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Vectors help!

  1. Sep 18, 2005 #1
    Ok Tomorrow I have a test in my physic's class and my teacher was nice enough to give us a study guide with the answers, but I can't figure out for my life how to get a few of the answers. I have 3 problems that I am having trouble with.

    1. One force of 60N and a second of 30N act on an object at Point P. Graphically add the vectors and find the magnitude of the resultant when the angle between them is as follows:
    a. 0
    b. 30
    c. 45
    d. 60

    Her answers: a. 90N (I got that one)
    b. 87N
    c. 84N
    d. 79N

    2. A person weighs 612N. If the person sits in the middle of a hammock that is 3.0m long and sags 1.0m below the points of support, what force would be exerted by each of the two hammock ropes?

    Her answer: 457.3N

    I will put the 3rd on in a little bit, just because their is a diagram that goes with it. Though I really need help with the 1st 2 b/c I just don't get one part of the 3rd question. Thanks!

    Edit: since I was informed that I needed to show the troubles I having with the problems here they are:

    I know in part a I just have to add the two vectors together. Then in Parts b-d I thought I was suppose to break up the vectors to have right triangles then use trig functions then Pythagorean's theorem to solve the equation.... but when ever I do that I keep on getting answers that are like 10N off. I am not sure if I am doing right. Can someone explain the steps I need to take into solving the equation.

    For problem 2 ... I was wondering if someone can explain how to set up the diagram.
     
    Last edited: Sep 18, 2005
  2. jcsd
  3. Sep 18, 2005 #2

    Gokul43201

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    You must show what trouble you are having with these problems. It is against the policy to help you unless you show your work first.
     
  4. Sep 18, 2005 #3
    Ok sorry.. Kind of new and didn't know that. I will get right on it. Thanks for telling me
     
  5. Sep 18, 2005 #4

    Gokul43201

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    Last edited: Sep 18, 2005
  6. Sep 18, 2005 #5
    I am sorry, but read what first?
     
  7. Sep 18, 2005 #6

    Gokul43201

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    Oops...fixed it.
     
  8. Sep 18, 2005 #7
    I did read it, but I guess after looking at a few other people's post I just still didn't get what they were looking for. Is my 1st post right now?
     
    Last edited: Sep 18, 2005
  9. Sep 18, 2005 #8
    :cry: I have a feeling no one will help me because I don't understand the format of posting here.... I really need help b/c I don't get it. I have been trying for 2 hours to get these 2 questions, but of course I have an evil textbook that does not explain anything. :cry:
     
  10. Sep 18, 2005 #9

    Gokul43201

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    keita : Your textbook (which one is it ?) has a pair of formulae for adding two vectors of magnitude F1 and F2 when they are separated by an angle [itex]\theta [/itex].

    There's one formula telling you the magnitude of the "Resultant" vector ([itex]F^2 = F1^2 + F2^2 + 2F1F2cos \theta[/itex]). There's another one telling you the angle at which the resultant works. All these results pop out of the parallelogram approach to adding vectors. I suggest you read this part of your text.
     
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