Engineering for Writers #3

Today we are going to be thinking about force and pressure.

Every so often I come across a writer who uses one when they mean the other, so I thought I would do the writing world a favour and explain the difference.

Force is a measure of how much you push something, or how much you bear down on the ground, or the turning effect of a car engine on the wheels. We feel the force of the wind as it's gusts make us stagger, or as it sends a boat creaming through the water. We talk about being forced to do something if we are compelled to do it when we would not otherwise have done so.



Pressure is a measure of how much that force is spread out. The smaller the area over which the force acts, the greater the pressure. The force of the wind spread out over the area of a sail creates a pressure that can be seen in the way that the sail billows, but it is the force that drives the boat along.



Here's an interesting fact. When your kick-ass heroine stomps on the foot of the goon holding her with her size 5 stilettos, she is exerting more pressure than the articulated lorry that is concurrently running over the foot of the goon's side-kick. For proof the lorry weighs 1200 times as much as your delectable MC, but the lorry's weight is spread over 12 tyres, each tyre making contact with the ground over an area 800 times the size of a stiletto. So in total 1200 times the weight spread over 9600 times the area, meaning the stiletto exerts 8 times the pressure. If you didn't follow that, don't worry. Just remember to make your goon's eyes water.

A politician is forced to resign due to intense media pressure. There is a force applied by the media spread over a period of time (rather than area, as we talked about before)

So a force is an active, doing word that has results (which might be action or it might be a bruised toe). Pressure is a more passive word which describes how the force acts. You are knocked over by the force of a wave, not by it's pressure.

So on to today's MYTHBUSTER



Weight is also a force. It is a measure of how much we press down upon the earth. When we stand on our bathroom scales they measure our weight, yes, but the reading it gives us is NOT our weight. Bathroom scales read in lbs or kg which are units of MASS not WEIGHT. All the while we are running around on the face of the earth, this little deception doesn't really matter. However if you took your bathroom scales to the moon, instead of your usual 120lbs (55kg) of mass, it would tell you that your mass had reduced to 20lbs (8.5kg). As the only way of losing mass is a diet or having bits chopped off, such a loss of mass would be catastrophic. Correctly your scales, back in your bathroom, should tell you that you weigh 120lbf (pounds force being a measure of, you guessed it, force) or 550N (Newtons - 10 newtons is the force exerted by 1kg of mass in standard earth gravity). Technically, when you overindulge at Christmas you put on mass, but if you tell your work colleagues that you put on a little mass at Christmas, they'll think you've gone all religious!