It is perfect for kids still at school as well; it can be made by dads-and-lads together, or by competent older children alone, and it can be calibrated in Newtons and used to do home experiments to reinforce schoolwork (any UK readers with kids in Years 6 or 9, this will help them get ready for their SATs).
Step 1: Materials Needed.
Step 2: Preparing the Tubes.
Drill or melt holes in the tubes. You need two in the wider tube, both near one end, opposite each other, and four in the narrower tube, all at the same end, spaced every 90 degrees around the tube).
Smooth the edges of the holes to stop friction wearing through the rubber bands (you will not need to do this if you melt the holes).
Hold a nail in pliers, and heat it in a flame. A candle or lighter flame will do, but this is a case of "hotter is better", so I use a blowtorch or a bunsen burner. Use the hot nail to poke a hole in the plastic tube, being careful to avoid inhaling the noxious fumes.
Step 3: Main Assembly.
Using a straightened-and-bent-in-half paper-clip as a "needle", thread the rubber band through the four holes in a line.
Use metal washers to stop the rubber band slipping back out of the holes. Poke the band through the centre of the washer, then bring it back around the outside of the washer (congratulations, you just tied a lark's-head hitch).
Step 4: Adding a Hook.
Thread the wire through the two unused holes in the inner tube and tie a knot to hold it in a loop.
The wire I used was quite smooth, so required three over-hand knots in opposite directions to lock. (I may still add a drop of glue to the knot, just to be sure.)
Step 5: Calibration.
Stick a piece of tape along the visible part of the narrower tube. Make a mark on the tape where the bottom edge of the wider tube is. This is your "zero" point.
Add a known weight. Either hang it off the hook or balance it on the narrower tube. It's a lot easier for you if the weight is sensible number (say 100g, 200g, 50g), as you will be using it to draw your scale. Make a mark on the tape.
If possible, repeat with a couple of other known weights, and mark those on the tape as well.
The Science Bit:
What you've been adding to the meter is not weight - it's mass. The weight is the pull on the mass due to the local gravity-field (i.e. Earth's gravity). A mass of 100g actually weighs a smidge under one Newton (1N). A mass of 1kg weighs 9.8N.
Label the marks you made in multiples of 1N. If a mark was due to a 100g mass, label it 1N. A 50g mark should be labelled 0.5N etc.
Either by eye or my measuring and marking, you should now be able to complete the scale on your force meter, filling in gaps and extending the scale to the end of the narrower tube.
The exact range of your meter will depend on the thickness, length and age of your band, and on the length of your tubes.
Step 6: Caveat.
That is why the scale is drawn on tape - you can easily change or replace it when the band needs replacing. It would be a waste of effort to draw a permanent scale directly onto the narrower tube.
Step 7: Using the Meter.
Objects can be hung from the loop or hook to find weights, or pulled with them to measure the force needed to move it (say, if you were investigating friction). Turn the meter round, and (still holding the outer tube) you can use the inner tube to measure the force of a push, or to weigh something that won't hang on the loop, such as balls or eggs.
You can also use the meter to investigate the effects of known forces, pushing or pulling until the meter reads a certain force and observing the effects. A fun one is to fire toy cars across the floor; how far will it travel when fired with a force of X Newtons?
Step 8: Hacking the Meter.
You may find yourself needing to measure forces that are outside the range of any single meter, so you might want to make a range of meters, using different bands or pieces of bungee or shock-cord.
The materials may not be to your taste, so change them - you can make a nice meter with a piece of copper tube for the outer, and a length of dowel replacing the inner tube (which makes attaching a hook a lot easier, since it will simply screw into the end of the dowel, just make sure you don't screw it into the band).
Anyhoo, the exact materials you use don't really effect the operation of the meter, as long as they are stiff enough not to bend much when you use the meter, and you avoid rough edges that will saw through rubber bands.
(Perhaps you would like to add a comment with ideas for other uses for this instructable?)