About this blog

Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Tuesday, 2 August 2011

Recoil and Other Unwanted Forces

Not that I have ever used a gun myself, and I imagine that it’s not very common here in the UK, but for gun users around the world, apart from the obvious problems of getting shot, it’s the recoil that is the biggest problem. What I mean by recoil is the way that the gun jumps back away from the bullet as you fire, causing many an injury to an unsuspecting user. And of course the reason for this: Physics!

This is an example of Newton’s Third Law which states that for every action there is an equal and opposite reaction. So, the gun forces the bullet to fly out, but the bullet also forces the gun to move backwards. But why doesn’t the gun injure you as much as a bullet would? Well, the gun is a lot bigger, so the force has less effect.

When you think about it, Newton’s Third Law (sometimes abbreviated to NIII) is totally logical. Whenever you interact with something physically then you feel the force exerted on you by the object, and the object is subjected to the same force from you. If we take a less violent example, imagine trying to push a large box on a slippery surface. If you’re wearing socks (so there’s no friction with the ground), you might actually move backwards rather than pushing the box forwards. You’re both experiencing the same force, just the friction on the box’s base is higher than on your own base (your feet).

This is useful to remember whenever you come across force problems, because sometimes not all the forces will be marked in. What I like to do when trying to solve such problems is to make a free body diagram, which shows all the forces that are acting on one body, isolated from the rest.

Look at the truss system below. It’s far too complicated to work out what’s happening, say, at joint A if you look at it in its entirety, but extract the joint and then work out what is acting on in and you’ll be on your way to finding a solution!

So next time you clobber your little brother or sister around the ear and it hurts your hand too, you’ll know it’s not their fault, just their ear exerting an equal but opposite reaction on you!

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