Problems in Statics Involving Moments related to rigid bodies

by jnbfive
Tags: bodies, involving, moments, rigid, statics
 P: 47 I have a couple problems that I can't seem to get. The first I'm really close, I can tell, because my professor did one that was similar in class. The answer the book gives is 250 lbs, I'm coming up with 225. The second I think is similar to another problem he did in class but I'm not quite sure. I'll post it in the next reply. Attached Thumbnails
 P: 47 This is the problem that I think is similar to the one labeled 3.71. I just want to know if I'm correct in my assumption the two problems have a similar premise. Attached Thumbnails
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 Quote by jnbfive This is the problem that I think is similar to the one labeled 3.71. I just want to know if I'm correct in my assumption the two problems have a similar premise.
The premise seems similar (Moment = force times perpendicular distance). For the force on the nails in part 1, one of your arrows points in the wrong direction.

 P: 47 Problems in Statics Involving Moments related to rigid bodies I need help on part c of 3.71. I have the angle from b, which I believe I need to use. I originally thought that the way to set it up would be 86.2 = x(22.36)*sin(53.1) ^22.36 coming from the sqrt of 17.6^2+ 13.8^2 Any help on what I'm doing wrong would be appreciated.
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 Quote by jnbfive I need help on part c of 3.71. I have the angle from b, which I believe I need to use.
how did you determine the angle that requires the min tension?
 I originally thought that the way to set it up would be 86.2 = x(22.36)*sin(53.1)
check angle and position vector
 ^22.36 coming from the sqrt of 17.6^2+ 13.8^2
looks like it should be 15.2^2 + 13.8^2
 Any help on what I'm doing wrong would be appreciated.
Note that if you are trying to minimize the tension, you want to maximize the perpendicular distance from the line of action of the force. Forget the r*T*sin theta approach.
 P: 47 *I took the inverse tangent of 11.4/15.2 *The angle of 53.1 is correct; thats the answer the book got. *Why only 15.2? *Thanks
 P: 47 If I could actually ask for help in another question, any help would be appreciated. Excuse my chicken scratch work. I solved for the angle at which the force is acting at, which is 30 degrees in the xz plane. I solve for the actually distance from the x axis that said force was, which was about 387 mm or 100*sqrt(15). Given this I solved for the distance from the origin, 100*sqrt(19). I found my force, the set up a cross product. My answers are listed as such. I have the i vector correct, j is off by 8 (should be 22) and k is off by 1 (should be 39.1). I feel as if I'm really close to this one, was just wondering what I'm doing wrong here. Attached Thumbnails