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Three quick experiments on friction

These experiments will open your eyes and get you thinking about how the world works. They'll get you ready for the theoretical models of friction.
Here are some basic friction tests you can try for yourself. They are simple to set up. We’ll put a book on a flat horizontal surface and push it. Gently at first, and then harder until the book moves. Often, once the book starts moving, you need to push less to keep it moving. You could try it on various surfaces to see how it varies.
Now we’ll add a few more books and repeat the test. We would expect the same behaviour but with larger forces. We can show the same thing by increasing the inclination of a surface until the book slides, like this. We’ll try reducing the angle a bit and give the book a push. Yes, it keeps sliding. We’ve just encountered limiting static friction and kinetic friction. We’ll explore that more with the analysis. Here’s something else to try. We’ll take a tall box– this magazine box will do– and try pushing it at the top. If the surface is rough enough, the box will pivot about the bottom front edge and tip over. Like this. Now we’ll try pushing near the bottom. Right. It slides.
You might have noticed the same behaviour if you’ve tried to move furniture along a carpet. Now for something quite different. Here are the two weight pans joined with a length of string.
Each pan has 10 washers. Next, we’ll hang them over a circular rod, like this. Now we’ll swap weights over from one pan into the other until the system moves. We’ll record our results.
Now we’ll take a turn around the rod and swap the weights until the system moves again. We’ll record our results, again. We’ll supply the results in the next activity when we analyse this behaviour using a simple theory of friction.

These experiments will open your eyes and get you thinking about how the world works. They’ll get you ready for the theoretical models of friction that are to come. They cover ‘dry friction’.

Dry friction is quite different from the fluid friction that is found in lubricated bearings for example, or the friction drag as a truck pushes its way through the air. We’ll look at air friction in Week 3.

You can download instructions to the experiment in the Downloads section below. Data for the rope around a bollard experiment can be found in the upcoming analysis step.

Talking points

These experiments are effective and so simple that we assume that you have done them.

  • What did you notice in your experiment with the book as it started to move?
  • What did you make of the tip/slide phenomenon? Perhaps you have noticed this while moving furniture on a carpet or shifting a refrigerator?
  • Those of you who use ferries might have seen how the deck-hand takes many turns around a bollard to secure the vessel against the wharf. Any sailors among you will know to take multiple turns around a winch to increase friction. Did anything else come to mind?

Share your experiment

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).

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Through Engineers' Eyes - Expanding the Vision: Engineering Mechanics by Experiment, Analysis and Design

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