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You have seen how forces change motion. Now follow Mia and Cole on a seesaw. Mia's end moves down, and the bar turns around its fixed pivot.
What happens to Cole's end on the other side?
Levers, Pulleys & Gears
Recognise that some mechanisms including levers, pulleys, and gears allow a smaller force to have a greater effect
What a learner can do afterwards
- Describe a lever's effort, load and pivot, and explain how their positions affect lifting
- Explain how a pulley changes the direction of a force and can make lifting easier
- Describe how matching gear teeth pass on turning and change its speed, direction or turning effect
1 · Read
A force is a push or a pull. You have seen how forces change motion. Mechanisms can change a force's direction or help you move a load with less force. The force you apply is the effort. Effort can be a push or a pull. A machine does not make energy.
Try it together
A lever uses a stiff bar that turns around a pivot, also called a fulcrum. The bar must resist bending when effort is applied. Effort is your applied force; the load is what you move. Mia and Cole sit on opposite sides of a seesaw's pivot. When Mia's end moves down, Cole's end moves up. Mia supplies the effort and Cole is the load. Other levers can have effort and load on the same side of the pivot.
Try it together
A flagpole has a fixed pulley: its wheel stays at the top. Pull the rope down and the flag rises. This changes the pulling direction, so you can work from the ground. Ignore friction and the rope's weight. To raise a load at steady speed, the pull equals its weight because one rope section supports it. Starting it moving upward needs a little extra force.
Gears are toothed wheels. Matching teeth fit together and pass on turning. Two gears touching at their outer edges turn opposite ways. With matching tooth sizes, a bigger gear has more teeth. A small gear driving a big one makes it turn slower; a big gear driving a small one makes it turn faster. Ignore friction: the bigger driven gear has a greater turning effect at its axle, not a greater push between the teeth.
A movable pulley rises with the load. In our ideal examples, ignore friction and the weights of the rope and pulleys. Each section of the same rope pulls equally. At steady speed, two vertical sections supporting the moving load each carry half its weight, so your pull is half the weight. Count supporting sections, not wheels. Extra fixed wheels can redirect the rope without reducing the pull.
Try it together
In the chart's lever example, ignore friction and the bar's weight. Mia pushes farther from the pivot than Cole sits. At steady speed, lifting Cole takes 10 force units directly or 4 with this lever. His weight stays the same. Mia's end travels farther than Cole's end. Moving the pivot closer to a load can reduce the effort. On an equal-arm seesaw, a larger downward force gives a larger turning effect.
A force chart compares the force needed in different arrangements. Read the label to find the arrangement you need, then read its value. To find how many fewer force units one arrangement needs, subtract its value from the larger value. If direct lifting needs 9 units and a lever needs 3, the difference is 9 - 3 = 6 units.
Levers can reduce lifting force. Fixed pulleys change pulling direction; movable systems can reduce force. Gears change turning speed, direction or turning effect.
Original readings and flashcards written for this dataset. Source material studied: OpenStax College Physics, section 9.5, Simple Machines, and wikiHow's How to Create a Gear Assembly Using Siemens NX 12.
2 · Watch
3 · Play
Try it yourself
Slide the weights in and out until the seesaw balances.
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
24 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.