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: Change in Kinetic Energy

  1. Dec 11, 2013 #1
    1. The problem statement, all variables and given/known data

    If the resultant force acting on a 2.0kg object is equal to (3i + 4j) N, what is the change in kinetic energy as the object moves from (7i -8j)m to (11i -5j)m

    2. Relevant equations

    w = Fd
    Ek = 1/2mv2

    3. The attempt at a solution

    I assumed that Ek = w so I tried to solve it by getting the change in displacement and then getting the dot product using the given force (did not use the mass at all)

    d= (11i -5j) - (7i -8j)
    = (4i +3j)

    w= (3i +4j)(4i + 3j)
    = (12i + 12j)

    |w| = 17 J

    ... this answer was totally wrong, as the answer should be 24 J.

    I can't really grasp the relationship with force and kinetic energy.. would appreciate some help. thank you.
  2. jcsd
  3. Dec 11, 2013 #2
    Your dot product is incorrect. Remember it's also called the scalar product, because it produces a scalar, not a vector.

    The dot product of two vectors, [a,b] and [c,d], is ac+bd, a simple number. In your case you have everything else correct.

    Gotta kick that habit of tacking i and j back on there after evaluating the dot product!

    You can also see how this is wonky because you're saying w, work, is a vector quantity, which energy is not!
  4. Dec 12, 2013 #3
    Ohhh!! thanks so much!! so i really didn't need the 2.0kg?
  5. Dec 12, 2013 #4
    No, you really don't. The force over the distance is the work done, or change in kinetic energy. If you have a very massive object, the change in kinetic energy would be the SAME, but the resulting velocity would be lower!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted