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Work and energy

  1. May 29, 2016 #1
    1. The problem statement, all variables and given/known data
    A body moves a distance of 10 meter along a straight line under the action of a force of 5 newtons. If work done is 25 joules, the angle which the force takes with the direction of motion of the body is:
    a: 0 degree
    b: 30 degree
    c: 60 degree
    d: 90 degree

    2. Relevant equations
    work=fdcostheta

    3. The attempt at a solution
    what formula should we use to get the degree or how to figure out it by making a picture?
     
  2. jcsd
  3. May 29, 2016 #2
    Based on the work's definition, to be honest, the more precise version of what you wrote in the Relevant equations, you have to find ##F\cdot d,## since the force is a constant vector here. Hence by what you had, you can get ##W=F\cdot d=|F||d|\cos\theta,## which can lead you to what you want.
     
  4. May 29, 2016 #3
    work = FDcos
    25=5*10*cose
    25=50*cos
    cos=25/50
    cos=0.5

    but what now? 1/2 or 0.5 is equalt to what?? help
     
  5. May 29, 2016 #4
    work = FDcos
    25=5*10*cose
    25=50*cos
    cos=25/50
    cos=0.5

    but what now? 1/2 or 0.5 is equalt to what?? help
     
  6. May 29, 2016 #5
    The value ##0.5## you get is the cosine of the angle ##\theta,## which the the angle of the force and the displacement. Do you realize these?
     
  7. May 29, 2016 #6
    Inverse cos (0.5)=60
     
  8. May 29, 2016 #7
    yes but on calculator the value of cosetheta is 0.99?
    can you get the answer of the given options above?
     
  9. May 29, 2016 #8
    Now the question is given ##\cos\theta=0.5## and you have to find the ##\theta## between ##0## to ##180## degrees. Right?
     
  10. May 29, 2016 #9
     
  11. May 29, 2016 #10
    yes it is 60 by using inverse cosine theta !
     
  12. May 29, 2016 #11
    That's how you find x in cos(x)
     
  13. May 29, 2016 #12
    You don't have to apply the idea of inverse function, though the same, but just have to know what you want, with some basic knowledge of trigonometry, like ##\cos 60^{\circ}=\frac{1}{2}.##
     
  14. May 29, 2016 #13
    25=(5*10)cos(x)
    25/(5*10)=cos(x)=0.5
    cos^-1 (0.5)=60
     
  15. May 29, 2016 #14
    oh! thank you for your quick help. appreciat
     
  16. May 29, 2016 #15
    yes you are right if have this value memorized i would have done thi question in seconds thankyou i will memorize cos30 cos60 and cos90 values right?
     
  17. May 29, 2016 #16
    thanks
     
  18. May 29, 2016 #17
    If you keep learning other physics knowledge, then you'll keep being with these basic concepts, and finally you will memorize, or say acquire, them naturally.
     
  19. May 29, 2016 #18
    60°
     
  20. May 29, 2016 #19
    yes you are right , this question is solved
     
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