I need urgent help in Statics

gjt01

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

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:

gjt01

Heres the first two problems.

Attached Thumbnails

   

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 [itex]\vec{M}=\vec{r}\times\vec{F}[/itex], 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.