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Little help with some simple Vector physics

  1. Sep 15, 2006 #1
    I just wanted to make sure I'm doing my homework correctly, please let me know if any of the following is incorrect, if anyone knows. most the questions are multiple choice but im just posting what I found to be the answer. thanks,:

    1- What type of quantity is characterized by both magnitude and direction?

    Vector

    2- When we add a displacement vector to another displacment vector, the result is:

    a displacement vector

    3- A student adds two vectors. One has a magnitude of 200 and the other has a magnitude of 40. Which of the following is the only possible chice for the magnitude of the resultant?

    ok, so im assuming the vectors are at a right angle so I used the phthalgorym therom or whatever (A squared + B squared = C squared) and I came out with 204, and one of my choices was 200 (the other choices were 100,260, and 40) Im just a little scetchy on this answer and wondering if I should have gone about it a different way.


    Any input is greatly appreciated. Thanks.
     
  2. jcsd
  3. Sep 15, 2006 #2
    the first two are correct. you got the correct answer for the third one, but went about it incorrectly. Take a look at the answers given. With those two vectors could you add them in any way (doesn't have to be a right triangle) to make a vector of that length. For example, a vector of magnitude 260 could not be made from one of 200 and 40. For if they were added in the same direction it would only equal 240, the same type of logic applies to the other two incorrect ones. The answer of 200 could be made by a triangle (but not a right triangle) with two sides of length 200 and one of length 40.
     
  4. Sep 15, 2006 #3

    radou

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    Homework Helper

    Regarding 3 - the only possible choice seems to be 200. You can draw a vector in any direction, and say it has a magnitude of 200. Then, you draw a circle of radius 40 with the centre on the 'top' of the vector whose magnitude is 200 (on the arrowhead :biggrin: ). Then it's obvious that the least vector has a magnitude 160, and the greatest 240. So, 100, 40, and 260 fall off.
     
  5. Sep 15, 2006 #4
    You got the first two ok. For the last one, I think you got the right answer, but why did you assume they are at right angles? The maximum possible magnitude is if both vectors are pointing in the same direction, in which case the magnitude would be 240, and the minimum is if they're going in opposite directions, giving 160. So you know the answer must be between 240 and 160.
     
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