Friday's Physical Law - Motion

Welcome to Friday’s physical law (FPL) - a weekly look at how physics governs everything, and as this is the first of this series I will be starting somewhere we hopefully can all understand with simple motion.

Everyday experience would seem to indicate that the natural state of motion of an object is a rest. An object in motion will slow down and stop unless a force is acting on it. This was the reasoning of Aristotle. Of course this is what we see here on Earth in the presence of what we now recognise as friction.

We can quite easily see the truth of what happens if we remove the force of friction, imagine if you will you are on the ISS and you push off one wall, you keep drifting until you hit the other wall, this is because now the friction that is acting on you in now minimal. We can see examples of reduced friction on Earth such as skating on ice, or ball bearings.

This leads us to the understanding that we now have, Newton’s first law: That an object remains in a state of steady linear motion unless a force acts on the object. This means that an object at rest remains at rest and an object moving in a straight line will continue at the same speed and direction of motion unless a force acts on it

And our experience of friction that slows down the object is recognised for what it is: a force on the object acting against its motion. It arises from the interaction of the object and the media (substance) through/on which the object is moving. We can tell definitively about the existence of friction by the heating of an object as it slows down, the friction dissipates the motion (kinetic energy) of the moving object turning it into heat.

The effect of friction itself depends of only two things:

1. The object which is moving (i.e. what it is made of, its shape and weight)
2. The media through/on which the object is moving

As you can see the former is really a combination of a number of factors - the most important being those listed in the bracket. Obviously if changing what the object is moving through makes a difference then so will changing the object, the shape is important, the less contact there is the less friction there will be (think of a ball bearing), and the greater the mass the stronger the interaction between the object and the surface it is moving on (ok so to be completely honest the weight factor only applies to motion across a solid surface and not to motion through a fluid).