Forces · Model
What forces do to motion
A resultant force does not make things move. It makes whatever they are already doing change — and that is a much stranger idea.
Start here
Nothing is pushing it, and it keeps going anyway.
A curling stone slides down twenty metres of ice at a steady speed. Nobody is touching it. No engine, no rope, nothing pushing it forwards at all.
So why does it keep moving?
It keeps moving because nothing is stopping it. Moving needs no force at all — a resultant force is only needed to change what something is doing. On very smooth ice there is almost nothing left over, so almost nothing changes, and the stone carries on at the same speed in the same direction for a very long way.
A resultant force can do exactly three things to motion: start it or speed it up, slow it down or stop it, or change its direction. If nothing is left over, none of those happens — whatever the object was doing, it carries on doing.
At the bench · trolley and light gates
Same trolley, same start, four resultants
Change a control to begin
Every run starts with the trolley already travelling at 2.0 m/s to the right, timed through a light gate. Choose a resultant force, choose how long it acts, and take a second reading.
Commit first. The resultant force is 0 N and the trolley is already moving. What does the second gate read?
The resultant force
How long it acts
Gate 1
2.0 m/s
Gate 2
—
What changed
—
What stayed the same
—
Three things, and only three
Everything on the bench was one of these.
Start it, or speed it up
A resultant force in the direction of travel makes the object go faster. From rest, it is what gets it moving at all.
Slow it down, or stop it
A resultant force against the direction of travel makes it slower. Leave it acting and the object stops, then goes the other way.
Change its direction
A resultant force across the direction of travel bends the path. The original motion is still there underneath.
Or, with 0 N left over: nothing
At rest, it stays at rest. Moving, it carries on at the same speed in the same direction, with no push needed.
Two things also decide how much changes: how big the resultant force is, and which way it points. A bigger resultant changes the motion faster, and the direction it points is the direction the change happens in.
Key fact
A resultant force is needed to start something moving, to speed it up, to slow it down or stop it, or to change its direction. Moving at a steady speed in a straight line needs no resultant force at all.
Think again
“If something is moving, there must be a force pushing it along.”
This one is worth taking seriously, because everyday life agrees with it. Stop pedalling and the bike slows; stop pushing the box and it stops. So it really does look as though motion needs a constant supply of push. What is hidden is friction: on a bicycle, on a floor, in the air, something is always pushing backwards, and your forward push is cancelling it rather than causing the motion. Remove the friction — a curling stone on ice, a puck on an air table, a spacecraft between planets — and the object needs nothing at all to keep going. Voyager 1 has had its engines off since 1980 and is still travelling at 17 kilometres a second.
“A force sideways makes it go sideways instead.”
It does not throw away what the object was already doing. On the bench above, a sideways resultant left the trolley still travelling to the right — it simply added a bend to the path, so the trolley ended up going right and sideways at once. This is exactly why the Moon goes round the Earth instead of falling into it: the Earth's pull is sideways-on to the Moon's motion, so it bends the path into a circle rather than stopping the Moon and dragging it in. A resultant force adds a change; it does not replace the motion.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply
A cyclist is freewheeling and slowing down. What must be true about the forces on the bicycle?
Rung 2 · The one that catches people
A ball is thrown straight up. At the highest point, just for an instant, it is not moving. What is the resultant force on it then?
Rung 3 · Explain
A curling stone slides twenty metres across ice at almost the same speed the whole way, with nothing touching it. Explain why it keeps going, and why it does eventually stop.
Rung 4 · Take it somewhere new
A satellite circles the Earth at a steady speed with its engines switched off. Its speed never changes, but there is a resultant force on it the whole time. Explain how both of those can be true, and say which way the force points.
Key note
Forces change motion; they do not maintain it. A resultant force can start something moving, speed it up, slow it down, stop it, or change its direction, and how much it changes depends on how big that force is and which way it points. With balanced forces nothing changes: something at rest stays at rest, and something moving carries on at the same speed in the same direction.
Going further
For roughly two thousand years the best minds available believed that motion needed a cause, and they were not being stupid — they were describing the world in front of them, where everything does stop. Aristotle's version lasted until the 1600s, when Galileo tried rolling balls down one slope and up another and noticed that the smoother he made the surfaces, the further they went. He drew the conclusion nobody had drawn: that with the friction removed entirely, the ball would never stop, and therefore motion needs no cause at all. Nothing about that can be seen directly. It is reached by making an experiment cleaner and cleaner and then imagining the limit, which is one of the most powerful moves in science and worth recognising when you next meet it.
The same trolley pushed with the same resultant force does not always change by the same amount, and what makes the difference is its mass. Load the trolley with bricks and 1 N barely alters it; empty it and the same 1 N makes a much bigger change. That relationship — resultant force, mass and how fast the motion changes — is a real equation you will meet at GCSE. At this stage the useful half is the shape of it: bigger force, bigger change; bigger mass, smaller change.
Before this lesson
Connects to
At GCSE this becomes
- Newton's first and second laws, acceleration, and the equation linking resultant force, mass and acceleration.
Where to next
Ask Mr Badmus AI
Want to test the idea on a situation of your own?
The trolley bench is a teaching model: the gate readings are the values a 1 kg trolley would give with a steady 1 N resultant and no friction, rounded to one decimal place. Real trolleys have friction, so real second readings are always a little lower.
Lesson content © MrBadmusAI.