1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: How to solve this Cosine Law Equation?

  1. May 31, 2015 #1
    Question is regarding Scalars and Vectors Article.
    Q: One of the two forces is double the other and their resultant is equal to the greater force . The angle between them is ?
    Ans : Its answer is cos^-1 (-1/4)

    My solution :
    The formula for Cosine law is
    R= √A²+B² +2ABcos theta

    Our teacher told that when talking about force we can write :
    F= √F1² + F2²+F1²X F2² cos theta
    F= √F1+F2+2F²cos theta
    F=√1+cos theta

    But, what I did to solve problem is this below :-

    1: I plugged in values . A = F1 , B = 2F , Resultant "R" = 2f

    2: I got this equation :
    = 2F = √F1² + 2F² +2F(2F) cos theta
    = 2F= √F²(cancel) + 2F²(cancel) + 4F² cos theta : F² F² cancel each other
    = 2F = √2(cancel)+4(cancel)F² cos theta : 2 and 4 are divided by 2 and cos theta shifts
    = F/2F²= √ 1+ cos theta
    = 1/4= √1 + cos theta
    = cos ^-1 (-1/4)

    But I know the way solved is not it is meant to be solved because I am in Pre- Medical and i have less knowledge of mathematics , so Kindly tell me how to deal with such numericals and please solve it and tell me shortest route to solve it . thanks in advance
    Last edited by a moderator: May 31, 2015
  2. jcsd
  3. May 31, 2015 #2


    Staff: Mentor

    This answer doesn't make sense. If the smaller force is F and the larger force is 2F, then we have the vector equation 2F + F = 2F, so 3F = 2F.
    If the resultant of two vectors is equal to one of the vectors, that says something about the other vector.
    Your sign of the 2ab cos(θ) term is wrong. The Law of Cosines is ##c^2 = a^2 + b^2 - 2ab cos(\theta)##, where θ is the angle between the sides a and b.

    Furthermore, what triangle are you working with? The two vectors and their resultant don't form a triangle.
    The above isn't right.
    ?? Why do you think they cancel?
    Last edited: May 31, 2015
  4. May 31, 2015 #3

    Thanks alot for reply ,
    ok see this other solution sir , this is what I did now but in a simpler way.

    2F² = (F)² + (2F)² + (2xF) (2F) cos theta
    2F² = F² + 4F² + 4F² cos theta
    2F² = F² + 8F² cos theta : numbers added 4F² + 4F² = 8F² and F² cancelled with F² of (8F²)
    2F² = (1 + cos ) 8
    2F²/8 = 1 + cos theta)
    so cos inverse ^-1 (-1/4) answer

    I still think I am wrong please help me out how to get the answer
  5. May 31, 2015 #4
    Yes exactly such numericals appear in our tests without any thing about triangle , I know law of cosine is obviously to find angle of triangle but I dont know why they are using law of cosine here?

    If you know appropriate method then please help me
    Last edited by a moderator: May 31, 2015
  6. May 31, 2015 #5


    Staff: Mentor

    Simpler, yes, but still wrong.
    • The sign (marked in red) is wrong.
    • On the left side, it should be (2F)2.
    No, you can't do this. 4F2 + 4F2cos(θ) ≠ 8F2cos(θ).
    That's like saying that 4 + 4*7 = 8*7, which isn't true.
  7. May 31, 2015 #6
    I have actually used plus sign because If I would have used negetive sign than in answer sign would have been automatically reversed if cos >120 degree i think .

    Btw please tell me how to solve it now ?
  8. May 31, 2015 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You're on the right track. You just need to pay more attention to the details because you're making careless mistakes. Mark already pointed out various errors you made. Fix those and try again.
  9. Jun 1, 2015 #8

    Ok here is what I did more bro

    Thanks alot , I found that I was doing careless simpler mathematical mistakes .
    So I found this the correct solution .

    = 2F = √F²+(2F)² + 2(F) (2F) cos theta
    = (2F) = (√F²+(2F)² + 2(F) (2F) cos theta )² : squaring both sides
    = 4F² = F² + 4F² + 4F² cos theta
    = 4F² = 5F² + 4F² cos theta
    = -5F² + 4F² =4F² cos theta
    = -F² = 4F² cos theta
    = -F² / 4F² = cos theta : F² cancelled with F²
    = -1 / 4 = cos theta
    = -1 / 4 / cos theta
    = cos inverse ^ -1 (-1/4) . Final Answer
  10. Jun 1, 2015 #9


    Staff: Mentor

    In post #1 you gave this as the Cosine Law:
    This formulation is applicable if the angle ##\theta## is greater than 90° (see http://en.wikipedia.org/wiki/Law_of_cosines, in the discussion of Figure 2). What made you think that the angle between the two vectors was larger than 90°? Did you draw a sketch of the possible orientations of the two vectors?

