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: Projectile Motion: explaining why Vi=Vf

  1. Nov 29, 2009 #1
    How can i show that vi=vf in projectile motion using variables?

    Could i just say that since its a parabola we know that vx isnt going to change, but that vy is because it is affected by gravity. so when it reaches max. height, and vy=0, you know that half of the flight is done, and that the object will follow the opposite path it took to get up there, to go back downwards resulting in the same velocity since gravity and vx is constant.

    except i dont know how to show this using variables..
  2. jcsd
  3. Nov 29, 2009 #2
    you may have to answer why Vx does not change... any forces?

    And you could use one of your kinematic equations to show that in the vertical that Vyo is equal to - Vyf when in free fall and assuming the change in position in the vertical is equal to zero.
  4. Nov 29, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi shocklightnin! Welcome to PF! :smile:
    (I assume you mean for the initial and final heights being the same?)

    One of the standard constant acceleration equations will do it. :wink:
  5. Nov 29, 2009 #4
    Re: Welcome to PF!

    You could show that the time needed for the mass to get up to height "h" is the same time it takes for the mass to get from height "h" to the ground. Then using this time calculate Vf, and it will = Vi.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook