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Demonstrating physics: forces
Forces is one of the most important topics in KS3 and KS4 physics — and one where good demonstrations pay the highest dividend. Newton's laws are counter-intuitive in ways that pupils rarely fix through calculation alone. They need to see forces at work.
About this video
Pupils' intuitions about forces are shaped by lived experience of friction-dominated everyday physics. Objects need a constant push to keep moving; things slow down "naturally". Neither is true in the idealised Newtonian picture, but pupils don't experience an idealised picture. That gap is the pedagogical problem: how do you get pupils to reason with a model that contradicts their everyday experience?
The best demonstrations don't just show the phenomenon — they isolate the variable. Air-track gliders, dry ice pucks, hover boards, low-friction trolleys: all reduce friction enough that pupils can see what "no net force" actually looks like. Suddenly Newton's first law isn't counter-intuitive; it's obvious. Pair the low-friction demonstration with a discussion of why the world usually doesn't look like this (friction, drag) and pupils have the right mental model.
For KS4 the focus shifts to quantitative work — F = ma, resolved forces, terminal velocity — but the intuitions still matter. A pupil who can calculate an acceleration but doesn't believe an object needs zero net force to move at constant velocity is one shove away from a wrong answer on a real problem.
Demonstrations that build the right intuitions
- Low-friction motion. Air track, dry ice puck, low-friction trolley. Show what constant velocity actually looks like without friction to slow it.
- Balanced vs unbalanced forces. A trolley with equal weights pulling in opposite directions vs unequal weights. Discuss what pupils predict and observe.
- Newton's third law pairs. Two pupils on skateboards pushing each other — both move. Draw the force pairs explicitly.
- Terminal velocity. Paper cake cases falling — they reach terminal velocity almost immediately, unlike heavier objects. Pupils can time falls to demonstrate the relationship between weight, drag and terminal velocity.
Discussion prompts
- What misconception about forces most stubbornly survives your teaching, and what do you try to shift it?
- How do you help pupils reason with an idealised model when their everyday experience contradicts it?
- Which of your force demonstrations are actually about the physics, and which are mostly showing "cool motion"?
Try this in your classroom
- Run an air-track or low-friction trolley demonstration. Ask pupils to describe what they see and explain it in terms of forces. Collect their explanations — the misconceptions are diagnostic.
- Give pupils a moving object and ask them to draw all the forces acting on it. The gap between what they draw and what's actually there is where the teaching happens.
- Ask pupils to predict what happens when a moving trolley enters a friction-free zone. Then run it. Then discuss.