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: About impulse and directions

  1. Dec 29, 2008 #1
    1. The problem statement, all variables and given/known data

    a tinnes ball of mass 150 gm moves with velocity 11.2 m/s to collide with a racket , after collision it is rebounded with vilocity 7 m/s ...find :-the impulse of the racket on the ball.
    2. Relevant equations

    I = m (delta) v

    what i want to know .. is
    when do we use the vilocity with the negative direction ??

    i mean when we answer it

    can we calculate

    0.15 * ( 11.2-7)= 0.63 kg.m/s


    0.15* (11.2-(-7))=2.73 kg.m/s

    so what is the right answer ???!
  2. jcsd
  3. Dec 29, 2008 #2


    User Avatar
    Gold Member

    Think about it. If we define the direction of the initial velocity to be positive, than what must the sign of the resultant final velocity be?

    Hint: The ball's final velocity is opposite in direction to it's initial velocity.
  4. Dec 31, 2008 #3
    i know

    i thought that second answer is the right one as it is opposit in direction

    but i found different answers in the books so i got confused

    but also i want to understant ... we want to find the change in velocity so when we give the final one -ve sign we will add them so how ???

    im sorry im asking alot of questios

    thank u
  5. Dec 31, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The change in velocity is always Vfinal - Vinitial.

    The direction that you call negative is arbitrary. For example, if you call toward the racket as positive, then away from the racket must be negative.

    Pick a sign convention, then determine Vfinal and Vinitial.
  6. Jan 3, 2009 #5
    thank u
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook