Levers
A lever is a beam that hinges on a single fixed point called a fulcrum. If the fulcrum is in the middle (as in a seesaw) equal weights placed on both ends will balance each other. If the fulcrum is placed off-center, a small weight on the long end of the beam can lift up a large weight on the short end.
On a small scale, it’s easy to try this out by balancing a ruler (the beam) across a fat pencil or a dowel (the fulcrum) and piling different numbers of standard stackable weights (very small cans of food, magnets…) on different ends of the beam. Check out how the number of weights you need to place on each end to make the ruler balance changes as the fulcrum moves farther from the center. On a larger scale, if you have a long plank for a beam and a log/ cinderblock/ anything else for a large fulcrum, a small child can lift a parent so long as the parent stands on the short end of the lever. This will become easier to do as the fulcrum moves farther from the center.
Inclined Planes and Wheels
An inclined plane is simply a slope, and it’s easy to demonstrate how changes in the steepness of that slope affect much force it takes to move a weight up the slope, or how much momentum that weight picks up as it moves down. You can do this at any scale: use a large book as your ramp and a small toy car as your moving object, or a long board and a larger vehicle, depending on what you have available and how much noise, drama, and effort you want.
To see the difference in momentum, set up a plane of the same length at different angles with a long flat smooth surface beyond its lower end. Let the vehicle roll from the top of the plane…don’t push it, just let it go. Measure how far the vehicle rolls across the flat before it comes to a stop at different steepnesses.
There are different ways of measuring the effort it takes to pull your vehicle up a steeper or gentler slope. Tie a string or rope around your vehicle. If you have a spring scale, tie the string to the end of that and see what weight reading you get as you pull your vehicle up the slope. If you don’t have a spring scale, you can tie objects to the other end of the string and let them dangle down over the vertical space at the top of the inclined plane; add weights until they’re heavy enough to pull the vehicle up the slope. See how the weight needed to pull the vehicle uphill changes as the steepness changes.
This setup also shows just how effective wheels are. Find another wheelless household object that weighs about the same amount as your vehicle. Something fairly smooth is best, to minimize friction. Try sliding it down the slope and see how far it goes compared to the wheeled vehicle. Try pulling it up the slope and see how much effort is required.
Pulleys
A pulley is a wheel over which a rope is looped. You can easily demonstrate how much easier it is to raise a load with a pulley than without. Tie a weight to the end of a string or rope; let your child lift that and feel the effort. Then run the string or rope over a pulley and let the child pull down on the other end to lift the weight. Then run the same rope over two pulleys and see how the effort required changes.
If you don’t have actual pulleys on hand to use (say, off a clothesline) you can use other objects you may have around the house. Rolling pins and casters are already complete wheels with axles. You can also make a pulley by running a rod through spools which have held ribbon or thread, central parts out of tape dispensers, Tinkertoy rounds/wheels…
Other Simple Machines
I’m sure my father also did activities with us to help us see how wedges and screws worked, but I can’t remember those now, and since my mother didn’t do the simple machines projects with us she doesn’t remember either. If you have any ideas for wedge and screw activities, or further ideas for simple machines projects, please post those in the comments.