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Recently, one of the structures on the bridge linking Oakland to San Francisco collapsed. Since this happened soon after maintenance had been done it was an embarrassment for the city when the bridge had to be closed.

What's the big deal? How come you can't just slap and dash up a bridge and be on your merry way? Physicists and engineers know that every load bearing member of a bridge has forces on it. A bridge, a static structure, demonstrates one of Newton's laws of motion. Every object in motion remains in motion and every object at rest remains at rest unless acted upon by an unbalanced force. When a bridge is designed, an engineer applies Newton's laws on every bolt and makes certain that the forces are balanced in every direction.

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

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