Introduction: 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).

Step 1: Materials Needed.

Two PVC tubes (one has to fit easily inside the other - I used offcuts of drain and overflow pipe), two small metal washers, a length of wire, a thick rubber band about as long as the widest tube's diameter, plus a small hacksaw, wire cutters, pliers, and a rotary tool

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.

Step 3: Main Assembly.

Put the narrow tube inside the wide tube, with the holes at the same end, and the outer holes lined up with two of the inner holes.

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.

As it stands, the force meter will only measure pushes. To measure pulls, you need a hook or loop. I'm adding a loop:

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.

Your meter is essentially finished, but you cannot yet actually measure any forces with it. It needs to be calibrated.

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.

Rubber bands age and perish, and they stiffen as they do, so you will need to check the calibration of your meter every few weeks. They also snap occasionally, especially if stretched to their limits.

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.

Hold the meter by the outer tube.

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.

Well, not hacking, as such, but variations on a theme.

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.

Enjoy.

(Perhaps you would like to add a comment with ideas for other uses for this instructable?)

Comments

author
static (author)2008-08-22

<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.

author
shannonlove (author)static2016-09-12

"So force meter is a fancy term for a scale here?"

Strictly speaking a "scale" measures mass/weight only in respect to the to pull of gravity. A force-meter will measure will measure the amount of force needed to move an object along any vector. A scale will not work in micro-gravity but a force meter/guage will continue to function. A force meter responds to friction, moments of inertial etc.

author
Kiteman (author)shannonlove2016-09-13

You did see the <grin>, yes?

A "scale" refers to a particular use of force-meters as weighing devices - yes, they measure the force placed upon an object by the mutual gravitational attraction of the object and the Earth, but the scale is graduated in whatever units the user finds convenient, be they Newtons, pounds or even kilograms.

Most specific to this kind of meter would be the fishing scale, upon which one suspends one's catch and the end of the trip.

author
Kiteman (author)static2008-08-23

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).

author
gserrano701 (author)2011-10-15

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.

author
80$man (author)2011-10-04

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.

author
80$man (author)2011-10-04

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!)

author
nafango22 (author)2008-06-06

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.

author
Patented (author)2008-03-25

How can we get the Instructables tool?

author
Kiteman (author)Patented2008-03-26

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.

author
thematthatter (author)2007-07-24

Can you use U.S. measurements like lbs to calibrate this force meter directly or would you have to convert to slugs first?

author
Kiteman (author)thematthatter2007-07-24

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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