chadd
07-16-2007 13:32:57
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Re: horsepower of a farmall 450 in reply to Janicholson, 07-14-2007 17:19:09
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Am I right in understanding that you disagree that a force is equal to an object's mass mulitplied by its acceleration? Correct me if I am wrong, but is this not stated in Newton's second law of mechanics as follows (taken from Vector Mechanics for Engineers, page 4) "F=ma; where F, m, and a represent, respectively, the resultant force acting on the particle, the mass of the particle, and the acceleration of the particle expressed in a consistent system of units." If you simplify a porch on a house by a cantilever beam, it does experience forces and moments, as you stated. The downward force is the mass of the porch, (which acts at its center of mass) multiplied by the acceleration due to gravity (usually either 9.8 m/s^2 or 32.2 ft/s^2, although gravity is a weak function of elevation). The reason the porch doesn't fall is that it is balanced by an equal and opposite force upward at the base of the cantilever. However, because the two forces do not act at the same point, there is a bending moment present. Because the porch or canilever is static, it must be balanced by an equal and opposite moment at the base of the cantilever. I am not trying to play devil's advocate, and instead am just trying to understand.
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