1. Not finding help here? Sign up for a free 30min 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!

Motion equation

  1. Dec 15, 2008 #1

    Im trying to make an equation for a projectiles path, but this is only over a very short distance, so the projectiles curve aren't taken into account. Im including the air resistance.
    This is what i have done:
    m*dv/dt=-k*v2 Differential: dv/v2=-k*dt integrated: -1/v=-k*t+C when: t=0 og c=-1/v0 and we get: v=m*v0/(k*v0*t+m)
    I have gotten some help doing this, but i am wondering if anyone could tell me if this is right. Another thing is that i seem to have forgotten "m", could anyone please tell me where "m" should be in the differentaded and integrated equation? i have a hard time explaing actually what i have done, but maybe you could help me.
    By the way, im from Denmark so sorry for my bad english.

    - Michael
    Last edited: Dec 16, 2008
  2. jcsd
  3. Dec 16, 2008 #2
    Anyone might be able to tell me where this: m*dv/dt=-k*v2 comes from?
    Last edited: Dec 16, 2008
  4. Dec 16, 2008 #3


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Michael ! Welcome to PF! :smile:

    (use the X2 tag above the reply field to make v2 :wink:)

    (and what's "og"? :confused:)

    Yes, that's fine :biggrin:

    you lost the m (though somehow you put it back in the right position :rolleyes:) when you went from

    which should have been mdv/v2=-k*dt. :smile:
  5. Dec 16, 2008 #4
    Thanks :), i was wondering where this comes from too: m*dv/dt=-k*v2
    Oh, and "og" is a danish word for "and", my bad :blushing:
  6. Dec 16, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper

    ah, i didn't see your second post :redface:

    mmm … for me, it usually comes from the question :rolleyes:

    if you want to find why air resistance is like that, wikpedia is always a good place to start. :smile:
  7. Dec 16, 2008 #6
    I was meaning the formula m*dv/dt=-k*v2 what's the original formula? or what is it calculated from? i mean dv/dt = a ... so might be F=m*a newtons second law, but i don't know, i would appreciate some help here :)
  8. Dec 16, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    oh i see! :smile:

    yes, it's good ol' Newton's second law :biggrin: … F = ma = mdv/dt …

    so if the force is given as -kv2, then that's mdv/dt = -kv2 :wink:
  9. Dec 16, 2008 #8
    Thank you very much, I appreciate it. :smile:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Motion equation
  1. GR equations of motion (Replies: 4)