KS3 · Physics
Using force arrows in diagrams
You cannot see a force, so how do you draw one? A sledge slides right across ice, yet one of its arrows points left. By then it will make sense.
Physics · Force arrows
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The sledge is sliding to the right. Gravity starts at the centre and points down. The ice pushes back up and rubs against the sledge, and both of those arrows start on its lower surface, where it touches the ice. The push-back points up; friction points left, against the movement. Only the sledge is drawn: the ice and the Earth appear only in the labels.
Pick a view. For each arrow ask: where does it start, which way does it point, and what is it called? (Arrow lengths here are not to scale; that is the next block.)
Physics · Force arrows
Drag the bar to change the force. The bar stands for the shaft of your arrow: watch its length follow the force, and notice that it never gets thicker.
Physics · Force arrows
Which way must the arrow point?
Pick a force, then choose the direction its arrow must point. Commit first; the reason comes after.
Still to sort
Down, towards the centre of the Earth (0)
Starts from the centre of the object.
Against the movement (0)
Opposite to the direction the object is moving.
Pushing back on an object resting on a surface (0)
The surface pushes back on the object.
Where the line is: Both this and upthrust can point up. The difference is what is doing the pushing: a solid surface the object rests on, or water under a floating object.
Up, on the bottom surface of a floating object (0)
Water pushing up from underneath.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Where an arrow starts, which way it points, how long it is, and what to call it.
What you need to know
- A force is a push or a pull that objects exert on each other. You cannot see forces, so we draw an arrow to represent one.
- Draw each arrow so that it starts on the object the force acts on, at the point where the force acts.
- The arrow points the way the force acts, which is not always the way the object is moving.
- The length of the arrow shows the size of the force in newtons (N). The width of the arrow shows nothing.
- Label each arrow as the force of B on A, and draw only the forces acting on the one object you are studying.
The big picture
A force arrow is a drawing with rules, not a doodle. Where the arrow starts shows what the force acts on, which way it points shows which way the force acts, its length shows how big the force is, and its label says which object is pushing or pulling which.
Key points
Worked example
Problem
A tennis racket hits a ball with a force of 6 N to the right. Draw the force arrow, using a scale of 1 cm for 1 N, and label it.
⚠ Watch out
Drawing the arrow the way the object is moving. A force acts in the direction of the change it causes, so the friction force of the ice on a sliding puck points backwards, even though the puck moves forwards.
Memory hook
Four checks for every arrow: Start, Direction, Length, Label. Start on the object, point the way the force acts, measure the length with a ruler, and label it "B on A".
Check yourself
A book slides right across a table. Where does the friction arrow of the table on the book start, and which way does it point? (It starts on the lower surface and points left.)
Flashcards
(12)What is a force?
What does the length of a force arrow show?
On a scale of 1 cm for 1 N, how long is the arrow for a 7 N force?
Does a thicker arrow mean a bigger force?
Where does a force arrow start?
Which way does a force arrow point?
Where does the arrow for the gravitational force start, and which way does it point?
Which way do friction and drag arrows point?
What is the normal contact force?
What is upthrust, and where does its arrow start?
How should a force arrow be labelled?
Which forces go in a diagram of one object?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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