Simple forces - web

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1. Dec 2, 2015

Heres2hoping

1. The problem statement, all variables and given/known data
A spider builds a web using 4 strands as support.
Strand 1: 21mN [20°E of N]
Strand 2: 16mN [60°E of S]
Strand 3: 18mN [40°W of S]

Determine the force of tension in strand 4 assuming the web is stable.

2. Relevant equations
r(cosθ) - N/S components
r(sinθ) - E/W components
The pythagoren theorem might be of use along with equations used in simple trig.

3. The attempt at a solution

North-South components:
Strand 1: 19.7mN [N]
Strand 2: 8mN
Strand 3: 13.7mN

East-West components:
Strand 1: 7.1mN [E]
Strand 2: 13.9mN [E]
Strand 3: 11.6mN [W]

Other than that I've determined that the direction will be in the North-West segment. I'm just having issues determining the total force acted by the 3 given strands. Any pointers would be much appreciated. :)

2. Dec 2, 2015

Heres2hoping

Strand 2 is 8mN
Strand 3 is 13.7mN

( I'd forgotten the direction )

3. Dec 2, 2015

azizlwl

Draw a vector diagram.

4. Dec 2, 2015

Heres2hoping

Ok, I'll do that.
But what formula should I use if I wanted to determine the total force and to what degree?

5. Dec 2, 2015

azizlwl

From there we can get rough idea about magnitude and direction of required force AND we can make calculation base on the drawing.

6. Dec 2, 2015

Heres2hoping

So, I can assume that the answer is going to be in the north-west segment, right?

7. Dec 2, 2015

azizlwl

I hope you can show the vector drawing so that we can analyze it.

8. Dec 2, 2015

Heres2hoping

I'm not sure how to... Sorry

9. Dec 2, 2015

Heres2hoping

I got it! I think...

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10. Dec 2, 2015

azizlwl

For a stable web, the forces are in equilibrium means total sum of forces equal to zero. A+B+C+D=0
Given vector A,B and C, so you need to find vector D.
What you have to do is to join(add) the vectors(arrows)
Vector can be moved anywhere as long you do Not rotate or modify size.
So you have to draw another vector diagram with head join to another vector tail.

11. Dec 2, 2015

Heres2hoping

do you mean for each individual strand to become its own little triangle (in a sense)? Or more of one big one?

12. Dec 2, 2015

azizlwl

There are 3 arrows, you label any sequenece as you like as A,B and C.Move B, without size adjustments and rotation, so that tail of B touches head of A. Then move C so that tail of C touches head of B on new position(after adding to A)

13. Dec 2, 2015

Heres2hoping

Oh my goodness! It makes so much more sense! Thank you so much! I'm going to go try that out

14. Dec 2, 2015

Heres2hoping

Ok, so where do I go from there? I don't have the angles of the new triangle, so do I need to figure them out?

15. Dec 2, 2015

azizlwl

Hope you can show the vectors addition diagram.
Then we can discuss adding vectors algebraically.

16. Dec 2, 2015

Heres2hoping

Actually, I think I'll try to figure out the rest tomorrow. I understand the problem more than I had, so thank you for your time :)

17. Dec 2, 2015

Heres2hoping

im having problems ioading the picture, so maybe I'll try again tomorrow (it's really late for me)thank you for you help :)