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You have drawn force arrows and learned about friction. Mia's book starts still. She pushes right with 5 units while friction acts left with 5. These are its only horizontal forces; the other forces cancel.
What happens to the book?
Balanced & Unbalanced Forces
Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object
What a learner can do afterwards
- Explain that balanced forces cancel overall and leave motion unchanged, while unbalanced forces do not cancel overall and change motion
- Plan a fair test investigating how different force sizes affect an object's motion
- Use results as evidence to explain when forces are balanced versus unbalanced
1 · Read
You have drawn force arrows before. Equal forces in opposite directions cancel each other. All the forces on an object are balanced when they cancel overall. Its motion then stays the same: a still object stays still, and a moving object keeps the same speed and direction. If a force is left over, the forces are unbalanced and motion changes.
Try it together
The numbers measure force in units. A newton, written N, is one force unit. In this example, Mia pushes her still book right with 8 units while friction acts left with 5. The other forces cancel. The leftover force is 8 - 5 = 3 units right, so the book starts sliding right. If it is already sliding right, it speeds up.
A fair test changes one thing at a time. To test push size, use the same object and surface. Start with the object still and in the same place, and push for the same length of time. Change only the force size. To test surfaces, change only the surface. Keep the object and the way you push it the same.
Friction can act on a still object. It matches a gentle push, up to a limit. A push above that limit makes the object start sliding. In repeated fair tests, a box stays still at 10 units but slides at 11. Its greatest friction while still is at least 10 but less than 11 units. That limit is not the friction in every trial.
The net force is the leftover force after combining all forces. Along one straight line, add forces pointing the same way. Compare the totals pointing opposite ways, then subtract the smaller total. Equal totals cancel. A cancelling pair does not cancel an extra force: right 2 and left 2 cancel, but another left 1 leaves a net force of 1 left.
Balanced forces cancel; speed and direction stay unchanged. Unbalanced forces leave a net force that changes speed or direction: acceleration. Along a straight line, add same-direction forces and subtract opposite totals. The net force points toward the larger total. With the motion, it speeds the object up; against the motion, it slows the object before any reversal. A rolling ball with friction as its only horizontal force slows and stops.
Before a trial, write a prediction: what you expect to happen. Afterward, record the result: what you saw. Note the force size and the motion before and after. To compare travel distances, measure from the same starting mark, use the same unit, and measure after the same length of time.
Repeat each trial with the same conditions and record every result. Repeats help you see whether a result happens again. If the same push sometimes starts motion and sometimes does not, repeat and check the setup. Do not pick a friction limit from results that disagree.
Balanced forces cancel overall and keep motion unchanged. Unbalanced forces change speed or direction. Test one change at a time, record what happens, and repeat.
Reading, extra questions and flashcards written for this dataset.
More to read
Checking forces in a fair test
A force can act without making an object move. A gentle push on a still book can be matched by friction. Gravity also pulls the book down while the table pushes up. To decide whether the forces balance, check all of them.
Balanced forces leave motion unchanged. That means a still object stays still, but a moving object keeps its speed and direction. Unbalanced forces leave a force that changes motion. They can speed an object up, slow it down or turn it.
To investigate push size, use the same object and surface. Begin each trial at the same mark with the object still. Keep the push time the same and vary the force size. Record the force and what happens to the motion. If you measure distance, use the same unit and measure after the same length of time.
Repeat each trial and keep every result. Friction can match different gentle pushes up to a limit. If a box stays still at one push size but starts sliding at a larger one, those results help locate the limit. If repeated results disagree, check the setup and test again before drawing a conclusion.
2 · Watch
3 · Play
Try it yourself
Change either pull and predict what happens in a new trial from rest.
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
28 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.