KS3 Science · National Curriculum · Physics: Motion & Forces
Mini-Lesson
Motion & Forces
This mini-lesson walks you through the KS3 physics of motion and forces: how we measure speed, read a distance–time graph, describe relative motion, and how forces push, pull, balance, turn and press.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Describing motion
Speed tells you how fast
An object that is moving covers distance as time passes. Speed tells you how much distance it covers each second:
speed = distance ÷ timespeed (m/s) = distance (m) ÷ time (s)
Everyday speeds are often given in metres per second (m/s) or kilometres per hour (km/h). A brisk walk is about 1.5 m/s; a fast sprinter reaches about 10 m/s.
Average vs instantaneous: most journeys speed up and slow down, so the equation gives the average speed over the whole trip — not necessarily the speed at any single moment.
Worked example
Using speed = distance ÷ time
Worked example
A cyclist rides 300 m in 20 s. What is her average speed?
speed = distance ÷ time = 300 ÷ 20 = 15 m/s
Rearranging: the same triangle gives you the others — distance = speed × time, and time = distance ÷ speed. Cover the one you want to find.
d = distance, s = speed, t = time. Cover the quantity you want to find.Calculate
Your turn — average speed
1A car travels 400 m along a straight road in 25 s. Calculate its average speed.
m/s
Hint: speed = distance ÷ time = 400 ÷ 25.
Calculate
Your turn — rearranging
2A train travels at a steady 30 m/s for 12 s. How far does it travel in that time?
m
Hint: distance = speed × time = 30 × 12.
Distance–time graphs
Drawing a journey as a graph
A distance–time graph shows how far an object is from its start at every moment. Time goes across the bottom; distance goes up the side.
On a distance–time graph the gradient (steepness) is the speed. A steeper line means faster; a flat line means stopped.
Key idea: a straight sloping line means a steady (constant) speed. A horizontal line means the object is not moving — the distance isn't changing.
Quick check
Reading the graph
?On a distance–time graph, what does a flat, horizontal line tell you about the object?
Quick check
Which line is faster?
?Two objects are shown on the same distance–time graph. Object A's line is steeper than object B's. What does this tell you?
Relative motion
Speed depends on your point of view
How fast something seems to move depends on what you compare it to. This is relative motion.
Two trains going the same way pass each other slowly; going opposite ways they flash past.
To a passenger on a moving train, someone walking down the aisle seems slow — but to a person standing on the platform they are moving at almost train speed.
Quick check
Passing trains
?Two cars drive along a motorway in the same direction, one at 30 m/s and one at 25 m/s. How fast does the faster car move relative to the slower one?
What is a force?
Forces are pushes and pulls
A force is a push or a pull. A force always comes from an interaction between two objects — one object pushes or pulls another.
Your hand pushes a door — hand and door interact.
A magnet pulls a paperclip — magnet and clip interact.
The Earth pulls you down (gravity/weight) — you and the Earth interact.
Watch out: a single object on its own can't have a force "in" it. Forces always involve two objects interacting. We measure force in newtons (N) using a newtonmeter (a spring scale).
A newtonmeter stretches a spring; the bigger the force, the further the pointer moves.Contact & non-contact
Two families of force
Forces come in two families, depending on whether the objects must touch:
Contact forces — the objects touch: friction, air resistance, the push of a table (normal contact force), tension in a rope, applied pushes and pulls.
Non-contact forces — act at a distance, with no touching: gravity (weight), magnetism, electrostatic force.
Sort it
Contact or non-contact?
Tap a force, then tap the box it belongs in.
✋ Contact
🧲 Non-contact
Balanced & unbalanced
When forces balance
Objects usually have several forces on them at once. We show each force as an arrow: the longer the arrow, the bigger the force.
Left: equal, opposite forces are balanced. Right: a bigger force one way is unbalanced.
Big idea: when forces are balanced (equal and opposite), they cancel out. The object either stays still OR keeps moving at a steady speed in a straight line. When forces are unbalanced, the motion changes — it speeds up, slows down, or changes direction.
Common mistake
Balanced does not mean stopped
Lots of people think "balanced forces" always means "not moving". That's only half true.
The truth: balanced forces mean the motion doesn't change. A car cruising at a steady 30 m/s on a flat road has balanced forces (driving force = friction + air resistance), yet it is definitely moving. Balanced forces = "no change in motion", which can mean stationary OR steady speed in a straight line.
So next time you see equal, opposite arrows, don't assume the object is still — it could be gliding along at a constant speed.
Sort it
Balanced or unbalanced?
For each situation, tap whether the forces are balanced or unbalanced.
Quick check
Cruising along
?A skydiver has reached terminal velocity: her weight (down) exactly equals air resistance (up). What is her motion doing?
Turning forces
Forces that turn things — moments
A force can also make something rotate around a pivot. The turning effect of a force is called its moment.
moment = force × distancemoment (Nm) = force (N) × distance from pivot (m)
A seesaw balances when the moments on each side are equal: force × distance is the same both ways.
Why door handles are on the edge: a handle far from the hinge gives a big distance, so even a small push makes a large moment — easy to open. Push near the hinge and it barely turns.
Quick check
Using a spanner
?A stiff bolt won't turn with a short spanner. Using a longer spanner (same push) makes it easier. Why?
Pressure
Pressure spreads a force over an area
Pressure tells you how concentrated a force is — how much force pushes on each bit of area:
pressure = force ÷ areapressure (N/m²) = force (N) ÷ area (m²)
The same force over a smaller area gives a bigger pressure. That's why a sharp knife (tiny area) cuts, while lying on a bed of many nails (large total area) doesn't hurt.
Same weight, different area: a point concentrates the force; a flat base spreads it out.
Pressure in fluids: liquids and gases also push. Pressure in a fluid gets greater the deeper you go, and it pushes in all directions — that's why your ears hurt at the bottom of a swimming pool.
Calculate
Your turn — pressure
3A box pushes down with a force of 200 N over an area of 4 m². Calculate the pressure it exerts.
N/m²
Hint: pressure = force ÷ area = 200 ÷ 4.
Calculate
Your turn — average speed again
4A runner completes a 100 m race in 12.5 s. Calculate her average speed.