Science - It's a dog's life

Teach terminal velocity with the help of man's best friend

Simon Porter

Convincing a row of Year 11 girls that a knowledge of the physics of terminal velocities could one day save their lives (when they are falling from a helicopter, James Bond-style, having just rescued the population of Rome, perhaps) is not always easy. But humour - particularly the "running gag" - can be a powerful weapon.

Teaching terminal velocity requires a staged approach, where the teacher can explain each development in the motion, and a interactive whiteboard lends itself to this tactic.

A dog (accidentally) falling from an aeroplane at first has only the force of gravity pulling it down, so it will accelerate (slide 1). As the dog goes faster (slide 2) the force of air resistance increases but, because it is still less than the force of gravity, the forces are unbalanced and the dog continues to accelerate. Eventually (slide 3), the force of air resistance becomes equal to the force of gravity, so the forces become balanced and the dog falls at a constant speed (called terminal velocity).

The staged approach means the teacher can go back and forth when students need a point repeating or reinforcing. Of course, the dog meets a nasty end. ("Oh, Sir - why don't you like dogs?")

The staged approach - and unfortunate dogs - can also be used to explain orbital motion and centripetal acceleration.

A dog can be kicked horizontally off a table only a certain distance (slide 1). A small cannon is required to fire the dog further horizontally, but the dog is still pulled down by gravity (slide 2). A larger cannon (slide 3) will fire the dog even further than expected as the curvature of the earth now has to be taken into account, although of course gravity continues to pull (and thus accelerates) the dog towards the earth. A very large cannon will fire the dog even further around the earth before gravity pulls it down (slide 4). Eventually, a "humungous" cannon (slide 5) enables the dog to complete one whole orbit while still "falling" towards the earth.

Again, the use of the interactive whiteboard means teachers can go over each step as often as they like. Slides can even be printed out instead of pupils wasting time copying diagrams.

My use of the dog helps to introduce humour and a deeper understanding of some physical processes to my classes. And no dogs were harmed in the writing of this article.

Simon Porter teaches physics at the British School in Warsaw

What else?

To use Simon Porter's "dogs falling from aeroplanes" and "dogs in orbit" resources, download them free from TES Resources.

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Simon Porter

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