Geometry - oblique triangle law question

[tyler.hansen02]

Hello,
Here is the question and my work so far. I just don't know if I'm doing it right since it mentions to use the oblique triangle law (which I know are the sine and cos law). I'm having difficulty finding out where the 5lb vector should be.
Thanks!

 

[Dr.Peterson]

Here is the question and my work so far. I just don't know if I'm doing it right since it mentions to use the oblique triangle law (which I know are the sine and cos law). I'm having difficulty finding out where the 5lb vector should be.

Clearly you are using components rather than oblique triangles, so you are not using the required method.

But also, I don't think you've answered the question (though the question confuses me a little). It doesn't say there is a 5 pound force along the bone, but that there is another force in an undetermined direction, such that, I believe, the resultant of the three forces will be in the same direction as the bone (15 degrees).

You might first find the resultant of the two 10 pound forces shown (which is easy), and then draw in a third force (direction unknown) and use an oblique triangle in which the resultant is in the required direction. Maybe you should show us a picture showing your interpretation of the problem.

 

[tyler.hansen02]

Here's what I have so far using the oblique triangle laws. I'm just not sure where the 5lb force comes into the whole scenario.

 

[Dr.Peterson]

Here's what I have so far using the oblique triangle laws. I'm just not sure where the 5lb force comes into the whole scenario.

That's part of the work; but what I suggested is to answer your last question first: What does the problem mean? We can't accomplish much until we figure that out. So I was hoping for a picture of where you guess the 5 pound force might fit!

So here's my guess:



You're given vectors [MATH]u[/MATH] and [MATH]v[/MATH], whose resultant [MATH]r_1[/MATH] you've found; this pulls at an angle to the bone, so it would tend to rotate upward. You want to drag it straight along the dotted line. So ...

you need to add in a third vector [MATH]w[/MATH] so that the resultant [MATH]r_2[/MATH] is along that line:



Does that interpretation sound right to you? If so, you can now try to solve the problem. If not (or if you are told it's wrong), offer an alternative.

I do wish the problem were clearer.