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Wednesday, 10 October 2012

Indiana Jones

Vi ricordate il giovane Indiana Jones che salta giù dal treno del circo?

"An especially interesting case arises when a projectile is hurled from the rear of a fast-moving train or other vehicle. Let us suppose that someone throws a stone, horizontally, down the track from the rear platform of a train speeding along at 60 miles per hour. And suppose that the stone is thrown at an initial speed of 60 miles per hour (relative to the train, of course) . Then, to the people on the train, the stone will appear to follow a perfectly normal parabolic path. But how will it seem to a person standing on the ground alongside the track? Remember that velocity is always relative. The forward motion of the train will just cancel the backward motion of the stone. In other words, the stone will plummet straight down to the ground, with no motion at all in the horizontal direction.
A similar situation arises when a bullet is fired from a speeding aeroplane. A revolver bullet, for instance, has a muzzle velocity of only about 500 miles per hour. If such a bullet is fired from the rear of a modern warplane speeding along at 500 miles per hour, the two velocities cancel, and the bullet at first stands still momentarily then falls straight down as though it had been dropped. On the other hand, if the bullet is fired from the front of the plane, the velocities add, and the speed of the bullet relative to the earth is 1,000 miles per hour. Of course, the machine guns used in warfare fire their bullets at speeds much greater than 500 miles per hour. Moreover, if the target is another moving plane, it is the speed of the bullet relative to this moving target that counts in determining the damage done not the speed relative to the earth. It makes no difference at all whether a revolver bullet stands still with respect to the earth and you run into it with a speed of 500 miles per hour, or whether you are standing still with respect to the earth and the revolver bullet strikes you with this speed. In both cases the effect is the same, and unpleasant for you."

From PHYSICS TELLS WHY, An Explanation of Some Common Physical Phenomena 

the Monkey on the String

Imagine a string passing over a pulley, with a monkey hanging on one end of the string, and an iron bob on the other end balancing the monkey. Monkey and bob are equal in weight, and both are initially at rest. The weight of the string and the friction in the pulley can be neglected.
What happens to the iron bob when the monkey begins to climb up the string? In other words, will the bob rise with the monkey, will it descend, or will it remain stationary?

To solve the problem we must apply Newton's Laws of Motion. When the monkey begins to climb, he is accelerated upward. Therefore, according to Newton's Second Law, the string must not only support the monkey's weight, but it must supply additional force for the acceleration. As a test of this conclusion, you might stand  on bathroom scales sometime when you are going up in an elevator. You will find that as the elevator starts upward, the scales will register several pounds more than your weight. The added push upward on the bottom of your feet serves to accelerate your body. For a simpler experiment, one which can be done less conspicuously, hang a weight on a string, and jerk upward. You will feel a sudden added tension in the string as the mass is accelerated.

Even though the monkey moves upward by his own efforts, there must be an added tension in the string to provide force for the acceleration. By Newton's Third Law the tension in the string must pull equally on the iron bob. Therefore, the bob is accelerated upward just like the monkey. The solution to the problem, then, is this: the monkey and the bob rise together.

When the monkey stops climbing, and thus decelerates, the tension in the string is decreased, and the bob comes to rest at the same time as the monkey. Likewise, if the monkey turns and starts down the string, the bob descends with the monkey.

From PHYSICS TELLS WHY, An Explanation of Some Common Physical Phenomena 

Does a Flying Bird Weigh Anything?

Does a Flying Bird Weigh Anything? ... Suppose that a bird weighing one pound is flying around in a five-pound cage. If you hung the cage on a spring balance,  would the scales record the weight of the cage alone, or the weight of the cage plus the bird? 

There is a story connected with this problem. Some years ago, a graduate student in physics at a large university decided to have some fun at the expense of two of his professors. A newspaper reporter was made a party to the scheme, and was persuaded to call each of the two professors on the telephone in order to ask his expert opinion on a scientific question.

Professor A was asked the following question: If a one-pound bird is flying in a five-pound cage made of thin wire, how much will the combination weigh? "Five pounds," Professor A told the reporter.

Professor B was then called, and a similar, but slightly different question was put to him: If a one-pound bird is flying in a five-pound cage made entirely of glass, how much will the combination weigh? "Six pounds," replied Professor B without hesitation.

The next day, much to the embarrassment of the two prominent professors, headlines appeared in the local paper: UNIVERSITY PROFS DISAGREE ON SCIENTIFIC QUESTION. A carefully misworded account of the questions and answers followed, with the words wire and glass omitted. No doubt everyone would agree that the bird and cage together would weigh six pounds, provided the bird were sitting stationary on its perch. But which of the professors was right in the case of the flying bird? The answer is that they were both right.  Since the bird is not falling, it must be supported by something. That something is the air. Because of the flapping of the bird's wings, the air pushes up on the bird with a force of one pound. The bird must then push down on the air with an equal and opposite force. This downward force of one pound is transmitted through the air to the first solid surface available. Since the wire cage would not have solid walls or floor, the air would push down, not on the cage, but on the ground below. Therefore, as Professor A said, the wire cage plus bird would weigh only five pounds. On the other hand, the glass cage would be impermeable to air, and in this case the weight of the bird must be borne by the cage. Professor B was absolutely correct when he said that the scales would then read six pounds. There is a moral to this story about the bird in the cage. It illustrates the necessity for precise statement in a scientific problem. 

From PHYSICS TELLS WHY, An Explanation of Some Common Physical Phenomena