# So how much does air weigh?

Started off the day thinking it was a Monday, meaning it was pretty slow going. Anyway, sorted out an issue which I then found out actually wasn’t so ended up reverting it. Then implemented rolling resistance, which is basically just a coefficient of the downward force. So the harder something is forced into the ground the more resistance it gives.

Took the dog out for a run as it was really nice weather, still very cold, but the sun was out. It’s the first time we’ve done that in months. It was fun but she does slow me down quite a lot, but will give it ago again if the weather is nice.

Then came back to do aerodynamic drag. Now this is quite interesting, aerodynamic drag is the force an object is subjected to when its travelling at a velocity. The faster the object, the more the drag. The drag can build up and be a very important force as it’s actually the square of the velocity multiplied by a coefficient. So rolling resistance is important up to a certain velocity and then aerodynamic drag takes over. So what is this magic force? Well it’s actually the weight of air. So what’s the weight of air? Well first a distinction, weight and mass are not the same thing. Weight is actually mass under the force of gravity, so w=mg where m is the mass and g is the gravity. Gravity is normally taken as 9.8 or 10 for simplicity, it’s an acceleration so it’s units are meters per second per second or ms^-2. So anyway, back to air, what’s it’s mass? Well this actually depends on air density or how close the particles are together and this depends on temperature. So air at about 15 degrees centigrade is about 1.2 kilograms per meter cubed, which when you think about it is quite heavy. So no wonder when you drive a Volvo at 100Mph the aerodynamic drag is quite a force to be reckoned with.

Did another Japanese lesson, it’s getting very tricky and got a bit tired towards the end.