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Education and outreach

Education and outreach

Amazing science demo four: Ball River Bobsleigh

22 Apr 2015
Taken from the April 2015 issue of Physics World

This is the fourth in a series of “five amazing physics demonstrations” presented by science-demo guru Neil Downie and his adept assistant Matthew Isbell.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how you would present each demonstration to an audience. Full details of how to access the digital edition are available at the bottom of this article.

In this fourth demo from the series, Downie and Isbell go head to head as they race their homemade vehicles along a track lined with ball-bearings. Downie’s vehicle is a propeller-driven half-pipe, while Isbell has constructed a pressure vessel from a soft-drinks bottle and a valve. Watch the video to find out whose vehicle wins!

Ball River Bobsleigh

So what’s this all about? Believe it or not, ball-bearings make the world go round. Drive a car, ride a bike or travel by bus or train, and you’re rolling along on ball-bearings. Run an electric motor and – unless it is very small – rotation is assured by ball-bearings. In fact, these tiny metal spheres have been so important to industry that ball-bearing factories have been wartime military targets. This project shows how a vehicle moves on a bed of loose ball-bearings.

What bits and pieces do I need? You need a long length of plastic guttering that’s normally used to collect water draining off a roof – the kind that’s got a U-shaped cross section. Spread evenly inside the guttering lots of small, round glass beads about 3–4 mm in diameter and then tape up either end of the guttering to stop the balls getting lost. The simplest “vehicle” is a plastic soft-drinks bottle. Put a car-tyre valve in the base of the bottle and a pinhole in the screw cap so that, once you’ve pumped it up, air will be released in a continuous stream over a minute or two. Place a small weight in the bottle – such as toy figures as passengers – so that it doesn’t topple over. Another option is to make a vehicle from a smaller piece of guttering, to which you attach a propeller powered by a small electric motor.

How do I get going? Simply place the vehicle on the ball-bearings at one end of the guttering – and let go. With luck, the vehicle will quickly accelerate to a few metres per second, possibly swaying from side to side and with a few beads occasionally bouncing into the air.

And what physics will I learn? Ordinary “sliding” friction is huge – the force needed to move an object on a horizontal surface can be 30–60% of the vertical force (i.e. its weight). But “rolling” friction, which this experiment shows, is hundreds of times less and can be engineered to be less than 0.1% of the vertical force. The experiment also illustrates the complex physics of ball-bearings, which concerns how the surface of the bearings and the surface they’re rolling along deform. Ball-bearings come in many different forms, and to keep friction down, the key is to use materials such as steel that are hard yet spring back efficiently. Big balls are also better. As for the top speed of the vehicle, it depends on various factors, including rolling friction, air drag, the pitch of the propeller and the kinetic energy needed to accelerate the balls that it rolls over, which it doesn’t get back in full.

  • If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full access to Physics World both online and through the apps
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