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: 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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook