PPT9a10a - Is there Pressure in Phys 151? An interesting...

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Is there Pressure in Phys 151? An interesting aside: When a force is applied it is usually over a large surface since real objects are not point-like. This brings in the concept of pressure - again not the usual definition. In physics, pressure P is defined as the force F per unit area A, i.e. P = F/A F It is pressure that explains how a number of interesting phenomena occur. 1. I can lie on a flat bed of nails without any discomfort because my weight (a sizeable force) will be spread over a large area (my body in contact with the nails). 2. Bicyclists can become impotent! The small area of contact with the seat creates tremendous pressure around the genital area and on the nerve fibers there. 3. Eskimos use large “flapping” snow shoes to walk through soft snow. A
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What’s The Matter? As we mentioned in the previous slide, we have neglected the nature of the objects in our discussion of motion and pretended that they are like point, not material, objects. Water spun around in a bucket was treated no differently than a solid ball twirled at the end of a string. In truth there is shape, structure, color, etc to real objects. It has been possible to avoid these “extraneous factors” because for ordinary mechanics and with the accuracies we can expect to attain, only the mass is of paramount importance. Now, as we move to other topics, we have to bring in the other properties of matter. Objects in nature exist in three common forms: solids - like a block of wood, liquids - like water, and gases like the air in a balloon. Most materials can be viewed as being made up of a whole lot - billions upon billions - of very small molecules -almost like tiny spherical balls, too tiny for the eye to see.
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Earth, Water, Air, Fire If, in the microscopic world, they are tightly held together through stiff bonds in regular “lattices”, they are solids - hard, “heavy”, compact with a given shape in our world. If they are held together rather loosely without any form, they are liquids - “soft”, with a changing shape. If they are almost free of each other, they are gases in our world - “light”, given to “wandering around”. Solids, liquids and gases have
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This note was uploaded on 02/08/2011 for the course PHYS 151 taught by Professor Lim during the Winter '10 term at Drexel.

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PPT9a10a - Is there Pressure in Phys 151? An interesting...

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