Instructables

Be a scientist: make your own force meter.

Forces are the pushes and pulls of our universe. This neat little device can be built for almost nothing (I used plumbing off-cuts), but will let you measure pushes and pulls around you, as well as doing experiments with different-sized forces.

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).
 
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Step 2: Preparing the tubes.

Cut the tubes to convenient hand-held lengths, say 20cm for the narrower tube and 4 or 5cm for the wider. Smooth the cut edges with sandpaper, your rotary tool, or by scraping with a knife.

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).

Melting 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.
gserrano7012 years ago
Love it, nice and simple. Pictures are good, congrats for your effort. I guess you could replace the short lived rubber band with a couple of springs. Thanks.
80$man2 years ago
I like this meter! Will build one (with a tensile property testing rig, see below) and post during December holiday!

Different Application:
Tensile property testing rig:

Referring to step 8, you mentioned using different materials for different measurement applications. This gave me the idea: the known mass (i.e. known force) and the resulting displacement between the 'spring' material's ends could be used to plot the stress/strain characteristics of that material.

This could give a good indication of which material is most applicable to which mass/force range by considering the linearity of the stress/strain plots of the various materials tested.

Slight modifications could make the forcemeter quite accurate: Use of vernier calipers with spring materials used in highly linear ranges ect.
80$man2 years ago
Smile indeed! Good work, I will be making one of these with a mod of my own (see final paragraph for a possible modification)

Three comments:

Firstly, (as said below) the rubber band will change with use/age, replace rubber bands & calibrate often if you wish to use in a serious application.

Secondly, just to make sure:
F = m * a -> Force (caused by the weight of an object) = mass (grams/pounds meassured value ect.) acceleration ({constant, g} gravity = 9.81m/s^2)
Thus, 100g mass is pulled toward the earth by gravity with a force = 0.981N

Finally, your design could used to measure the max static force required to move an object and overcome the force it's friction with a certain surface can support, from which static friction constants can be derived. All you need to add is a slider that sticks to the scale. If you make the inner pipe steel you could use a small magnet, the idea being, as your measurement increases, the magnet slides, as the measurement decreases (due to the object sliding away), the magnet would remain at the maximum meassurement. (Simple Static/dymanic friction is quite interesting, fun to experiment with!)
static5 years ago
<Grinning> So force meter is a fancy term for a scale here? Should get the creative juices flowing for kids needing a project for school. Stating the obvious rubber bands deteriorate somewhat rapidly. Recalibration would be needed every time the bands are replaced. For the long term PVC pipe can be used to adapt a simple hardware store spring scale. http://en.wikipedia.org/wiki/Spring_scale to measure both pulling and pushing force. I have been keeping an eye out for a hydraulic cylinder that has a bore area of exactly of 1 square inch. That way a simple pressure gauge can be used to indicate the force in pounds.
Kiteman (author)  static5 years ago
Regarding rubber bands, see step 6. I did only design the meter to be a short-life project anyway, to help Year Six kids (age 11) do extra work to support their SATs (UK version).
nafango226 years ago
add yourself a few extra rubber bands, a linear potentimeter, and a pull-up resistor and you got yourself a nice little force-controlled voltage splitter.
Patented6 years ago
How can we get the Instructables tool?
Kiteman (author)  Patented6 years ago
It was a prize in the first Valentine contest I entered.

Some online suppliers do free engraving, if you want one with your name on.
Can you use U.S. measurements like lbs to calibrate this force meter directly or would you have to convert to slugs first?
Kiteman (author)  thematthatter7 years ago
You can use anything you want - hang a pound weight on there, draw a mark and call it pounds. You could even make up your own scale, based on any repeatable unit you want, but your results won't make as much sense to other people (although graphs etc would still be the same shape, so general conclusions could be reported). Your chosen units will depend on what you want to measure (the metric unit of force is the Newton), and what size thing you want to measure. You could make a big meter, out of timber and bungees, and measure the force needed to push your car on different road surfaces.
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