2 against 1.2; a mismatch means a missed mass or sign. The common final speed must be below the incoming speed; above it means the masses were swapped. Kinetic energy can only fall in an inelastic collision, so a rise means a sign error. Momentum is quoted in kg m s-1 or N s, never in joules. Convert grams to kilograms before substituting.

05 Independent practice (3 questions)

Question	Answer	What to watch
Common speed after 0.40 kg hits 0.60 kg and sticks	1.2 m/s	conservation of momentum
Average force over a contact time of 0.05 s	14.4 N	F = delta p divided by t
Fraction of kinetic energy lost	60 per cent	subtract the two half m v squared values

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using energy conservation for the final speed	answers 3.0 m/s	Stick-together collisions do not conserve kinetic energy, only momentum.
Forgetting to add the masses	answers 2.0 m/s	After the collision they move as one body, so the masses add.
Quoting momentum in joules	answers 1.2 J	Momentum is kg m s-1; joules belong to energy.
No minus sign for the oncoming body	answers 3.0 m/s	An oncoming velocity is minus 1.0, not plus 1.0.

07 Homework (paper, section and question numbers)

Secondary Physics chapter index — questions 5 and 8 of the same type
Prep guide home — other chapters in the subject
DSE Physics practice papers — mechanics set, questions 7 to 12

08 Exit check (last 5 minutes)

Question	Answer	Decision
The two sides of the momentum equation	equal totals	three correct means move on to impulse
Kinetic energy in an inelastic collision	always falls	one wrong means reteach next lesson
Impulse and average force	F = delta p divided by t	two or more wrong means back to act one

09 Tutor FAQ

Why is momentum always conserved but kinetic energy not?

Momentum conservation follows from Newton third law, while kinetic energy can leave as heat and sound, so an inelastic collision loses it.

How do I solve an elastic collision quickly?

Use relative speed: separation speed equals approach speed, then combine with the momentum equation to solve two unknowns.

What about collisions in two dimensions?

Write one momentum equation for the horizontal direction and one for the vertical; do not try to force a single equation.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.