Blossom'sGirl Posted October 12, 2011 Share Posted October 12, 2011 (edited) This is on p. 105 of the CPO Middle School Physical science book A box has a force of 60N up and 80N to the east. What is the net force? I get 100N at a 45 degree angle in a north east direction. The book gets 11.8N in the same direction. I haven't done these in years but I thought it was a simple a squared + b squared = c squared. Edited October 13, 2011 by Blossom'sGirl Quote Link to comment Share on other sites More sharing options...
Miss Tick Posted October 12, 2011 Share Posted October 12, 2011 A box has a force of 60N up and 80N to the east. What is the net force? I haven't done one of these in years either, but it seems to me that you need to have the forces acting from the same point, the "box" (i.e. don't start the 80N force from the end of the 60N arrow). Then, when you draw your 60/80/100 right triangle the resultant force will extend from the point ("box") to intersect the 100 side of the triangle (forming more right angles). Sorry I don't have time to try to solve it now. I looked at it a little - do you need to use the law of cosines? Quote Link to comment Share on other sites More sharing options...
Blossom'sGirl Posted October 12, 2011 Author Share Posted October 12, 2011 It is a very general problem designed for middle school students. My guys haven't even covered the Pythagorean Theorem yet, so I 'm not sure how they would solve it. The arrows shown on the diagram do not meet, they both extend from the box. I like your idea, Susan, but it seems too complicated for this level. The other problems are simple: A box with an arrow pointing up labeled 50N and one pointing down labeled 25N. Obviously the net force is 25N in the up direction. I was just wondering why I am not getting the same answer as the book. Quote Link to comment Share on other sites More sharing options...
regentrude Posted October 12, 2011 Share Posted October 12, 2011 I answered you in the high school forum: The forces are directed at right angles, so the net force can be found from the Pythagorean theorem: Fnet= sqrt( F_up^2+F_east^2)= 100N. (This is a 3-4-5 triangle.) The direction of the net force requires trigonometry. tan theta= 80/60, so theta=53.1 degrees east of up. Calculating this is beyond the scope of your course. The angle can NOT be 45 degrees, because that would require the forces to be equal. Are there any other forces? Weight? Because otherwise, how can they come up with the answers in the book? Quote Link to comment Share on other sites More sharing options...
Jen the RD Posted October 12, 2011 Share Posted October 12, 2011 (edited) Blossom Girl, I've already found incorrect answers in the first 2 chapters. I started a thread about this last week on the logic stage board. As far as this text, I'm really looking at it with skeptical eyes. Jennifer Edited October 13, 2011 by Jen the RD Quote Link to comment Share on other sites More sharing options...
Blossom'sGirl Posted October 13, 2011 Author Share Posted October 13, 2011 Blossom Girl, I've already found incorrect answers in the first 2 chapters. I started a thread about this last week on the logic stage board. As far as this text, I'm really looking at it with skeptical eyes. Jennifer I agree. We have found some errors also. I just was wondering where this problem came from as it doesn't seem to line up with anything previously presented. I think I will call this a misprint. regentrude - I will have to find and blow the dust off my statics and mechanics book to remember much about net forces, but you have a point that the net force would not be at a 45 degree angle. Quote Link to comment Share on other sites More sharing options...
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