# Explosion

Drop two eggs from about a metre high. One is a normal egg while the other has been emptied out so all that remains is the shell with a tiny hole from which the inside has been blown out. The normal egg will, as expected, smash into pieces making a mess. But the emptied shell, the hollow egg, will not crack. Why is that? It turns out that it’s because of the same reason why meteors catch fire when they rush through the atmosphere on their Earth bound trajectory. How? Let me explain.

Boom!

We are all familiar with the concept of mass and speed. It’s safe to assume that a golf ball is less massive than a bowling ball. A bowling ball is definitely less massive than a lorry. And you have meteors that are as massive as lorries. Fine, we all understand what mass is. As for speed, well, there’s this other quantity closely related to it and which, essentially, carry the same meaning. This quantity is called velocity. Whereas speed tells you about how fast the lorry is going, velocity gives you an additional piece of information about the lorry: it also tells you about the direction in which the lorry is going. So, velocity is speed and direction combined. Now why should the direction matter when we’re talking about moving objects? And why combine that with speed anyway? What else does that give us which speed doesn’t already tell us about the moving object?

There can be cases where the speed is constant while the direction of motion is changing. For example, when a lorry is going round a roundabout at constant speed. There is something going there which is related to the changing direction of the lorry as it goes round and round. To explain why the lorry can go round the roundabout in the first place we need to come up with, firstly, the concept of velocity (looking at both speed and direction) and, secondly, the concept of changing velocity. Because direction is always changing then, by extension, velocity must also be changing. And what happens when velocity changes? We get this new quantity called acceleration. So far, so good. These are the pieces of the puzzle we’re putting together to understand what’s going on when a lorry is going round the roundabout. Just bear with me a few more paragraphs and I’ll show you how this will all lead to the egg and meteor.

There’s another piece of the puzzle which is missing. It’s yet another quantity that we need to define. What that quantity does is combine velocity and mass. The product of mass and velocity gives rise to what we call momentum. Momentum is such a fundamental quantity in Physics that it’s got its own law. It’s such a key element that it holds a special place in the pantheon of physical quantities. Like energy, momentum is a conserved quantity. What that means is that, by the magic of physics, if a system starts off with a given amount of momentum then that amount will remain the same throughout the transformation of the system. A falling plate has a given momentum – simply multiply the velocity with which it’s falling with the mass of the plate. When it hits the floor it smashes into a million pieces – really, I counted. Yet, the amount of momentum which you had previously calculated will not change for that system of the fallen plate. If you consider each little piece of broken plate, determine their mass and their velocity, work out their individual momentum and, finally, sum it all up, you will end up with the same value you calculated for the whole plate just before it hit the floor. The total momentum got distributed among each single piece. Not only that but the smaller pieces will end up with the higher velocities while the larger pieces tend to be imparted with lower velocities. What comes out of this is a rather annoying situation: the small pieces are able to then travel further out than the larger pieces making it a nightmare to pick those tiny pieces up. They get lodged in the most inaccessible of places, the most remote corner, the deepest reaches underneath the furniture…

Now that we have the final piece of the puzzle we can put it all together to understand what’s going on. Momentum is closely related to force. The rate at which momentum changes determines the force needed to change that momentum. Assuming mass remains constant (just to make life easy for once), then what must change if momentum is changing is undeniably velocity. With changing velocity we have acceleration. This is how a force can cause an object to accelerate. This, after all, is Newton’s Second Law of Motion. That famous relationship between acceleration and force: F = ma. So, back to our lorry on a roundabout. Even though its speed is constant, it is nevertheless accelerating. Why? Because its direction is changing as it goes round. This implies a change in velocity which, in turns, determines the acceleration. And, because of the F = ma relationship, there must be a force allowing for this acceleration to exist. This force, it turns out, comes from the friction between the road and the tyres. Had there not been enough friction between the road and the tyres, the lorry would not have been able to go round the roundabout. It would, perhaps, have skidded. As counter-intuitive as it might seem, friction is necessary for motion!

Speaking of friction, this is what causes the meteor to blaze up as it dashes through the atmosphere. Friction is a whole area of study in itself but here it suffices to say that it manifests itself under different guises. The meteor approaches the Earth at a tremendous speed. When it hits the atmosphere it encounters a lot of resistance causing its initial high momentum to decrease in a short time. This is what a collision is, after all; a large decrease in momentum in a short period of time. This relatively big change in momentum is at the root of the heating up of the meteor. You see, because momentum has to be conserved then whatever the meteor loses in momentum, the air molecules surrounding it must gain. The air molecules having thus gained momentum will have their velocity increased significantly. As their velocity goes up, their temperature also goes up (see Thermal for an explanation of how this happens). So much so that they light up causing the meteor to burn up on its descent. If we’re lucky then the meteor will burn up completely in the atmosphere. If not then let’s hope the meteor or meteorite, to be precise, doesn’t crash in a populated area (see Crash).

Finally, we come to our pair of eggs. Why does the normal egg crack when it hits the ground but not the hollow one? Again, it has to do with how momentum changes and, therefore, how force is involved in all this mechanism. The hollow egg has less mass and so less momentum than the normal egg. As they both hit the ground, the hollow egg’s momentum will change from a small amount to zero while that of the normal egg will change from a larger amount to zero. The greater the change, the more the force needed for that to happen. The force to do so, thus, comes from the floor pushing against the eggs upon impact. For the normal egg, the force is large enough to cause it to crack. The hollow egg, on the other hand, will barely feel any force on it and will, most likely, remain intact.

The lesson to learn here is that, if you want to make the most damage then you have to cause the maximum change in momentum in as short a period of time as possible. The keyword here is momentum. This implies you have two variables to play with: mass and velocity. So, you can take a massive object, like a bowling ball, and drop it on the floor causing the floor to crack. Or to impart the same amount of damage to the floor, you can use a less massive object, like a golf ball, but throw it – not just drop it – but throw it with a very high velocity so that it imparts the same momentum or force to the floor as the bowling ball did. A bullet is even more lethal. Even though it’s not very massive, its velocity is so high that when we look at its mass and velocity combined then the momentum is large enough to cause serious damage.

This reasoning not only applies to collisions, by the way. The opposite situation is also based on the same principle. When you look at explosions, a large increase in momentum in a small period of time can be very dangerous indeed. So, if there’s one thing you have to bear in mind about this whole business of collisions and explosions it’s this: to make the most impact always think about how you can cause the greatest change in momentum in the shortest time. Boom!