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Finding the unknown magnitude of force

by leroyjenkens
Tags: force, magnitude, unknown
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leroyjenkens
#1
Jun18-10, 03:52 PM
P: 538
1. The problem statement, all variables and given/known data
Here's a link to the website that has the problem. It's right under example 1.
http://www.sparknotes.com/testprep/b...section3.rhtml


2. Relevant equations



3. The attempt at a solution
This is just for the first part; finding Curly's force.
For the sled not to move up or down, both Curly and Moe will each have to have a y component force that cancels out the other one. I calculated Moe's y component force to be 17.32 N, which means Curly would also have to have 17.32 N of downward force to negate Moe's force. Then I calculated Curly's x component of force from having his y component of force and got 29.999 N of force.

What they did is completely foreign to me. First, they got 8.660 N as Moe's y component of force because they multiplied .866, the sine, times the x component. I don't know why they did that.
Then they just divided the 8.660 they got by Curly's sine to get Curly's x component of force. I don't see why they did that and they don't explain why they were doing it.

Thanks.
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nrqed
#2
Jun18-10, 04:10 PM
Sci Advisor
HW Helper
P: 2,884
Quote Quote by leroyjenkens View Post
1. The problem statement, all variables and given/known data
Here's a link to the website that has the problem. It's right under example 1.
http://www.sparknotes.com/testprep/b...section3.rhtml


2. Relevant equations



3. The attempt at a solution
This is just for the first part; finding Curly's force.
For the sled not to move up or down, both Curly and Moe will each have to have a y component force that cancels out the other one. I calculated Moe's y component force to be 17.32 N,
Hi there!

How did you get that result? The y component cannot be larger than the magnitude, which is 10 N. You have to use

y component = magnitude times sin(angle measured with respect to the x axis)


which means Curly would also have to have 17.32 N of downward force to negate Moe's force. Then I calculated Curly's x component of force from having his y component of force and got 29.999 N of force.

What they did is completely foreign to me. First, they got 8.660 N as Moe's y component of force because they multiplied .866, the sine, times the x component.

Not quite. They multiplied the sin by the *magnitude* of Moe's force. And that's in agreement with what I wrote above.

I don't know why they did that.
Then they just divided the 8.660 they got by Curly's sine to get Curly's x component of force. I don't see why they did that and they don't explain why they were doing it.
They do not divide 8.660 by any sine. So I am not sure what you are refering to.

Hope this helps!
leroyjenkens
#3
Jun18-10, 04:38 PM
P: 538
How did you get that result? The y component cannot be larger than the magnitude, which is 10 N. You have to use
I assumed the magnitude was their x component force. So their magnitude was the total force they were exerting? Which would be the hypotenuse?
Not quite. They multiplied the sin by the *magnitude* of Moe's force. And that's in agreement with what I wrote above.
Oh, ok.
They do not divide 8.660 by any sine. So I am not sure what you are refering to.
I was just kinda reading what the last equation was that they did. I'm going to try this again and see if I can do it....


Ok thanks. I got 17.32 as his force, which is what they got. What confused me is the way they did it at the end. That and I thought the force they were talking about was the force just in the x direction. Don't know why.

Thanks again.


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