Physical Science Enhancement — Physics 4 — Forces, Momentum & Impulse

Introduction

Why do moving objects keep going, and what makes them stop? In this unit you will explore Newton’s laws to explain motion and forces, then build towards real-world applications involving momentum and impulse. You’ll analyse why safety features like seatbelts and crumple zones work, and apply these ideas to sport and collisions.

PS4.1 — Introduction to Force & Motion

Objectives

Review prior knowledge of motion (speed, velocity, acceleration)
Define force, mass, and acceleration
Introduce Newton’s First and Second Laws in simple terms

Starter (discussion)

When you kick a football, why does it eventually stop? Elicit ideas about friction and air resistance acting against motion.

Demonstration (teacher-led)

Push a trolley with no added mass, then with extra weights on top. Students observe that the heavier trolley is harder to start and stop. Use this to introduce inertia (resistance to changes in motion).

Key ideas

Force: a push or pull that can change motion
Mass: how much matter an object contains
Acceleration: the rate of change of velocity

Newton’s First Law: an object remains at rest or in uniform motion unless acted on by a resultant (unbalanced) force.

Newton’s Second Law (in words): greater force gives greater acceleration; greater mass gives smaller acceleration for the same force.

Activity

Classify the forces acting on different objects and decide whether they are balanced or unbalanced:

parked car
falling apple
runner at constant speed
satellite in orbit

Plenary

Each group shares one scenario and explains which forces act and whether they are balanced. Preview: next lesson focuses on inertia and everyday examples (seatbelts, crockery on tablecloths, etc.).

Summary

Forces cause changes in motion; without a resultant force, motion stays the same.
Mass affects how difficult it is to change motion (inertia).
Greater force → greater acceleration; greater mass → smaller acceleration for the same force.

Check your understanding

Why does a football eventually stop after being kicked?
In your own words, what is inertia?
A heavy and a light trolley are pushed with the same force. Which accelerates more? Explain.

PS4.2 — Newton’s First Law: Inertia

Statement of Newton's first law of motion

Every object continues at rest or in a state of constant velocity providing no resultant external force acts upon it

It's magic - Watch the video.

Why do the plates remain in place?

Activity

Perform the coin-on-card trick:

Place a card on top of a glass and balance a coin on the card.

Flick the card quickly — can you get the the coin into the glass.

Why does the coin not fly off with the card?

Can you think of any other tricks that use the same principle?

Activity

Worksheet: Match scenarios (e.g. car crash, trolley on slope, ball rolling on grass) to Newton’s First Law explanations.

Finish with a think–pair–share: “What does inertia depend on?”

Summary

Newton’s First Law states that motion does not change without a resultant force.
Inertia is resistance to changes in motion.
Inertia depends on the mass of the object.

Check your understanding

State Newton’s First Law.
Why does a coin fall into the glass when the card is flicked away?
Does a heavier object have more or less inertia? Explain.

PS4.3 — Newton’s Second Law: F = ma

Objectives

Understand the relationship between force, mass, and acceleration
Introduce and use the equation F = ma
Rearrange the equation to solve for force, mass, or acceleration

Starter

Pose the question: “Why is it harder to push a heavy trolley than a light one if you want the same acceleration?”

Explanation

Introduce the equation F = ma. - A larger force produces a larger acceleration. - A larger mass requires a larger force to achieve the same acceleration. Demonstrate rearranging the formula using the triangle method.

Worked examples

A force of 20 N acts on a 5 kg trolley. What is the acceleration?
A car of mass 1000 kg accelerates at 2 m/s². What is the net force?

Student task

Students complete short calculations on a worksheet: finding F, m, or a from given values. Encourage showing full working and correct units.

Plenary

Quick quiz: teacher calls out two numbers (e.g. force and mass), students respond with the third variable. Use whiteboards or quick-fire answers to check understanding.

Summary

Newton’s Second Law: F = ma.
Force, mass, and acceleration are directly related.
Formula can be rearranged to calculate any one of the three quantities.

Check your understanding

A 10 N force acts on a 2 kg object. What is its acceleration?
What net force is needed to accelerate a 1200 kg car at 3 m/s²?
If a trolley accelerates at 0.5 m/s² when a 5 N force is applied, what is its mass?

PS4.4 — Required Practical: Acceleration

Objectives

Investigate how force and mass affect acceleration
Collect and record data systematically
Interpret results to test Newton’s Second Law

Starter

Recap: F = ma. Ask: “If I double the force on a trolley, what should happen to its acceleration? What if I double the mass?”

Practical setup

Trolley or dynamics cart
Runway or smooth surface
Light gates and timer (or stopwatch if not available)
Mass hanger and slotted masses (force varied by adding weights)

Students work in small groups. First vary the force (by adding weights to hanger), then vary the mass of the trolley while keeping total force constant.

Method

Release trolley from rest, record acceleration using light gates/timer.
Repeat with different forces, keeping mass constant.
Repeat with different masses, keeping force constant.
Record results in a table.

Analysis

Students plot graphs: - Force (N) vs acceleration (m/s²) - Mass (kg) vs acceleration (m/s²) Discuss how results show proportionality of F and a, and inverse relationship with m.

Plenary

Class discussion: “Do your results support F = ma? What sources of error might affect the experiment?”

Summary

Acceleration is directly proportional to force.
Acceleration is inversely proportional to mass.
Experimental results support Newton’s Second Law: F = ma.

Check your understanding

What happens to acceleration if the force on a trolley is doubled?
What happens to acceleration if the mass is doubled but force stays the same?
Why does plotting force vs acceleration give a straight line through the origin?

PS4.5 — Momentum & Impulse: Definitions

Objectives

Define momentum as mass × velocity
Define impulse as force × time
Recognise that impulse equals change in momentum

Starter

Ask: “Which would be harder to stop — a moving bicycle or a moving car, both travelling at the same speed? Why?”

Explanation

Momentum = mass × velocity (kg·m/s)
Impulse = force × time (N·s)
Impulse causes a change in momentum (Δp)

Example: catching a ball. A larger mass or higher speed means greater momentum, requiring a greater impulse to stop it.

Worked examples

A 2 kg ball moving at 5 m/s has momentum of 10 kg·m/s.
A 1000 kg car moving at 20 m/s has momentum of 20,000 kg·m/s.
If a force of 50 N acts for 2 s, impulse = 100 N·s = change in momentum.

Demonstration

Perform a quick egg-drop demo (into a soft cloth vs a hard surface) or catch a ball gently vs abruptly. Discuss how increasing the time reduces the force, even though momentum change is the same.

Student task

Worksheet: Calculate momentum before and after collisions; calculate impulse from force × time; link to change in momentum.

Plenary

Class recap: momentum depends on mass and velocity; impulse is force × time; impulse always equals change in momentum.

Summary

Momentum = mass × velocity.
Impulse = force × time.
Impulse equals the change in momentum.

Check your understanding

Calculate the momentum of a 0.15 kg ball moving at 12 m/s.
A force of 200 N acts for 0.5 s. What is the impulse?
Why does catching a ball over a longer time reduce the force on your hands?

PS4.6 — Impulse in Real Life (Safety)

Objectives

Understand how impulse explains the design of safety features
Relate impulse to reducing force by increasing collision time
Apply the concept of impulse to real-life contexts

Starter

Show a short video clip of a car crash test. Ask: “Why don’t modern cars feel as ‘solid’ as older cars, yet are much safer?”

Explanation

Impulse = force × time = change in momentum. If the change in momentum is fixed (e.g. car going from 20 m/s to 0), the force can be reduced by increasing the time over which it acts.

Seatbelts stretch slightly
Airbags cushion impact
Crumple zones deform to extend collision time
Helmets use foam to absorb energy and lengthen impact time

Group task

Students work in groups to match a list of safety devices (seatbelt, helmet, airbag, crumple zone, parachute) with an explanation of how each increases impact time and reduces force. Share findings with the class.

Calculation practice

Example: A 70 kg person in a car at 15 m/s comes to rest. Change in momentum = 70 × 15 = 1050 kg·m/s. If stopped in 0.2 s, force = 5250 N. If stopped in 0.5 s, force = 2100 N. Longer time = smaller force.

Plenary

Class discussion: “Why do airbags not only save lives but also reduce injuries to the chest and head?” Summarise: impulse stays the same, but time is extended, lowering the force.

Summary

Impulse = change in momentum.
Extending collision time reduces the force experienced.
Safety features like airbags and helmets work by increasing impact time.

Check your understanding

Why does an airbag reduce the force on a driver in a crash?
How does a helmet protect a cyclist during an accident?
A 0.5 kg ball moving at 10 m/s is stopped. What impulse is required? If the impact lasts 0.25 s, what is the average force?

PS4.7 — Newton’s Laws in Sports

Objectives

Apply Newton’s three laws to real sporting examples
Explain how momentum and impulse affect performance and safety in sport
Analyse movements using force, mass, acceleration, and reaction forces

Group task

In groups, students choose one sport (football, rugby, athletics, baseball, basketball) and identify examples of all three laws in action.

Prepare a short presentation (3 slides) to the class.

Example calculation problems

- A 0.45 kg football is kicked with a force of 90 N.
What is its acceleration?
- A rugby player of mass 80 kg moving at 6 m/s collides and stops in 0.5 s.
What is the average force on the player?

Plenary

Discuss: “How do Newton’s laws help coaches and athletes improve performance and reduce injury?”

Summary

All three of Newton’s laws can be observed in sport.
Momentum and impulse explain forces in tackles, kicks, and catches.
Understanding these concepts helps improve performance and safety.

Check your understanding

Which law explains why a ball keeps rolling until friction slows it down?
How does Newton’s Third Law apply when a sprinter pushes against the ground?
A 70 kg basketball player jumps by pushing down with a force of 2100 N. What is their upward acceleration?

PS4.8 — Velocity/time graphs

Objectives

Interpret velocity–time graphs to find acceleration and momentum change
Interpret force–time graphs to calculate impulse
Relate graph shapes to Newton’s Laws and collision scenarios

Starter

Show a simple velocity–time graph (constant acceleration). Ask: “What does the slope represent?” → acceleration. “What does the area under the line represent?” → change in displacement.

Explanation

Velocity–time graphs: slope = acceleration; area under graph = displacement.
Force–time graphs: area under graph = impulse = change in momentum.

Discuss how real collisions produce force–time graphs with peaks rather than constant forces.

Activity

Provide students with sample graphs:

- A car braking (v–t graph, deceleration).
- A ball bouncing (F–t graph with sharp peak).
- A crumple zone (F–t graph with a longer, lower peak).

Students interpret what the graphs show about motion, impulse, and safety design.

Worksheet practice

Students calculate impulse by finding the area under force–time graphs. Include one example where the impulse is split into two triangles (build skill in calculating areas).

Plenary

Class discussion: “Why do real force–time graphs help engineers design safer cars and sports equipment?” Summarise key takeaways.

Summary

Velocity–time graphs show acceleration (slope) and displacement (area).
Force–time graphs show impulse as the area under the curve.
Impulse from F–t graphs equals change in momentum in collisions.

Check your understanding

What does the slope of a velocity–time graph represent?
What does the area under a force–time graph represent?
A force–time graph shows a triangular peak with base 0.2 s and height 500 N. What impulse is delivered?

PS4.9 — Review and Application

Objectives

Consolidate understanding of Newton’s Laws, momentum, and impulse
Identify and correct common misconceptions
Apply knowledge to mixed questions in preparation for assessment

Starter

Quick-fire Q&A on whiteboards: teacher calls out terms (force, inertia, momentum, impulse), students write the definition or equation.

Quiz game

Class split into teams. Use a quiz format (Kahoot, cards, or simple scoreboard) with a mix of conceptual and calculation questions: - “Which law explains why you move forward in a braking bus?” - “Calculate the momentum of a 1500 kg car at 20 m/s.” - “Why do airbags reduce force in collisions?”

Practice questions

Students attempt exam-style questions combining topics: - Using F = ma with numerical values - Interpreting a force–time graph - Explaining safety features using impulse - Applying Newton’s Third Law to everyday examples

Plenary

Class discussion: “Which concept do you find easiest? Which is still tricky?” Teacher addresses misconceptions, especially: - First Law misinterpreted as “objects need force to keep moving” - Confusion between momentum and inertia - Forgetting that impulse = change in momentum, not just force × time without context

Summary

Newton’s Laws, momentum, and impulse link together to explain forces and motion.
Safety features and sports applications are explained by increasing collision time to reduce force.
Graph interpretation and calculations are key skills for assessment.

Check your understanding

What is the difference between inertia and momentum?
How does Newton’s Third Law apply when a swimmer pushes off the wall?
A 2 kg trolley is accelerated at 1.5 m/s². What is the force acting on it?

Now test yourself

Click on the button below to access the self-tests for MYP9 and MYP10.

MYP Self-test