# DIY Force probe

I was thinking about some experiments that deal with friction and I wanted to show something with a force probe. The problem is that most people don't have one of these. So, I decided to try and make one out of simple things. In this case, I am using some straws, a rubber band and some paper clips. Let me draw a little sketch of how this thing works.

The basic idea is to use the rubber band to measure the force (by measuring the amount the rubber band stretches). The two paper clips do two things. First, it allows you to hook up the device to something (like hanging some Lego bricks on it) and it gives the straws a place to connect. The straws (one of those smaller coffee straws) slide and let me measure the stretch. I made a video of how I put one of these together. Not sure how useful the video is - I am not actually a professional hand model.

The only tricks: I used a string to pull the rubber band through the straws. Also, I bent one of the paperclips so the straw would fit snugly over it. The only thing holding the straws to their corresponding paper clips is friction. If that is not working for you, you could tape it or something.

Great, but what next? It needs to be calibrated somehow. In my quest to use everyday things, I am going to use Lego bricks - the big ones. I hung various numbers of bricks from the force scale and used a marker to mark on the small straw the location that the two straws meet. Here is a picture of one set of hanging bricks (I made a video, but you couldn't see everything).

After hanging several different Lego masses, this is what my force scale looked like (I have decided to call it a 'scale' instead of a 'probe' - you know, because of the car from Ford).

Wow. I didn't realize just how poorly that picture came out. Note to self: stop using your phone to take these kinds of pictures. Maybe you can't read the ruler, but hopefully you can see the marks on the inner straw. Here is a quick plot of the distance the rubber band is stretched vs. the number o Lego bricks.

Since the amount of stretch is linear with respect to the force, I can just use a ruler and measure amount of stretch to determine the force. Note: I am assuming that each Lego brick as the same mass. Of course, if you don't have those big Lego bricks, you could use washers or nuts or anything that has a fairly consistent mass.

If I did want to use the distance to determine the force, I would have to know the slope of that line. From the fit above, I get a slope of 0.847 cm/Lego. Thus after measuring the stretch, my force would be:

This would give the force in units of Lego bricks. Then if you know the weight of one Lego brick, you could convert this to Newtons.

• Does the rubber band stay consistent? As you use it, does it need to be re-calibrated?
• Does the stretch per cm change with temperature (hint).
• Can you use different rubber bands to measure different forces? (thick vs. thin rubber band)
• Could you use two rubber bands in parallel? What would happen then? Could you use two rubber bands in series?
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You need to be careful here about the linearity assumption. Rubber-bands don't work like springs and don't always obey Hooke's law so well. I'm actually surprised your data points looks as straight as they do.

I'm actually surprised your data points looks as straight as they do.

Rubberbandgate!

@dzdt
Polymer chains may act decidedly non-Hookian, but macroscopic collections such as represented by a regular rubber band, in a small enough displacement vs k do behave pretty darned close. (Make sure you keep the rubber band at constant temperature and try and let the stretching be adiabatic.

The linearity assumption works well in the regime the author tested it in. Perhaps dzdt, you could repeat the experiment and include error bars?

I agree. I was surprised about the linearness of the data - but like Michael said, it is a small displacement at constant temperature.

My original plan was to make marks even if they were not linear. This could still be used as a force probe (just not as easy to interpolate) - and it would still be simple.

This would be a great science fair project. Just sayin.

Have you tried to overload the probe and have the students check to see if the calibration is still valid? I think this could make a very good lab to help students understand calibration and drive home the point that everything is a
harmonic oscillator to first approximation ;-)

a cool thing to do is to take a larger, say 3/8 to 1/2" wide rubber band. stretch it out. hold it stretched for about 10 seconds, then hold it lightly against your face just above your upper lip and let it contract. you ought to feel a temperature change. the rubber band cools very noticeably against your sensitive upper lip.

it is a demonstration of a temperature/entropy change.

@Ian,

Actually, I have not really done this as a lab yet. However, that is a great idea.