by Michael Skinner

Created on: November 07, 2009   Last Updated: June 20, 2010

A good bridge is static or non moving structure. Although there are forces acting upon it at all times, forces like tension and gravity, they are in harmony so the bridge does not move. There was a famous case of a bridge where in the forces were not in harmony and so the bridge moved - while there were people and cars on it. The bridge was called Galloping Gertie. The problem was that there were winds blowing on the bridge. On some level you can think of a bridge like a guitar or violin string. If struck or strummed in the right way the bridge reacts by vibrating. A force hits the bridge and the bridge moves. A properly built bridge has some sluggishness or dampers built into it. These dampers cause vibrations to dissipate. Galloping Gertie did not have dampers that would dissipate the energies of the fundamental frequency of the bridge. The fundamental frequency is the frequency at which the bridge wants to vibrate when struck. Now the wind that caused the bridge to vibrate kept blowing. It kept adding energy at the fundamental frequency. Each action of the wind caused a reaction of the bridge. Since the harmonics or vibrations were insufficiently dampened, the amplitude of the vibration grew. This means that the swings from side to side and up and down grew larger and larger. Any real structure can only bend and twist and vibrate to a finite degree. For Galloping Gertie that finite degree was met and surpassed and the bridge collapsed - taking some cars with it.

Newton's second law is Force = Mass * Acceleration. You are experiencing this law when your aircraft leaves the runway and rises through the atmosphere. That gentle force pushing you back into your seat is the thrust put out by the jet engines. Because you and the aircraft experience a force, you accelerate or move faster. Once the force stops you cease accelerating and you no longer feel that force pushing you back into the seat. As in all things, it's actually a bit more complicated than that. As the aircraft flies in level flight, forces are still required to keep it aloft but from your point of view as a passenger the forces are nearly balanced so you don't feel a push in any particular direction. If you didn't have a window you might not know you were moving at all.

Newton's third and final law is that every action has an equal and opposite reaction. A rocket ship blasting off to the moon is the easiest way to see this law in action. Physicists say that what the world wants to do is to reach equilibrium and harmony. Equilibrium literally means equal weight, or to us, it means the forces are balanced. The problem with a rocket is that before you lit that sucker nothing was moving. After the rocket engines fire it looks like everything is moving. This would appear to violate the principle of equilibrium. Somehow we must make the equations that describe the motion of the rocket when it was not moving look like the equations when it is moving.

You know what screwed everything up? It was when we lit that rocket engine. Suddenly hot gases were forced to all go in a single direction. That meant there was a whole lot of force all going in a single direction. We can't have that. We do not live in a universe where forces suddenly appear out of no where. The only way to balance the scales is for the universe to create an equal force in the opposite direction that the hot gases are flowing. Then, when we sum up all the forces, we will have our beloved zero, our equilibrium, once again. And so when the rocket ignites and the hot gases flow in one direction, an equal and opposite force appears and the rocket lifts off.

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