A teaching boast

Champion of constant speeds

A science teacher colleague, when talking about forces that add to zero and constant speed, always boasts to his class that he is a world champion in achieving constant speeds. The build-up to his claims of special expertise is so great that the pupils are soon clamouring to find out what he does to achieve these constant speeds, neither speeding up… nor slowing down.

And so the story goes:

Teacher: Well first of all I jump into my car and drive up onto the M62 motorway. Then I get into the outside lane and accelerate up to 70 miles per hour. Once at 70 mph, if the air resistance increases, and I start slowing, I gently press my foot down on the accelerator to increase the driving force. If I start speeding up, I think driving force greater than air resistance and I take my foot off the accelerator slightly to bring the forces back in line. It's such fun driving the car and keeping those forces in balance!

If my colleague is having a really good day, he goes further by claiming that he can also achieve constant speeds with pencil cases. With further great build-up he gets the class around a bench, borrows a pencil case and pushes the case at a constant speed across the surface.

Teacher: Ooops, just a little too much on the pushing side there, keep the push equal to the friction. Just look at that! Steady speed with no resultant force!

Amazingly his classes seem to enjoy these antics.