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U4Rdng-forcedgrm

# U4Rdng-forcedgrm - UNIT IV Reading Force Diagrams The...

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UNIT IV: Reading - Force Diagrams The analysis of a problem in dynamics usually involves the selection and analysis of the relevant forces acting on some object under consideration. An important first step in this analysis process is to carefully select the object of interest that will be the focus of our analysis. For purposes of this analysis, we will refer to the object under consideration as the system, .and everything else in the environment that might in any significant way affect the system as the surroundings . This analysis process can often times be greatly simplified by utilizing a technique of constructing force diagrams to assists you in selecting the relevant forces and appropriately representing these forces with vector notations. For the purpose of developing a force diagram for problem analysis, all forces will be categorized as either contact or long range forces. Contact forces are all forces acting on the system under analysis that result from the contact between the system and its surroundings at the systems boundaries. These forces include forces of static and kinetic friction, tension forces and normal forces . Long range forces result from the systems interaction with a force fields of some kind, such as magnetic, electric,or gravitational fields. Consider, for example, the forces acting on a block being pulled along a horizontal table top by a string attached to the block, passing over a pulley, and terminating with several masses attached to the string by a mass hanger. (See Figure 1). Figure 1 Notice first that the block makes direct contact with both the surface that it is sliding over and with the string. The block, additionally, interacts with its surroundings by way of its interaction with the earth's gravitational field. As the block slides along the surface, there is one component of force resulting form the sliding action that acts parallel to the surface and in a direction opposite to the direction of motion (resists the sliding motion). This component of surface contact force is designated as the force of kinetic friction * . In addition to the parallel

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