What the lesson is about
Teaching terminal velocity needs a staged approach where the teacher can explain each development in the motion - a smartboard can help, and a bit of humour, writes Simon Porter.
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 so 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 rapidly between the slides when pupils need to have a point repeated or reinforced, and slides can be printed out for them to stick in their books.
Taking it further
The same approach can 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 horizontally further, but the dog is still pulled down by gravity (slide 2). An even larger canon (slide 3) will fire the dog even further than expected, as the curvature of the Earth now has to be taken into account, although gravity continues to pull - and thus accelerates - the dog towards the Earth, and so on.
Where to find it
Simon's "dogs in orbit!" and "dogs falling from aeroplanes" can be downloaded free from TES Resources.