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 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jul 19th 2019, 08:37 PM #1 Junior Member   Join Date: Jul 2018 Posts: 22 Vector How to solve Q3. Attached Thumbnails
 Jul 20th 2019, 06:15 AM #2 Senior Member   Join Date: Aug 2010 Posts: 434 Do you know what the word "resultant" means?
Jul 20th 2019, 08:41 AM   #3
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 Originally Posted by HallsofIvy Do you know what the word "resultant" means?
Yes, resultant is the combined effect of two vectors. Am I correct?

 Jul 20th 2019, 10:22 AM #4 Senior Member   Join Date: Aug 2010 Posts: 434 Yes, so the "C" they refer to is the length |A+ B|. What is the relation between |A+ B| and |A- B|? Indranil likes this.
Jul 20th 2019, 11:26 PM   #5
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 Originally Posted by HallsofIvy Yes, so the "C" they refer to is the length |A+ B|. What is the relation between |A+ B| and |A- B|?
I don't understand the relation But as I know C = A + B (positive direction)
C = A - B A is in the positive direction and B is in the negitive direction

Jul 21st 2019, 07:46 AM   #6
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 Originally Posted by HallsofIvy Yes, so the "C" they refer to is the length |A+ B|. What is the relation between |A+ B| and |A- B|?
A+B is greater than A-B?

 Jul 21st 2019, 08:50 AM #7 Senior Member   Join Date: Aug 2010 Posts: 434 No, that is not necessarily true. First, do you understand the difference between "A+ B" and "|A+ B|"? Do you understand that for vectors, like A+ B and A- B, "less than" and "greater than" are NOT defined?
Jul 21st 2019, 08:56 PM   #8
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 Originally Posted by HallsofIvy No, that is not necessarily true. First, do you understand the difference between "A+ B" and "|A+ B|"? Do you understand that for vectors, like A+ B and A- B, "less than" and "greater than" are NOT defined?
A+B is the addition of two vectors and [A+B] is the magnitude of two vectors (addition of numbers) Am I right?

 Jul 22nd 2019, 06:40 PM #9 Senior Member   Join Date: Aug 2010 Posts: 434 Yes, that is correct. And while you cannot say, of two vectors A and B, that "A< B" or "B< A", you can say that |A|< |B| or |B|< |A|. But given two vectors (or even numbers) you cannot say "|A+ B|> |A- B|. For example, if B= -A then |A+ B|= 0 while |A- B|= 2|A|. topsquark likes this.
Jul 22nd 2019, 07:32 PM   #10

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 Originally Posted by HallsofIvy Yes, that is correct. And while you cannot say, of two vectors A and B, that "A< B" or "B< A", you can say that |A|< |B| or |B|< |A|. But given two vectors (or even numbers) you cannot say "|A+ B|> |A- B|. For example, if B= -A then |A+ B|= 0 while |A- B|= 2|A|.
Good catch. I hadn't thought of that.

-Dan
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