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Forums
Engineering
Mechanical Engineering
Signs of Tensile and Compressive Stresses
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[QUOTE="PhanthomJay, post: 6234362, member: 58874"] The plus and minus signs have multiple meanings in Physics and Engineering. Similar perhaps to your example, if you assume that a force acts down and you get a negative solution for that force, it means that you assumed the wrong direction for the force, and that it actually acts up. If you got a positive solution for that force, it means you assumed the correct direction for that force, even though it may be a compressive force. Forget about actual signage for tension and compressive axial forces in a free body diagram...I mean how do you assign a positive or negative values to a force acting at a 45 degree angle? The rule is this: if the force points inward toward the joint or member, it is a compressive force. If it points outward away from the joint or member, it is a tensile force. For the compression case , you call it a negative force, for the tension case, you call it a positive force, by convention. So don’t give it a sign until the end. If you assumed the direction of all forces correctly up front, you are going to get all positive numbers. Only after you determine if it is a pushing force or pulling force can you then give it a sign. Or don’t give it a sign, just say for sample that ‘the force is 10 N compressive’. [/QUOTE]
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Signs of Tensile and Compressive Stresses
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