I need help in Statics

[gjt01]

I need urgent help in Statics

I'm taking a Statics course in college right now. However, I wasn't able to get a book due to financial issues early on. Naturally I feel behind. Anyways, I have a test coming up and the teacher gave us four problems to solve as a study guide, saying similar problems would be on the test.

Looking over the assignment, I relized that I REALLY don't know what I'm doing. Is there anyone that could help me figure out how to do the four problems so that tomorrow night, when I take my test I'll have an idea of what I have to do?

If anyone can help, I'll try and post the problems. If someone could just guide me through the steps I would be VERY greatful!

 

[HallsofIvy]

So POST the problems already!

 

[Pyrrhus]

Sure, i'll help, maybe you want to review your vector algebra?

 

[gjt01]

ok... not really sure how since there is a picture... anyways

1)A 400-N force P is applied at point A off the bell crank shown. (a) Compute the moment of the force P about O resolving it's components.



A
B / /
\ \ <92> / /
\ \ / /
\ \ / /
42 \ O / 40

42 and 40 degree's off x Axis - and the Force - P, comes off of point A at 20 degrees (60 off axis) The smaller lenght (42 degree side) is 120 mm, and the 40 degree side is 200

Did I forget anything?

 

[gjt01]

Wow... spaces dont work in this forum.... ummmm fom the center point, there is a line going 120 mm at 132 degrees, and a line moving 200 mm at 40 degrees and the force is 400-N at 20 degrees off point A.

Point A is the end of the 200 mm line, B is the end of the 120mm line, and O is the axis.

 

[gjt01]

Would it be easier if I just scanned the 4 problems and sent the pictures?

 

[Tom Mattson]

Would it be easier if I just scanned the 4 problems and sent the pictures?

Yes, you can upload them to PF and attach them.

But spaces do in fact work if you use code tags (but without the spaces):

[ code ] (stuff) [ /code ]

Example:

With code tags, I can have as many 
s p  a   c    e      s    as I want!  Yay!

 

[gjt01]

Heres the first two problems.

 

[gjt01]

On the first problem, what throws me off is the force. I've solved problems where there were two forces on a pivot. Thats done by making the Triangle and using sin and cosine laws.

But I don't see how this one would be done the same. If it was a force of 120-N and 200-N, at those angles, I would be able to do it.

------------------

On the second problem, I'm sure you could use the vector triangles, but how would the triangles be set up since there are 4 forces on it.

I hate to sound dumb, but I really just don't know what I'm doing. Statics is VERY new to me.

 

[Tom Mattson]

The definition of a moment is $\vec{M}=\vec{r}\times\vec{F}$, right? So you're calculating the moment about O, which means that you can resolve both the displacement and the force vectors into rectangular components and compute their cross product. Have you tried that?

 

[gjt01]

The rectangle would have to be perpendicular to the Force. Since the force is at 60 degrees, perpendicular would be 110 degree. If I made a rectangle it wouldn't be perpendicular. So I wouldn't know how to go about doing that. I considered projecting downward from Point A and forming a right triangle. A lot of our example problems are done by making a right triangle and labeling the two sides, using the object as a hypotnus. I don't think I have enough information to determine the lenght of the sides using that method though.

(I'm telling you all this so you know that I really am trying, and not just looking for answers. I need to learn to do this on my own.)

You suggested a rectangle, and I'm sure you could easily do this problem. But since the pivot isn't a 90 degree angle, and the foce isn't perpendicular to the object, I don't understand how you would work it that way.

Could you explain?

 

[Pyrrhus]

For the first problem:
Simply obtain the radius vector from O to A, and then decompose the radius vector and the P vector into vectors in the parallel to the axis. Finally, do the
cross product as Tom suggested.

For the second problem:
Those forces are concurrent, therefore you can easily add them throught their components in order to obtain a resultant.

 

[gjt01]

...perhaps it's not to late to withdraw...

 

[Pyrrhus]

Nope, simply you need the theory behind this (the definitions and etc..). Look for a Statics book, i recommend Beers and Johnstons, try the library.