Year 11 · Science · Physical sciences
Motion and Newton's laws
Students calculate speed and acceleration, interpret motion graphs and apply Newton's three laws of motion to everyday situations such as car safety.
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These lessons were written around Australian Curriculum strands and matched to UK year groups by age (Year 1 is the year a child turns 6 in England). They are not mapped to the National Curriculum in England or the curricula of Scotland, Wales or Northern Ireland, so check them against your own plans.
This lesson uses some Australian examples (places, animals or contexts). Swap in local examples from the UK as you teach.
Learning goals
Learners will
- Calculate average speed and acceleration.
- State and apply Newton's three laws of motion.
- Use F = ma to solve simple problems.
Success looks like
- I can calculate speed from distance and time, and convert between m/s and km/h.
- I can calculate acceleration from a change in velocity.
- I can use Newton's laws to explain why seatbelts save lives.
The big idea
Speed = distance ÷ time. In science we usually use metres per second (m/s). To convert m/s to km/h, multiply by 3.6; to convert km/h to m/s, divide by 3.6.
Acceleration is how quickly velocity changes: a = (v − u) ÷ t, where u is the starting velocity, v the final velocity and t the time. A car going from 0 to 20 m/s in 5 s has an acceleration of 4 m/s².
Newton's first law: an object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it. This resistance to change is inertia.
Newton's second law: net force = mass × acceleration (F = ma). A bigger force produces a bigger acceleration, and a heavier object needs a bigger force for the same acceleration.
Newton's third law: when object A pushes on object B, B pushes back on A with an equal and opposite force. A swimmer pushes water backwards; the water pushes the swimmer forwards.
These laws explain road safety. In a crash, the car stops suddenly but your body keeps moving forward (first law). A seatbelt and airbag apply the stopping force over a longer time, reducing the force on your body. Australian speed limits are lower in school zones partly because stopping distance increases sharply with speed.
Worked examples
Calculating walking speed
You walk a 10 m track in 8 s. What is your average speed in m/s and km/h?
- speed = distance ÷ time = 10 ÷ 8 = 1.25 m/s.
- Convert to km/h: 1.25 × 3.6 = 4.5 km/h.
Answer: 1.25 m/s, which is 4.5 km/h.
Acceleration
A cyclist speeds up from 2 m/s to 8 m/s in 3 s. What is the acceleration?
- a = (v − u) ÷ t.
- a = (8 − 2) ÷ 3 = 6 ÷ 3.
- a = 2 m/s².
Answer: 2 m/s².
Using F = ma
What net force is needed to accelerate a 1200 kg car at 3 m/s²?
- F = m × a.
- F = 1200 × 3 = 3600 N.
Answer: 3600 N.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
A car travels 300 m in 15 s. What is its average speed?
20 m/s — 300 ÷ 15 = 20 m/s.
What is 72 km/h in m/s?
20 m/s — 72 ÷ 3.6 = 20 m/s.
Which of Newton's laws explains why passengers lurch forward when a bus brakes suddenly?
First law — Passengers' bodies keep moving due to inertia (first law).
A rocket pushes hot gas downwards. What pushes the rocket upwards?
The gas pushing back on the rocket — Newton's third law: the gas pushes on the rocket with an equal and opposite force.
A net force of 10 N acts on a 2 kg trolley. What is its acceleration?
5 m/s² — a = F ÷ m = 10 ÷ 2 = 5 m/s².
A runner covers 100 m in 12.5 s. What is the average speed in m/s? (number)
8 — 100 ÷ 12.5 = 8 m/s.
A car goes from 0 to 24 m/s in 6 s. What is its acceleration in m/s²? (number)
4 — a = (24 − 0) ÷ 6 = 4 m/s².
What is the unit of force? (one word)
newton — Force is measured in newtons (N).
Watch out for
- Many students think a moving object needs a constant force to keep moving. Without friction or other forces, it would keep moving forever (first law).
- Some think action–reaction forces cancel out. They act on different objects, so they do not cancel.
- Students often think heavier objects fall faster. Without air resistance, all objects fall with the same acceleration.
Tips for parents
- Measure the 10 m track carefully in a safe, flat outdoor area away from traffic.
- Encourage your child to write the formula, substitute values with units, then calculate: this habit prevents most errors.
- Safety: if using a skateboard, wear a helmet and test only on flat ground away from roads.
Go further
- Time family members walking, jogging and hopping 10 m and compare speeds in km/h.
- Roll a toy car down a ramp and time it over successive 50 cm sections to see it accelerate.
- Investigate how crumple zones in cars use the idea of increasing stopping time to reduce force.
About 60 minutes
You'll need
- A tape measure
- A stopwatch or phone timer
- Chalk or markers to mark a 10 m track outdoors
- A calculator
- A toy car or skateboard (optional)
Key words
- speed
- How far something travels per unit of time: speed = distance ÷ time.
- velocity
- Speed in a particular direction.
- acceleration
- The rate of change of velocity: a = (v − u) ÷ t, measured in m/s².
- inertia
- The tendency of an object to resist changes to its motion.
- net force
- The overall force on an object once all forces are combined.
- newton
- The unit of force (N). 1 N accelerates 1 kg at 1 m/s².
Still tricky?
For parents
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