Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Projectile motion

  1. Mar 1, 2008 #1
    is it possible to find the initial velocity of a projectile if you only know the Force applied (47N) to the projectile and the mass (0.04kg) of the projectile (assuming air resistance is negligible).. oh and the angle it was fired at was 30 degrees..

    thanks in advance
  2. jcsd
  3. Mar 1, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Good ol' Newton again!

    (Angle doesn't matter - the initial velocity will be the same for any angle!)

    Just use good ol' Newton's second law: force = change in momentum (and initial momentum is zero). :smile:
  4. Mar 1, 2008 #3
    im sorry i am not following
    newtons second law is basically impulse.... which is (delta) momentum / time
    but i don't have time or velocity...
    can you please clarify...

    your help is much appreciated
  5. Mar 1, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    Yes, you're right

    oops! I read your initial question as 47 Newton-seconds. :redface:

    Yes, you're right. The question says 47 Newtons, so you do need to know the time.

    (It must surely be Newton-seconds? - no-one in their right mind would fire a projectile with anything other than an impulse!! :smile:)

    Try it again, assuming that the total force is 47 Newton-seconds! :smile:
  6. Mar 1, 2008 #5
    ok will do
  7. Mar 1, 2008 #6
    ok so i just ran a few calculations and i came out with a number... not sure if it is correct because my knowledge of physics is pretty limited...

    well first i better explain the experiment i am doing (well yeah i guess it is homework and sorry to the mods for putting it in the section but it was never meant to tern into an explanation) .
    i was using a spring to fire projectiles and measure the distance and a bunch of other stuff... but i wanted to find the distance i would get if there was no air resistance (so that i could match it against my results)...

    anyways what i did was measure the distance the projectile was in contact with the firing pin(this was .14m). i used that in the impulse formula (using the .14m as the displacement, in velocity) and i worked out the time to be .011sec (the time it was in contact with the pin (well actually it was a bolt)).. anyways i used that time and displacement to work out the velocity (.14/.011) which equaled 12.7m/s... now to me that look like the figure i was expecting but i am not sure if the way i got it is correct.

    could anyone verify if it is correct or a load of BS..

    thanks alot tiny tim for your help
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook