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Homework Help: Initial Velocity of a Projectile

  1. Sep 23, 2007 #1
    1. The problem statement, all variables and given/known data
    "What is the initial velocity of a projectile"
    We did an experiment where we launched rockets and timed them from the time they were launched, to the time they hit the ground. Our "Problem" on the lab is to find the initial velocity.

    Here are the times we recorded with the first rocket:
    Trial 1: 4.38s and 4.5s
    Trial 2: 3.625s and the second stopwatch malfunctioned
    Trial 3: 3.69s and 3.82s
    Trial 4: 3.97s and 3.28s
    Trial 5: 3.81s and 3.81s
    Here are the times recoded with the second rocket:
    Trial 1: 5.35s and 5.31s
    Trial 2: 5.62s and 4.6s
    Trial 3: 5.18s and 4.6s
    Trial 4: 5.53s and 3.68s

    2. Relevant equations
    I was thinking, to get velocity in general I could use v = at since we know the times and acceleration = 10m/s due to gravity. But the question asks for Initial velocity, and we don't know the final velocity, so therefore our other equations wouldn't work

    3. The attempt at a solution
    v = (10)(change in times)
    *also, if for example, trial number 1 where i subtract 4.38-4.5 to get the change in time, would I write a negative time or just subtract 4.5-4.38?
  2. jcsd
  3. Sep 23, 2007 #2


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    Staff Emeritus
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    What do the two times represent? Is it launch to apogee, apogee to landing?

    There is a burn period in which the rocket accelerates, follow by slowing down with gravity and air resistance. After apogee, the rocket is in free-fall with gravity, but also with air resistance.
  4. Sep 23, 2007 #3
    for this lab I believe we were to disreguarding air restance, and to calculate the initial velocity with only the times and acceleration. We used air-powered rockets, and the two rockets actually represent different washers that we used to control the amount of air-buildup before the rocket was launched. The two times represent two different people timing the same rocket launch with different stopwatches. I hope that helps answer the questions!
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