Register to reply

Resultant Couple Moments

by VinnyCee
Tags: couple, moments, resultant
Share this thread:
VinnyCee
#1
Apr2-06, 05:20 PM
P: 492
Hi everyone.

I have this problem and I don't even know how to start it!

If you are familiar with my previous posts, you know that I usually have most of a problem completed before posting here with my difficulties, however, I cannot even start this one! I don't think there is enough information to get an answer!

The answer is an i-j-k vector, but how am I supposed to resolve the moments given into it's vectors and forces?

Please help!

Phys.Org News Partner Science news on Phys.org
'Smart material' chin strap harvests energy from chewing
King Richard III died painfully on battlefield
Capturing ancient Maya sites from both a rat's and a 'bat's eye view'
dav2008
#2
Apr2-06, 05:51 PM
PF Gold
dav2008's Avatar
P: 621
Your first step would be to write the moments as vectors.

Notice that the direction of the moment is given by the black arrows in the diagram, and the magnitude is given as 60 lb*ft and 80
lb*ft.

After you write them in vector notation I think you can figure out how to find their resultant, or sum.
VinnyCee
#3
Apr2-06, 06:23 PM
P: 492
How do I write the moments as vectors? There are no reference points. All I have are the directions of the vectors, but no lengths or anything!

Maybe [tex]M_1 = -60\,\hat{i}[/tex]

Cyrus
#4
Apr2-06, 06:29 PM
Cyrus's Avatar
P: 4,777
Resultant Couple Moments

I would advise you to go back and read about vectors in chapter 1 or 2. By the time you are working on couples, your expected to know that stuff.

Then post any new thoughts\ideas.
dav2008
#5
Apr2-06, 06:31 PM
PF Gold
dav2008's Avatar
P: 621
Quote Quote by VinnyCee
How do I write the moments as vectors? There are no reference points. All I have are the directions of the vectors, but no lengths or anything!

Maybe [tex]M_1 = -60\,\hat{i}[/tex]
You have the magnitudes (or lengths) of the vectors. They are simply the given torques in lb*ft.

Like Cyrus said, this is basically a vector addition problem.
VinnyCee
#6
Apr2-06, 06:33 PM
P: 492
I can do vectors. I have read those chapters, twice, and did all of the problems.

Are you hinting that the vector is to be a unit vector?

[itex]-\hat{i}[/itex] is the unit vector for [itex]M_1[/itex]. So does this mean that [itex]M_1 = \left(60 lb.\right) \left(-\hat{i}\right)[/itex]?
dav2008
#7
Apr2-06, 06:36 PM
PF Gold
dav2008's Avatar
P: 621
Almost. M1 points in the positive X direction.
nrqed
#8
Apr2-06, 06:38 PM
Sci Advisor
HW Helper
P: 3,015
Quote Quote by VinnyCee
How do I write the moments as vectors? There are no reference points. All I have are the directions of the vectors, but no lengths or anything!

Maybe [tex]M_1 = -60\,\hat{i}[/tex]
It's actually [itex] +60 \, \hat{i} [/itex].

A hint for the other: its z component is [itex] - 80 sin(30) [/itex], right? (the minus is because of the way it is pointing. For the x and y components, the simplest way to visualize is to first find the projection in the xy plane (which involves a factor of cos(30)) and *then* decompose that projection into the x and y components which will give an *additional* factor of sin(45) and cos(45) (which are of course equal).

Patrick
VinnyCee
#9
Apr2-06, 08:07 PM
P: 492
[tex]M_1 = \left(60 lb.\right) \left(\hat{i}\right)[/tex]

[tex]M_2 = \left[\left(-80\,cos 30\,sin 45\right) \hat{i} + \left(-80\,cos 30\,cos 45\right) \hat{j} + \left(-80\,sin 30\right) \hat{k}\right] lb. ft.[/tex]

Those are the vectors for the two moments, right?

Then I just algebraically add them together?

[tex]M_3 = \left(11 \hat{i} - 49 \hat{j} - 40 \hat{k}\right)[/tex]

Is that correct?
nrqed
#10
Apr2-06, 08:30 PM
Sci Advisor
HW Helper
P: 3,015
Quote Quote by VinnyCee
[tex]M_1 = \left(60 lb.\right) \left(\hat{i}\right)[/tex]

[tex]M_2 = \left[\left(-80\,cos 30\,sin 45\right) \hat{i} + \left(-80\,cos 30\,cos 45\right) \hat{j} + \left(-80\,sin 30\right) \hat{k}\right] lb. ft.[/tex]

Those are the vectors for the two moments, right?

Then I just algebraically add them together?

[tex]M_3 = \left(11 \hat{i} - 49 \hat{j} - 40 \hat{k}\right)[/tex]

Is that correct?
Looks right to me.

Patrick


Register to reply

Related Discussions
Values for magnitude of resultant vector Advanced Physics Homework 0
Resultant Couple and direction angle Introductory Physics Homework 3
Forces, Moments and couple question, Can you please check its right Mechanical Engineering 1
How to get mag. & angles of resultant couple moment Introductory Physics Homework 1