As mentioned before, even though the explosives put on an impressive show for crowds, it is really gravity which does most of the work. Doug Loizeaux, a renowned demolitions expert, once said: "Everything vertical to the horizon wants to fall. We just help it." Here we will look at the significance of gravity.
What is gravity?
At sea level, acceleration due to gravity is approximately 9.8 meters per second squared (or 32 feet per second squared), meaning that for every second an object falls, its speed increases from 9.8m/s after one second to 19.6m/s after two seconds to 29.4m/s after three seconds and so on until it reaches terminal velocity, the point at which the air resistance equals the weight of the falling object. To better comprehend the awesome force of earth's gravity, think of this: we have all heard of automobile ads which claim that their cars "can accelerate from zero to sixty miles-per-hour in six seconds." If one were to drop that same car off a really high cliff, after six seconds the car would be traveling more than 132 miles per hour!
The Controversy: Weight Vs. Mass
While all objects will fall at the same rate, no matter how massive they are (excluding wind resistance), the objects weight can vary greatly. It is important to understand that an object's weight is different from its mass. An object's mass is a measure of the amount of matter an object contains. In the SI system, the unit for mass is the gram. The weight of an object, however, is a measure of the force of gravity acting upon an object and its SI unit is the newton. Another way of distinguishing between the two is that an object's mass
never changes; an object with a mass of 500 grams on Earth will have a mass of 500 grams on the moon even though it is "lighter" on the moon than it is on earth.
There is one final term we must know before continuing, and that is force. I have already preluded to to this term when I defined weight. A force is the ability to accelerate or change the shape of an object, and, like weight, is measured in newtons under the SI system. The formula for force is:
Force = mass x acceleration.
For what we are interested in, the acceleration is the earth's gravity (9.8 m/s/s), which really is, in fact, the object's weight.
Getting back on the topic, we can now understand that a structure's weight applies force to whatever is supporting it. If one were to take away whatever is supporting the structure, the force of gravity will cause it to collapse or fall, similar to pulling a chair out from someone who is sitting on it. What the explosives do is to merely knock out what holds the structure up, allowing gravity to do the work of pulling it down and breaking it apart. Because of the fact that gravity does most of the work, in order for a building to be "blown up," it must be at least five stories tall to allow gravity to take effect on the structure. However, it is not just the simple case of removing whatever is holding the structure up. Getting the structure to fall just right is the primary objective of a blaster which involves careful timing, intensity, placement, and direction.