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Homework Help: Magnitude of a net force?

  1. Sep 17, 2006 #1
    I have these two problems and I cant seem to find the right answer...

    1)A person with a blackbelt in karate has a fist that has a mass of 0.43 kg. Starting from rest, this fist attains a velocity of 5.9 m/s in 0.25 s. What is the magnitude of the average net force applied to the fist to achieve this level of performance?

    2)When a 58 g tennis ball is served, it accelerates from rest to a constant speed of 43 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 39 cm. What is the magnitude of the net force acting on the ball?

    I know the answers are in N, but i've tried using F=ma which hasn't worked...maybe i am confused about what the magnitude of a net force is...please help!
  2. jcsd
  3. Sep 17, 2006 #2
    Show us what you've done and we would able to help you from there.
  4. Sep 17, 2006 #3
    You need to use a very important theorems here: the impulse-momentm theorem

    Impulse = average net force * time of application = net change in momentum
  5. Sep 17, 2006 #4
    well...i dre the fre body diagram for the first one...and labeled what i know. because i have the initial velocity and elocity and time i solved for the acceleration using: V=Vo+at and I found the acceleration to be 23.6 m/s^2.
    For the mass i know it is .43 kg
    I used F=ma and i got 10.15 N...
  6. Sep 17, 2006 #5
    That seems to be correct, although, technically, you shouldn't be using v = u + at, since it hasn't be explicitly mentioned that the accelaration is constant (in the first problem).

    [tex]<a> = \frac{\Delta v}{\Delta t}[/tex], where <a> is the average accelaration.
  7. Sep 17, 2006 #6
    so on the second one, i did the same thing only i solved for acceleration by using v^2=Vo^2+2ax and found the acceleration to be 23.7. I used f=ma again and got it to be 1.37...does that sound right?
  8. Sep 17, 2006 #7
    Yes, it does.
  9. Sep 18, 2006 #8
    It is not correct to use kinematical equation for constant acceleration here. Impulse-momentum gives the answer in a more technially correct form, and you have to less calculation.
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