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!

Initial Velocity - Projectile Motion

  1. Dec 24, 2013 #1

    My end goal is to calculate the vertical and horizontal displacement of an object that explodes in the air e.g. a firework. However, I am having difficulty deriving the initial velocity of the object. I know the approximate time is 5.0 seconds from the ground to explosion and the height is 600 ft (182.88m).

    So far I have used the equations v(i) = (d / t) - [(a * t) / 2] and V(i) = a*t. Any help would be greatly appreciated.
  2. jcsd
  3. Dec 24, 2013 #2


    User Avatar
    Gold Member

    The initial velocity to fire is the same as the velocity an object would be traveling at after falling from its peak altitude.

    Formulas and calculator can be found here:

    If the time is 5 seconds, the altitude would be 123 meters and the initial velocity would be 49 meters/second.

    If the altitude is 183 meters, the time is 6.1 seconds at an initial velocity of 59 meters/second.
  4. Dec 25, 2013 #3
    The 600 ft (182.88m) is the height in which the fireworks breaks and we do know from industry averages that a shell of this size takes 5.0 seconds to reach this height. It is likely that the maximum height is greater than 600 ft which would mean the shell still has a final velocity at the time of break. Is there a way to calculate for these two unknowns?
  5. Dec 25, 2013 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The first equation is fine. That will tell you the initial velocity (vertical component only) which will allow the projectile to reach a given height in a given time. Note that this neglects air resistance, which may well be significant.

    There's no way to determine the horizontal component of velocity from the given data.
  6. Dec 28, 2013 #5
    Thank you for your response it was a big help. If I wanted to become more precise and incorporate air resistance into my formula how would I do this. It was been a long time since I've done something like this.

    Known Values:
    Time = 3.8
    Aprox Height = 300 Ft
    Initial velocity = 140 Ft/s
    Launch Angle = 75 Degrees
    Weight of shell = 3.5 lbs
    Also, the radius of the sphere is 3 inches
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook