A free body diagram shows one object and every force acting on it, as arrows from the object: size by length, direction by the way it points. Add the forces along each direction to find the net force, then use Newton's laws to say what the object's motion will do.
A plane cruising at 900 km/h has four big forces on it, each hundreds of thousands of newtons. Yet the passengers can pour a drink without spilling it. How?
Put to work in:Pull or push(Engineering › Structures)
You can find a net force along one line. This lesson draws all the forces on an object at once.
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: the cruising plane
ExploreTry it on the board and compare what happens with your prediction.
Explore: draw it on the board
Up and down already balance; work on the forces across.
g is 9.8 N/kg here.
Try it yourselfYour edits are saved in this browser.
Change the rope's pull so the forces on the sled balance in both directions.
Use the board. This ticks when you get there.
A 500 kg lift moves up at a steady speed. Set the cable's pull so the forces balance.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: reading a free body diagram
In your book: draw a free body diagram for a cyclist riding at a steady speed on a flat road. Label four forces with their sizes, using made-up but sensible numbers that balance.
A 600 N skydiver has 450 N of air resistance acting up on her. What is the net force on her, in newtons, downwards?
A 2 kg box is pushed right with 14 N against 6 N of friction. What is its acceleration, in m/s²?
Elaborate TypicalUse the same idea in a new context.
Elaborate: four forces on a plane
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Set lift and thrust so the plane cruises: balanced in both directions.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: the skydiver's fall
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 free body diagram shows one object and every force acting on it, as labelled arrows from the object.
Work out the net force in each direction separately (up–down, left–right).
Net force zero in every direction: no change in motion (still, or steady speed in a straight line).
A net force: the object accelerates in that direction, with a = F ÷ m.
For teachers
Students draw free body diagrams for objects with several forces, find the net force in each direction, and use Newton's first and second laws to say what the motion does (AC9S10U05, node 6). About 25 minutes.
I can draw a free body diagram with every force on an object.
I can find the net force in each direction.
I can say what the object's motion will do from its free body diagram.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It follows Push the wall, and it pushes you? and pulls together the first and second laws. Integrating all three laws in problems (nodes 7 and 8) is a class task.
Not a hinge node, but students carry two errors into it: a moving object needs a net force forward, and forces the object exerts belong on its diagram. The plane prediction targets the first; the "what goes on it" question the second.
Board notes: the board draws forces from the object's centre (friction at the ground) with arrow length by size; large forces are capped in length, so read the labels. The plane's weight is given directly in newtons.
Running the lesson
Engage: tally the predictions. 'Thrust is bigger' is the Aristotelian idea.
Explore: ask which direction they balanced first, and why it didn't matter.
Explain: the two number checks and the lift question are the success criteria.
In your own words: the cyclist's balanced diagram is the evidence.
The hard part (on purpose)
Working one direction at a time. When a student mixes vertical and horizontal forces in one sum, ask them to cover half the diagram with their hand and work the other half first.
When students get stuck
A student adds weight to the horizontal forces. Ask: Which way does weight point? Fix: Down. Up–down forces balance separately from left–right ones.
A student draws the force the sled exerts on the rope. Ask: Is that force acting on the sled? Fix: No, on the rope. A free body diagram shows only forces on the object.
A student thinks the cruising plane needs more thrust than drag. Ask: Is its speed changing? Fix: No, so the net force is zero: thrust equals drag.
Stretch and support
Stretch: A 60 kg person stands in a lift accelerating upwards at 1 m/s². How hard does the floor push on them? (Net force needed = 60 × 1 = 60 N up, so the floor pushes 60 × 9.8 + 60 = 648 N.)
Support: Give a diagram template: a box with four arrow stubs (up, down, left, right) to label, and a two-row table for each direction's net force.
Where it leads
Next: problems using all three laws (node 7) and real applications (node 8) are class tasks.
In class: force meters on a block dragged at steady speed; drawing free body diagrams for photos (a climber, a kite, a car braking).
What each step is for
Engage: the cruising plane: Students who think a moving object needs an unbalanced force forward (the Aristotelian idea) or that 'no net force' means no forces pick the wrong options; the diagram makes both visible.
Explore: draw it on the board: Adjusting forces on a diagram until both directions balance makes 'net force zero in each direction' something students do.
Explain: reading a free body diagram: Drawing only forces on the object, working each direction separately, then reading the motion from the net force combines the first and second laws.
Elaborate: four forces on a plane: Balancing two directions at once with four forces is the full multi-force analysis in a real context.
Elaborate: the skydiver's fall: A changing force (air resistance growing with speed) shows why free body diagrams change during a motion.
Now you can
Draw a free body diagram with every force on an object.
Find the net force in each direction.
Say what the object's motion will do from its free body diagram.