Science Over the EdgeA Roundup of Strange Science for the MonthApplet credit: Ed Hobbs
January 2004 |
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This is a longtime favorite of physics teachers and it has two answers. In the "ideal" situation that Ms. Savant is talking about each of the cars has a certain amount of kinetic energy as they move (let's call this value E). If they cars are identical in every way (speed, mass, shape) and they do a perfect head-on collision the total energy involved will be 2 times E or 2E. Since there are two cars the energy will be divided in half and each one will be subjected to one E of kinetic energy which will deform the vehicles turning them into something that looks like a two-ton metal accordion. If we are dealing with one car hitting a wall then we will have one E of kinetic energy before the accident (The wall has no energy, it's not moving). If the wall is absolutely rigid and does not absorb any of the energy of the impact then again there will be one E of energy to deform one car. This is why safety crash tests simulating head-on collisions are done by launching a car into a rigid barrier. It is a good simulation of a head-on collision, but only requires one vehicle. If this seem unintuitive try it at home (not with cars, use tennis balls). Roll them at each other and when they hit note how far do they bounce back. Now roll one at the same speed toward a immovable object (like a brick). Did it bounce back the same distance? It should have it you gave it the same initial speed. The second solution involves what happens in a non-ideal situation. Very rarely in the real world would a car hit a totally immovable barrier. If the barrier absorbed some of the energy less would left over to damage the car. Also if two cars hit it is unlikely that they would be identical in speed and shape or be aligned perfectly head on. They would tend to bounce off each other and penetrate each other as well as spin off at an angle and these actions would absorb energy. For these reasons in it would be hard to say which accident would result in more damage to the car in a real life situation.
An acoustic shadow occurs when sound that would normally be heard is blocked. This can happen because the sound is absorbed by some material, or refracted when it meets a warm or rapidly moving layer of air. The refraction can not only bounce the sound away from a location but it can also direct it to another location where it would not normally carry. In these ways an acoustic shadow is to sound what a mirage is to light. Acoustic shadows played significant roles in several Civil War battles. Because there was no radio communication between a commander and his forward units in the 19th century, commanders often depended on what they heard of the battle to tell them what was going on up at the front. If an acoustic shadow existed it could alter what the commander heard and ultimately what decisions he made. It is said that sounds from the Battle of Gettysburg could not be heard just ten miles away, but were heard clearly in Pittsburgh 150 miles away. There is at least one book on the subject of acoustic shadows and their effect on the War Between the States. Check your local library for Civil War Acoustic Shadows by Charles D. Ross.
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LGM: Science over the Edge ArchivesLGM Archive 1998, 1999, 2000, 2001, 2002, 2003, 2004. Copyright Lee Krystek 2003. All Rights Reserved. |