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: Calculate the angle between the displacement vector and the force vector

  1. Mar 25, 2012 #1
    1. The problem statement, all variables and given/known data

    A force F = 8i-3i N acts on a particle that undergoes a displacement Δr = 2i + j m.
    (b) What is the angle between F and Δr? (state your answer to three significant figures)

    2. Relevant equations

    Angle = cos-1 (A.B/ AB)

    3. The attempt at a solution

    W=13.1 J
    AB= 8.5*2.23= 19
    cos-1 (13.1/19)= 46.4
  2. jcsd
  3. Mar 25, 2012 #2
    I will assume that the force is [itex]F=8\hat{i}-3\hat{j}[/itex]

    the dot product between F and delta-r:
    [itex](8,-3)\cdot (2,1) = 8\cdot 2 + (-3)\cdot 1 = 13[/itex]
    the norm of F:
    [itex]||F||=\sqrt{8^2 +3^2}[/itex]
    norm of delta-r :
    [itex]||\Delta r|| =\sqrt{1^2+2^2}[/itex]
    I got 47.12 degrees but it seems you have used different conditions :/
  4. Mar 25, 2012 #3
    I got 47.121 another time when I did not round, but even that gave me a wrong answer L: I am not sure what's wrong
  5. Mar 26, 2012 #4
    If you are sending it to some kind of online checking system:
    a. Check to see if they want it in radians.
    b. Check if all of the starting conditions are correct.
    c. Take into consideration that they make mistakes too.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook