MYP Physics

F.3 Newton’s Laws & Momentum

Applying Newton's three laws, exploring inertia, action-reaction pairs, and the conservation of momentum in collisions.

Questions to explore

? Why does a moving object keep moving unless something stops it?
? How do crumple zones and airbags use physics to save lives?

💡 Key ideas, explained simply

1️⃣ The three laws in plain English

1st law (inertia): an object keeps doing what it is doing — still or moving steadily — until a resultant force changes it.

2nd law: a resultant force makes an object accelerate, $F = ma$ .

3rd law: every action has an equal and opposite reaction — the two forces act on different objects.

💥 Momentum is conserved

Momentum $p = mv$ measures "how hard something is to stop". In any collision or explosion with no outside forces, the total momentum before equals the total momentum after.

This lets you predict speeds after a crash even without knowing the messy forces during the impact.

📖 Key terms

Inertia: The tendency of an object to resist a change in its motion; more mass = more inertia.
Momentum: Mass × velocity, $p = mv$ , in kg·m/s — a vector.
Impulse: Force × time = change in momentum, $F\Delta t = \Delta p$ .
Elastic collision: Objects bounce apart; kinetic energy is conserved.
Inelastic collision: Objects stick together; kinetic energy is not conserved.
Action–reaction pair: Two equal, opposite forces acting on two different objects.

1 Key Formulas

Momentum

p = mv

Impulse

F \Delta t = \Delta p

Conservation of Momentum

m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2

✏️ Worked example

Conservation of momentum

A $2\text{ kg}$ trolley moving at $3\text{ m/s}$ hits a stationary $1\text{ kg}$ trolley and they stick together. Find their combined velocity.

1

Momentum before

$p = mv = 2 \times 3 + 1 \times 0 = 6\text{ kg·m/s}$
2

Momentum after

They move together: total mass $= 3\text{ kg}$ , so $p = 3v$ .
3

Conserve momentum

$6 = 3v$ , so $v = \dfrac{6}{3}$

✓

v = 2 m/s — momentum is conserved even though this collision is inelastic.

🚗 Physics around you

Airbags and crumple zones use impulse. The change in momentum in a crash is fixed, but stretching the impact time over a longer moment makes the force ( $F = \Delta p / \Delta t$ ) much smaller — and a smaller force means fewer injuries.

🎯 Nail it in the exam

Momentum in Collisions – Past Paper Style

Momentum conservation is a favourite for ‘explain’ and ‘calculate’ questions. In any closed system, total momentum before = total momentum after.

Elastic collisions: objects bounce apart; kinetic energy is conserved.

Inelastic collisions: objects stick together; kinetic energy is not conserved (often transformed into heat/sound).

A typical 6‑mark question: “Use the principle of conservation of momentum to calculate the velocity of the combined cars after the collision.” Always write the full equation, substitute values, and solve.

Pro Exam Strategy

Always define the positive direction before substituting velocities.
If an object bounces backwards, its final velocity is negative.
Units for momentum are kg·m/s – examiners check them.
For inelastic collisions, mention that kinetic energy is not conserved, but momentum always is.

Impulse and Safety Features

Impulse = force × time = change in momentum. Crumple zones, airbags, and seat belts increase the time of impact, reducing the average force on the occupants.

MYP often links physics to real‑world applications (Criteria D: Reflecting on the impacts of science). Be ready to explain why an egg dropped onto a pillow doesn’t break, while one dropped onto concrete does – the pillow increases stopping time, reducing force.

Pro Exam Strategy

In graph questions, impulse is the area under a force–time graph.
When asked to ‘suggest’ or ‘explain’, always connect the science to the safety outcome.

🧠 Check your understanding

Tap an answer to see if you're right — and why.

Q1. A 2 kg ball moving at 3 m/s collides with a stationary 1 kg ball. If they stick together, what is their combined velocity?

$1\, m/s$
$2\, m/s$
$3\, m/s$
$6\, m/s$

Q2. Which law explains why a passenger lurches forward when a bus brakes suddenly?

Newton's First Law
Newton's Second Law
Newton's Third Law
Hooke's Law

📝 Exam-style questions

Try each one, then reveal the model answer.