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IB DemystifiedMYP Sciences

Newton's laws of motion

Why do passengers lurch forwards when a bus brakes? How does a rocket push itself into space? Why does an empty trolley speed up more easily than a full one? More than 300 years ago, Isaac Newton described three simple laws that answer all these questions and still guide the design of every car, aircraft and spacecraft.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

1000 kgdriving force 3000 Nresistance 1000 Nweight 10 000 Nsupport 10 000 NHorizontal arrows drawn at a larger scale than vertical arrows.
Figure 1. A free-body diagram shows every force acting on one object.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Newton's three laws
  6. Newton's laws in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • apply Newton's first law: balanced forces and inertia
  • apply Newton's second law: F = ma
  • identify Newton's third law force pairs
  • calculate weight and resultant forces
  • analyse force and acceleration data and design experiments
  • discuss vehicle safety using Newton's laws

Before you start

You will use these skills. If any feel shaky, review them first.

  • forces, balanced and unbalanced forces (see Forces and their effects)
  • speed and acceleration (see Acceleration)
  • rearranging simple equations

Key vocabulary

Resultant force
The single force that has the same effect as all the forces on an object combined.
Inertia
The tendency of an object to keep doing what it is doing: staying still or moving at constant velocity.
Acceleration
The rate of change of velocity, in m/s².
Newton (N)
The force needed to accelerate 1 kg at 1 m/s².
Weight
The force of gravity on an object: W = m × g.
Terminal velocity
The steady speed reached when resistive forces balance the driving force or weight.

Understanding the ideas

  1. The first law

    Newton's first law says an object stays at rest or keeps moving at a constant velocity unless a resultant force acts on it. This tendency to keep doing the same thing is called inertia, and it is why seatbelts are needed.

  2. The second law

    Newton's second law says the resultant force on an object equals its mass times its acceleration: F = ma. A bigger force gives a bigger acceleration; a bigger mass gives a smaller acceleration for the same force.

  3. The third law

    Newton's third law says that when object A pushes or pulls object B, B pushes or pulls A with an equal force in the opposite direction. These force pairs act on different objects, which is how rockets, swimmers and walkers move.

  4. Why does it matter?

    Engineers use these laws to calculate braking forces, design safety features such as seatbelts and crumple zones, and launch spacecraft.

  5. What does it connect to?

    Newton's laws link to speed and acceleration, momentum, pressure and energy in physics, and later to gravity, orbits and circular motion.

Newton's three laws

  • First law: zero resultant force → no change in motion (stays still or constant velocity).
  • Second law: resultant force = mass × acceleration (F = ma).
  • Third law: forces come in equal and opposite pairs acting on different objects.
  • Weight: W = m × g (g = 10 N/kg on Earth).
  • Remember: always use the resultant force in F = ma.

Newton's laws in the real world

Seatbelts, airbags and child car seats protect people because of inertia. Truck brakes must provide huge forces because of their large mass. Rockets launched from spaceports around the world rely on the third law.

Worked examples

Example 1: using F = ma

A 40 kg box is pushed with 100 N; friction is 20 N. Find its acceleration.

  1. Resultant force = 100 − 20 = 80 N.
  2. a = F ÷ m = 80 ÷ 40 = 2 m/s².

Example 2: third law pairs

A person pushes a wall with 50 N. What does the wall do?

  1. The wall pushes back on the person with 50 N.
  2. The two forces act on different objects, so they do not cancel out.

Assessment tips

Questions on Newton's laws often use free-body diagrams, calculations and everyday situations. Expect to:

  • Calculate resultant forces, accelerations, masses and weights, with units.
  • Explain motion using the first and second laws.
  • Identify third law pairs correctly.
  • Analyse trolley data and design fair experiments.

Common mistakes: using the driving force instead of the resultant force in F = ma; thinking a moving object needs a resultant force to keep moving; calling two balanced forces on one object a third law pair; and forgetting units (N, kg, m/s²).

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: falling with a parachute

Partially guided investigation · about 45 minutes · groups

Research question
How does the mass hanging from a paper parachute affect the time it takes to fall 2 m?
Scientific background
A falling parachute speeds up until air resistance balances its weight; then it falls at terminal velocity. A heavier load needs a larger air resistance, so it falls faster.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Paper or plastic-bag parachute with strings, paper clips or small masses as the load, metre rule, stopwatch, balance.
Method
  1. Weigh the load and attach it to the parachute.
  2. Drop the parachute from a fixed height of 2 m without pushing it, and time the fall.
  3. Repeat three times and find the mean time.
  4. Add more paper clips and repeat for at least five different loads.

Safety. Drop from a safe, stable position (an adult should hold any step or stool) and keep the area below clear.

Then evaluate: how could light gates or a video make the timing more accurate?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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