Find resultant vector and equilibrant

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Discussion Overview

The discussion revolves around determining the resultant and equilibrant of two forces acting on a particle, specifically a force of 6 N and another of 8 N acting at an angle of 60 degrees to the first. The focus is on the application of vector addition and the correct use of the cosine law in this context.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant claims to have calculated the resultant vector as 7.2 N but notes a discrepancy with the expected answer of 12.7 N, questioning the use of angle 120 degrees instead of 60 degrees.
  • Several participants suggest that the vector addition was performed incorrectly and recommend drawing the vectors to visualize the problem better.
  • Another participant expresses confusion about the angle used in the cosine law, asserting that the angle between the two forces should be 60 degrees as stated in the problem.
  • There is a suggestion that the vectors should be drawn with the tail of one vector starting from the head of the other, rather than both originating from the same point.
  • One participant indicates that the diagram drawn may be incorrect, implying that this could lead to misunderstanding the angle used in calculations.

Areas of Agreement / Disagreement

Participants generally agree that there is a misunderstanding in the vector addition process, but there is disagreement regarding the correct angle to use in the cosine law and the proper method for drawing the vectors.

Contextual Notes

There are unresolved assumptions regarding the correct interpretation of the angle between the forces and the method of vector addition. The discussion highlights potential confusion in applying the cosine law based on the diagrammatic representation of the forces.

rrosa522
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A force, f1, of a magnitude 6 N acts on particle P. A second force, f2, of magnitude 8 N acts at 60 degrees to f1. Determine the resultant and equilibrant of f1 and f2.

I use cosine law and found the resultant is 7.2 but the answer is 12.7
I used angle 60 and my solution manual says I had to use 120, but I don't understand why that it
 
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You did your vector addition wrongly. Try drawing out the two vectors f1 and f2 as well as their resultant vector.
 
Fightfish said:
You did your vector addition wrongly. Try drawing out the two vectors f1 and f2 as well as their resultant vector.
i did but i am not getting the right answer
 
rrosa522 said:
i did but i am not getting the right answer
I'm quite sure you drew both vectors as having the same origin (i.e. tails originating from the same point) and then trying to find the line that completes that triangle. This is not correct. The tail of one vector has to start from the arrowhead of the other.

(I'm kinda lazy to attempt to sketch figures - so take a look over at http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html)
 
Fightfish said:
I'm quite sure you drew both vectors as having the same origin (i.e. tails originating from the same point) and then trying to find the line that completes that triangle. This is not correct. The tail of one vector has to start from the arrowhead of the other.

(I'm kinda lazy to attempt to sketch figures - so take a look over at http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html)
yeah that is what I did, I just don't get why its cos120 and not cos60. the question says A second force, f2, of magnitude 8 N acts at 60 degrees to f1, doesn't that mean f1 and f2 form an angle of 60 degrees.
 
2v3mveb.png

What you probably drew is the incorrect diagram on the left.
 

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