A force is a vector: it has a size and a direction. Forces along one line add if they point the same way and subtract if they point opposite ways. What is left is the net (resultant) force, and that is what changes motion.
A car's engine pushes it forward with 2000 newtons. So why doesn't it keep speeding up for ever?
In Year 7 you drew forces as arrows. This lesson adds them up.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: three forces on a car
ExploreTry it on the board and compare what happens with your prediction.
Explore: add and subtract
Use + and − to change a force. The net force is shown under the board.
A net force backward on a car moving forward slows it down.
Try it yourselfYour edits are saved in this browser.
Change the driving force until the net force is zero.
Use the board. This ticks when you get there.
Now make the net force 1000 N backward (to the left). You can change both adjustable forces.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: forces as vectors
In your book: draw a force diagram for the car in the prediction, with all three forces as arrows to scale. Write the calculation of the net force, with its direction.
Two people push a stalled car forward with 300 N and 250 N. Friction acts backward with 400 N. What is the net force, in newtons? (forward is positive)
In a tug of war, Team A pulls left with 1800 N and Team B pulls right with 2100 N. What is the size of the net force, in newtons?
What will the motion do?
Net force 500 N forward
Net force 200 N backward on a moving bikeIt slows down.
Forward 900 N, backward 900 N
A puck sliding on ice with no frictionNo forces along its path: no net force.
Contact or non-contact force?
Friction between the tyres and the road
Air resistance on a cyclistThe air touches the cyclist, so it is a contact force.
A table pushing up on a book (the normal force)
Gravity pulling a skydiver down (her weight)Gravity acts across a distance, with nothing touching.
A magnet pulling a paperclip from a few centimetres away
Elaborate TypicalUse the same idea in a new context.
Elaborate: a four-person tug of war
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Change Ana's or Cai's pull until the rope doesn't move: net force zero.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: driving at a steady speed
Try it yourselfYour edits are saved in this browser.
The car starts at 20 m/s. Set the driving force so it keeps a steady speed, then press Play and run it for at least 3 seconds.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A force is a vector: it has a size (in newtons) and a direction.
Along one line, forces in the same direction add; forces in opposite directions subtract. Choose one direction as positive.
What is left is the net force (resultant force), in the direction of the bigger side.
A net force changes motion (speeds up, slows down, turns). A net force of zero leaves the motion unchanged: still, or at a steady speed.
For teachers
Students represent forces as vectors and add or subtract forces along one line to find the net force, then say what the net force does to motion (AC9S10U05, node 1). About 24 minutes.
I can find the net force from several forces along one line.
I can say whether a net force will change an object's motion.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It revisits Year 7 force arrows (AC9S7U04) with numbers, and leads straight into Does a moving thing need a push? (first law, node 2).
Not a hinge node, but the slip it targets returns in every later lesson: adding all the forces as numbers, ignoring direction. The prediction's 2800 N catches it.
Board notes: the board shows horizontal forces only (and weight and support when set); the net force is horizontal, positive to the right. In the Wider step the car moves along a track with a speed–time graph. Real air resistance grows with speed; here it is a fixed force.
Running the lesson
Engage: tally 1200 / 2800 / 2000 / zero. The 2800s added every force.
Explore: ask what one click on a force did to the net force (changed it by the same amount).
Explain: the two number checks are the success criteria; the skydiver links zero net force to steady motion.
In your own words: the scaled force diagram with the calculation is the evidence.
The hard part (on purpose)
Keeping track of direction. When a student adds every force, ask them to sort the forces into two sides first, without calculating.
When students get stuck
A student adds all the forces (2000 + 500 + 300). Ask: Which way does each force point? Fix: Forward forces and backward forces work against each other: add each side, then subtract the smaller side from the bigger.
A student gives the net force without a direction. Ask: 1200 N which way? Fix: A force is a vector: always say the direction (forward, left, up), or use a sign with a stated positive direction.
A student thinks zero net force means the object is still. Ask: What did the car do on the board when the net force was zero? Fix: It kept its speed. Zero net force means no change in motion, which can be steady motion.
Stretch and support
Stretch: A boat's engine pushes forward with 5000 N while a current pushes it sideways with 1200 N. Why can't you just subtract these? (They are not along one line: they combine at an angle, which needs vector diagrams in later years.)
Support: Give the frame: 'Forward: … + … = … N. Backward: … + … = … N. Net: … − … = … N, …(direction).'
Where it leads
Next: Does a moving thing need a push? (Newton's first law, node 2).
In class: force meters on a trolley pulled in opposite directions; a tug of war with spring balances; drawing force diagrams to scale on grid paper.
What each step is for
Engage: three forces on a car: Students who add every force regardless of direction (2800 N), or pick the biggest, show they are treating forces as numbers, not vectors.
Explore: add and subtract: Changing one force and watching the net force move by the same amount makes 'add along one line, subtract opposite ways' something students do, not recite.
Explain: forces as vectors: Working net forces by hand, with a sign or a direction every time, and then tying the net force to changes in motion, is the node and the link to Newton's laws.
Elaborate: a four-person tug of war: Four forces, two adjustable, make students combine same-direction forces before subtracting, the full one-axis method.
Elaborate: driving at a steady speed: Running the board with a zero net force shows the car keeps its speed, the bridge to Newton's first law next lesson.
Now you can
Explain why force is a vector.
Find the net force from several forces along one line.
Say whether a net force will change an object's motion.