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.
Things to think about
- 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?