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posted by  holl on 9/16/2008 10:50:48 PM  |  status: Live  

STATICS QUESTION

Course Textbook Chapter Problem
Engineering Mechanics Vector Mechanics For Engineers (8th Ed) 2 N/A
Question Details:
I was wondering if anyone could tell me how to decide which way the force goes in a tension problem. So for example, in problem 2.109 in the Vector Mechanics for Engineers (8th edition) The forces go toward A. In other words, the forces in the figure go BA, DA, CA. Why do the forces not go AB,AD,AC? I mess the direction of the forces up almost every time I do a tension problem, so if someone could give me some tips I would greatly appreciate it. Thanks.
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posted by Anonymous on 9/16/2008 11:20:37 PM  |  status: Live
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Remember Newtons third law states that forces are equal and opposite. There is a force pulling back due to the wood. However, in this problem it is in equilibrium. Therefore because of the weight of the wood the point A is your concern because it is the opposing force causing a zero net force.

When you lock your fingers and try to pull away both hands feel an equal and opposite force. One hand is pulling one way. The other is pulling equal and oppositely to maintain equilibrium. If your thinking about your right hand it has the force of your arm pulling one way and the force of your left hand pulling the other way. Incorporate this into the chains holding the wood up.
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posted by Anonymous on 9/18/2008 9:06:53 AM  |  status: Live
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your problem is very easy to answer..
just remember that when you are dealing with tensions, the direction of the tensional force is always
directed away from the body... it is because lines and cables do not exert a compressional force..
when you draw your free-body diagram, the tension is always away from the body in study..

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