Engineering Statics Problem - Determining Magnitudes

1. Aug 29, 2009

deathcap00

1. The problem statement, all variables and given/known data

Two cables are used to secure the overhang boom in position and support the 1500N load. If the resultant force is directed along the boom from point A towards O, determine the magnitude of F_C(FsubC), F_B, and the resultant force, F_R. Set x= 1 and z= 4 .

2. Relevant equations

3. The attempt at a solution

I am not catching on very quickly in my statics class so my notation is probably messed up. Thanks for the help!!!

2. Aug 29, 2009

nvn

What are you trying to achieve by computing Fac = Fc - Fa? There is no Fa shown on your diagram, and no Fac nor Fab shown. I guess I do not understand what you are doing yet, or why. And why did you write Fr = Fb + Fc? That is also a false statement. What about using equilibrium?

3. Aug 29, 2009

djeitnstine

Your work is not quite right. It asks you to find the magnitudes of the forces. So what you have to do is split your forces up in x,y, and z components. And also sum the moments about a particular point.

Cross products may be easier in this case.

Try again.

4. Aug 29, 2009

deathcap00

That is the problem, I don't know what I am doing, just need some guidance on where to start.

5. Aug 29, 2009

djeitnstine

Well all of your forces have components along the x, y and z axes. You should know from class that $$\sum{\vec{F}}=0$$ is the equilibrium condition. Split that up into scalar components and you have $$\sum{F_x} =0$$
$$\sum{F_y} =0$$
$$\sum{F_z} =0$$

And the same goes for moments.

6. Aug 29, 2009

djeitnstine

Just a hint: When they ask you to find the magnitude of something, you usually don't need to use vectors. Unless its easier that way, but your final answer is never a vector.

7. Aug 29, 2009

deathcap00

We haven't learned about equilibrium yet, so I am not sure about that.

8. Aug 29, 2009

deathcap00

We haven't been taught about moments or cross products yet either. I think I understand about splitting the x,y,z components up, but the 1500N force doesn't "terminate" anywhere so how do I determine that? Also, wouldn't I need one of the forces of AC or AB to determine the resultant force? Thanks for the help, I am determined to get this problem.....

9. Aug 29, 2009

deathcap00

10. Aug 29, 2009

djeitnstine

$$\sum{\vec{F}} = 0$$ IS equillibrium.

I do not know what you mean by terminate.

You could start by rereading posts #3 (save for moments) and #5. I am sure you know what vectors and their scalar components are.

Simply by looking at the diagram the only forces that have any contribution in the z direction are the 1500N, Fb and Fc. This should get you started.

11. Aug 29, 2009

djeitnstine

Last edited: Aug 29, 2009
12. Aug 29, 2009

deathcap00

How do I incorporate 1500N into the process? Don't one of the other cables need a value in newtons for me to calculate? By terminate, I meant that the 1500N force isn't anchored on one end so I don't know how to use that value in my calculations. Hopefully, that clears it up...

13. Aug 29, 2009

djeitnstine

You have a few misunderstandings you need to get rid of.

First of all the 1500N force vector is clearly given to you. namely $$(-1500 \vec{k}) N$$ Study vectors again. Any vector along a x,y,z axes, given its magnitude will only have the directions, i,j or k respectively.

You should also learn the difference between a vector and a scalar.

http://en.wikipedia.org/wiki/Moment_(physics [Broken])

Last edited by a moderator: May 4, 2017
14. Aug 29, 2009

nvn

deathcap00: The 1500 N applied load terminates at point A. Start by using equilibrium. In your case, that means summation(Fx) = 0, summation(Fy) = 0, and summation(Fz) = 0. Or, if you want to use vector notation (i, j, k), that is perfectly OK (your choice), in which case the above would be summation(F) = 0.

Start by writing the above equations. Include the unknowns. You have three unknowns. Solve the equations simultaneously for the three unknowns. You do not need summation of moments. Try it again.

15. Aug 30, 2009

deathcap00

Thanks to all for the help, I will try and work this problem again this evening with the provided help info. Sorry if I am not that smart about this stuff, I am taking Statics for the first time and vector calc too so it's all very new to me and I am having a hard time getting some of the concepts down the first time around. Hopefully, I will be able to help someone on here one day! Thanks again, I will post and let you guys know how it goes.