# Cross product

1. Oct 1, 2015

### goldfish9776

1. The problem statement, all variables and given/known data
Inq this question, I am given that the u dot ( rb x tb + re x w) =0.
I knew that the rb x tb give a resultant moment pointed inside the book , which is parallel to AB.... Then it cross with r AD, we will get the resultant moment.
But when re x w , I will gt the moment pointed in dx direction.... How can it be related to AD?

2. Relevant equations

3. The attempt at a solution

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2. Oct 1, 2015

### haruspex

$\vec{r_b}\times \vec{T_B}$ is the moment of the tension about D, while $\vec {r_E}\times\vec W$ is the moment of W about D.
To these we could add the moment of the forces at A about D and end up with 0, for equilibrium. However, that moment is orthogonal to $\vec u$, so if we take the dot product with $\vec u$ that moment disappears, leaving the given equation.

3. Oct 1, 2015

### goldfish9776

what do u mean by we could add the moment of the forces at A about D here ? i'm confused...forces at A are not taken into calculation here ....

4. Oct 1, 2015

### goldfish9776

you mean by doing u dot ( rb x tb + re x w) , it means the moment of the forces at A about D

5. Oct 1, 2015

### haruspex

I mean that the full "sum of moments about D = 0" equation looks like :
moment of the tension about D + moment of W about D + moment of the forces at A about D = 0

If we then take the dot product of that with u then the last of those three terms vanishes, leaving:
u.(moment of the tension about D + moment of W about D) = 0

6. Oct 1, 2015

### goldfish9776

why vector rDA is used here? why not rE ? this is weird becoz , moment of the tension about D + moment of W about D only pass thru rE ...

7. Oct 1, 2015

### haruspex

I don't understand either of those points.
Where is rDA being used that you think is wrong? It would be used for the forces at A, yes, but rDA is in the same direction as u, so when we take the dot product of u with (rDA x forces at A) we get 0.
What do you mean about moments "passing through" rE? Forces pass through points, moments turn about points. All the moments being calculated here are about D.

8. Oct 2, 2015

### goldfish9776

for them , why u is necessary ? aren't them have the vector rB and rE already ?

9. Oct 2, 2015

### haruspex

Taking the dot product with u is done to eliminate the forces at A from the equation.

10. Oct 2, 2015

### goldfish9776

That's only for force a right? Why tb and w also need to multiply u ?

11. Oct 2, 2015

### haruspex

We have an equation. We cannot multiply some terms of the equation by something and not other terms.

12. Oct 2, 2015

### goldfish9776

i still dont understand . why shouldnt thtotal moment about D = rB x tB + rE x W + (resultant force at A) x rDA ?

13. Oct 3, 2015

### haruspex

It does (except that you have the arguments of the last cross product in the wrong order; the distance vector should be on the left). But since the system is in equilibrium that total moment is zero. Write that as an equation. Now take the dot product of each side of that equation with u. What do you get?

14. Oct 3, 2015

### goldfish9776

sorry , i mean D = rB x tB + rE x W + rDA x (resultant force at A) ... why shouldnt the equation look like this ?

15. Oct 3, 2015

### haruspex

I just answered that question. That equation is quite correct, but we need to move beyond it.
Step 1: since the system is in equilibrium, total moment about D is zero:
rB x tB + rE x W + rDA x (resultant force at A) = 0.
Step 2:
We want to eliminate the unknown (resultant force at A). We can do that by taking the dot product of both sides of the equation with u:
u.(rB x tB + rE x W + rDA x (resultant force at A)) = 0
u.rB x tB + u.rE x W + u.rDA x (resultant force at A) = u.0 = 0
Now, u.rDA x (resultant force at A) = 0
(can you see why?)

16. Oct 3, 2015

### goldfish9776

we already know that the resultant force at A should be parallel to rDA , right , this will bring the rDA x (resultant force at A) = 0 ? So, why would we have to do one more step(step 2 ) ?

17. Oct 3, 2015

### haruspex

No. Why do you think that?

18. Oct 3, 2015

### goldfish9776

the resultant force of Az and Ay should be in perpendicular to Az and Ay(so, it's Ax) , right ? so it's Az . Az x rDA = o , since they are parallel to each other .

19. Oct 3, 2015

### haruspex

Eh?!
The resultant of adding a force in the z direction and a force in the y direction will be somewhere in the YZ plane. It will certainly not be perpendicular to either. Even if it were perpendicular to both, as you claim, that would put it in the X direction, not the Z direction. And even if it were in either the X or Z direction that would not make it parallel to DA.

20. Oct 3, 2015

### goldfish9776

yes , becoz u=rDA , they are parallel to each other