A force has a size and a direction, so we draw it as an arrow. In a force diagram every force on an object gets an arrow of the right length, pointing the right way, with its name.
Push a shopping trolley gently, then hard. Same direction, different force. Push it from the other side: same size, different force. Two things to get right every time.
Put to work in:Levers(Engineering › Mechanisms) · Pull or push(Engineering › Structures)
Forces are invisible, so scientists draw them. This lesson is about drawing them precisely.
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.
Practise: A few on your own, to check you have it.
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: what does an arrow tell you?
ExploreTry it on the board and compare what happens with your prediction.
Explore: push the trolley
Use the Push slider. Watch the push arrow, and watch friction too.
Try it yourselfYour edits are saved in this browser.
Make the push 30 N.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: reading a force diagram
In your book: draw the trolley with all four forces, each arrow the right length, labelled.
Match each force to what causes it.
Weight
Support (normal force)
Friction
Air resistance
Push or pull
PractiseA few on your own, to check you have it.
Draw it right
The book isn't moving up or down. What does that tell you about the table's push?
Try it yourselfYour edits are saved in this browser.
A 5 kg pile of books rests on a table (its weight is 5 × 9.8 = 49 N). Set the table's push so the diagram is right.
Use the board. This ticks when you get there.
In a tug of war, Team A pulls with 300 N and Team B with 300 N. Set both pulls.
Use the board. This ticks when you get there.
A 50 kg skydiver (weight 490 N) has reached a steady speed: air resistance now equals her weight. Set it.
Use the board. This ticks when you get there.
Elaborate TypicalUse the same idea in a new context.
Elaborate: four situations
Which diagram fits each situation? (Forward is the way the object moves.)
A cyclist speeding up
A car driving at a steady 60 km/hSteady speed: the forces cancel.
A sled slowing down on flat snow
A box being pushed but not movingFriction matches the push.
A runner speeding up out of the blocks
Elaborate WiderUse the same idea in a new context.
Elaborate: a boomerang
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 has a size (newtons, N) and a direction, so it is drawn as an arrow.
The arrow's length shows the size; it points the way the force acts.
A force diagram shows every force on one object, each one named.
Common forces: weight (down), support (up from a surface), friction (against sliding), air resistance (against motion through air), pushes and pulls.
For teachers
Students represent forces as arrows with the right size and direction, and name them (AC9S7U04, node 1, a hinge node). About 25 minutes.
I can draw a force as an arrow with the right direction and relative size.
I can name the forces acting on an object.
I can draw a force diagram that fits a situation.
Before class
A concept tutorial for introducing or revising the idea; it does not replace force meters and real pushes. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is imprecise diagrams: wrong direction, missing forces (especially support), sizes that don't match. Every later node uses diagrams to reason, so the Practise step matters.
Board notes: arrows start at the object's centre, the usual convention for a force diagram. The boards use g = 9.8 N/kg, as the weight lesson does; masses are chosen so the weights are whole numbers (5 kg weighs 49 N).
Running the lesson
Engage: ask what else an arrow could show; 'which way it moves' is the misconception the trolley exposes (friction points backwards).
Explore: ask students to make the push smaller than 10 N and notice friction shrinking with it.
Practise: the book task is the commonest omission (the table's push). Stop and discuss it.
In your own words: collect the trolley diagram.
The hard part (on purpose)
Drawing every force with the right length and direction. When a force is missing, ask what stops the object falling or sliding.
When students get stuck
A student leaves out the support force. Ask: The book isn't falling through the table. What stops it? Fix: The table pushes up: support (normal force), equal to the weight when the book is at rest.
A student draws friction in the direction of motion. Ask: What does friction try to do to sliding? Fix: Stop it: friction acts against the sliding (or the push).
Arrows all the same length. Ask: Which force is bigger? How would someone see that from your drawing? Fix: Make the bigger force's arrow longer, in proportion.
Stretch and support
Stretch: Draw the force diagram for a ball at the very top of its flight, just after you throw it straight up.
Support: Give a template: a box with dotted guide arrows (up, down, left, right) to trace over and label.
Where it leads
Next: node 2, balanced and unbalanced forces: what the diagram tells you about the motion.
In class: force meters pulling a block along the bench; tug of war with two force meters.
What each step is for
Engage: what does an arrow tell you?: Students who think only direction matters, or only size, will choose those options; the board shows the arrow carrying both.
Explore: push the trolley: Changing one force and watching its arrow grow makes 'length = size' concrete before any rules are stated.
Explain: reading a force diagram: Naming each force and reading size and direction from arrows is the precision skill every later node uses.
draw it right: Repeated diagrams with immediate feedback build precision: correct direction, relative size and a name for every force.
Elaborate: four situations: Matching diagrams to situations (at rest, speeding up, slowing down, falling) uses arrows as reasoning, not decoration.
Elaborate: a boomerang: A shaped object in flight shows forces that aren't pushes in contact, extending the list of forces students can name.
Now you can
Draw a force as an arrow with the right direction and relative size.