You know you’re a physics geek when the first thought that comes in to your head when watching all those car crashes on the news is: Now that’s what I call Newton’s first law of motion!
Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force.
Now most people think that this force simply slows the car down, but in the world of physics the word velocity covers two quantities; speed and direction. So it could be that the force merely changes the direction of the moving object, and has (little or) no effect on its speed. In fact that is what is happening when an object is moving in a circle (like a stone tied to the end of a rope circling over your head; the stone is moving at constant speed yet we still say that it is accelerating because its direction is changing). In this case the force is provided by the tension in the rope acting inwards.
We demonstrate this in class with an air-track, which is a bit like an elongated air-hockey table. A nice way to re-inforce the concept is to discuss why, when moon-bound rockets leave our gravitational field they can turn off their propulsion system and will remain moving at that speed untill they reach the moon’s gravitational field (although technically both gravitational fields are infinite – but that’s for another day).
Galileo himself (for it was he and not Newton who first promoted this) had great difficulty persuading others of the importance of this discovery.
The response of the students to the air-track demo is a reminder of how strange this idealised world of no friction actually is to us.
But every now and again we get to experience it for ourselves.