dynam2 - Physics 53 Dynamics 2 For every complex problem...

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Physics 53 Dynamics 2 For every complex problem there is one solution that is simple, neat and wrong. — H.L. Mencken Force laws for macroscopic objects Newton’s program mandates studying nature in order to discover general laws of force. Newton showed how powerfully successful this could be when he found the Law of Universal Gravitation, which we will discuss later. Classical physics describes objects we can observe using our senses directly. These are often called macroscopic objects. Since the 19 th century we have learned that the truly fundamental force laws refer to interactions in the microscopic world of atoms, nuclei and sub-nuclear particles. Behavior of an everyday object is the average or net effect of a very large number ( 10 23 ) of interactions of these microscopic particles. Because the number of particles is so large, the statistical Fuctuations of the average behavior are extremely small, so classical physics gives very reliable results. Except for gravity, all of the forces we experience directly through our senses are in fact electromagnetic at the microscopic level. The details of electromagnetism will be studied in the next course. Here we deal with everyday forces which are macroscopic effects of microscopic electromagnetic interactions. Catalog of force laws Here is a catalog of force laws used in applications of Newton’s laws: Name Symbol Formula Notes Gravity (weight) F g , W F g = m g Near earth’s surface g is toward the center with magnitude g 9.80 m/s 2 Physics 53 1 Dynamics 2

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Normal force F N , N This is a repulsion between atoms at the interface between two objects, to prevent interpenetration. It is normal (perpendicular) to the interface surface. In our applications its strength is as much as needed, and deformation of the surfaces is neglected. Tension F T , T A “pull” on an object attached to the end of a string or rod. It is away from the end and toward the center of the string or rod. The strength is as much as needed, up to the point of breaking or signiFcant deformation. Compression F N , N A “push” exerted by a solid object, basically like a normal force. The direction is away from the center of the solid object. Normal forces, tensions and compressions do not act at a single point, but are distributed over the contact surface, in the form of stresses . Elastic force
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This note was uploaded on 10/19/2011 for the course PHYSICS 53L taught by Professor Mueller during the Spring '07 term at Duke.

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dynam2 - Physics 53 Dynamics 2 For every complex problem...

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