    There are several things wrong in your work above.
    1. Each line in your work starts with '='. Except for the last two lines, each other line is an equation that already has '=' in it. The first 8 lines should NOT start with =.
    2. The right side of the first equation above is √F²+(2F)² + 2(F) (2F) cos theta. What you wrote isn't what you meant, which was that the square root is of the whole expression, not just of F or F2. Use parentheses to indicate what you're taking the square root of, as in √(F²+(2F)² + 2(F) (2F) cos theta).
    3. In the second line, where you indicate that you are squaring both sides, you show the left side as (2F), but this should be (2F)2. You don't "square an equation" -- you square each side of an equation.
    4. In the 8th and 9th lines you have
      = -1 / 4 = cos theta <--- this is an equation
    5. = -1 / 4 / cos theta <--- this is NOT an equation
    6. What do you mean by -1/4/cos theta? Are you dividing -1/4 by cos(theta)?
    7. In the last line you have cos inverse ^ -1 (-1/4). Ignoring the fact that you have cos inverse AND an exponent that indicates the inverse of the cosine, what happened to theta? The last line should look like "theta = <some number>"
  11. Jun 1, 2015 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    I think you meant 90 degrees, not 120 degrees. Anyway, that's not a good reason for changing the sign of that term, nor is it a valid reason.

    Draw a sketch and make sure the ##\theta## in the equation actually represents what you think it does.
  12. Jun 1, 2015 #11
    Thanks for reply sir ,
    but at 90 degree angle force doesn't work , so I don't think this is meant to be used , I found this formulae in our textbook , which is used in all local examinations here , so if I will play with this formula then answer will not be desired , Thanks alot
  13. Jun 1, 2015 #12
    Oh , actually I solved it in a manner we do in our exam mcqs where we have 1 minute to solve 1 question , let men explain you Sir.

    This was my work
    = 2F = √F²+(2F)² + 2(F) (2F) cos theta
    = (2F) = (√F²+(2F)² + 2(F) (2F) cos theta )² : squaring both sides
    = 4F² = F² + 4F² + 4F² cos theta
    = 4F² = 5F² + 4F² cos theta
    = -5F² + 4F² =4F² cos theta
    = -F² = 4F² cos theta
    = -F² / 4F² = cos theta : F² cancelled with F²
    = -1 / 4 = cos theta
    = -1 / 4 / cos theta
    = cos inverse ^ -1 (-1/4) . Final Answer

    so yes it was my mistake that I didnt wrote ² on (2F) so it is (2F)² = 4F²
    and = yes it was wrong to mention it

    and yes i divided -1/4 with cos theta to get cos inverse by shifting it . That was taught by teacher in our class . because answer isn't further solved , questions are weird in our local examinations they ask weird question which have weird solutions . Thanks for reply
  14. Jun 1, 2015 #13
    Sir , angle wasn't mentioned in question because these are quick quesitons which come in exam and at that time one is practically not in position to calculate in that way so they have tweaked cosin law for us and taught us to apply in that way , reason they haven' told because textbooks here are written by team to which it is difficult to speak or communicate their people
  15. Jun 1, 2015 #14


    Staff: Mentor

    This is what you have in post #1.
    So the bottom line should be something like "θ = cos-1(-1/4)"
  16. Jun 2, 2015 #15

    ops sorry by angle I mean Triangle diagram or something like that wasn't mentioned in the question Sir .

    Yes exactly this is the answer you are right .

    I also have an quick alternate to solve this question

    I found this method more quicker and applicable to variety of questions . See below I have made F understoodable in both 1F and 2F . So below is the less reliable but faster method .

    simple use numbers without using "F"
    1: 2 = √ 1² +2² + 2 X 2 cos theta
    2: 2² =(√1² + 2² + 4 cos theta)² " square cancelled with underroot and 2 on left side squared to 4 .
    2 : 4 = 1² + 4 + 4 cos theta
    3: 4 = 5 + 4cos theta
    4: -5+4 = 4 cos theta
    5: -1 = 4 cos theta
    6: -1/4 = cos theta
    7: θ = cos-1(-1/4) Answer

  17. Jun 2, 2015 #16


    Staff: Mentor

    That looks a lot better
  18. Jun 8, 2015 #17

    Sorry for late reply sir , actually internet was down since days .

    Yes this looks way better and way easy too because we have atleast 20 chapters of Physics with too much number of articles , so we have to find such a way that keeps thing in our mind for long term and save time , Thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted