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Homework Help: Need Help - velocity measuring problem

  1. Sep 23, 2008 #1
    Hey i am doing an experiment for a school physics assignment on projectile motion. I have three variables which i want to change so i can see how i can obtain a maximum range (distance). I have chosen mass of projectile, angle of launch and initial velocity of mass as my three variable.

    My problem is that i have not been able to come up with a way to measure the initial velocity. My experimental setup at the moment is; small mass, rolls down ramp to a small inclined "jump" and then gets projected out and travels a certain distance. Is there some device that will help me measure the initial velocity or can i calculate it by conservation of energy (PEg to KE).

    Any input will be awesome

  2. jcsd
  3. Sep 24, 2008 #2
    Conservation of energy definitely makes sense--subtract the potential energy at the height of the ramp from the initial potential energy at the height of the decline and you will get your kinetic energy, and then you can solve for the velocity, v. Frictional forces are likely to have some effect, so you could try including those in your calculations.

    Are you familiar with the equation that gives the range of a projectile (without air resistance)?


    If not, do your experiment first, and then interpret your results in regards to the variables that appear in the equation. Good luck with the experiment.
  4. Sep 24, 2008 #3
    Thx that is realy helpful. Just talking about other variables, i am hoping to include things like air resistance and friction as negligible. i am estimating to get a distance of about 20cm tops so the mass will not be airborne for very long.

    You said using cons of energy;

    PEg = mgh
    KE = 1/2mv^2


    KE (when leaving ramp) = PEg (at drop height) - PEg (when leaving ramp)
    1/2 mv^2 = mgh1 - mgh2

    v^2 = 2(mgh1 - mgh2)/m

    Then square root and you should have your initial velocity assuming that all potential energy is converted into kinetic and there is no loss of energy to friction or air resistance.

    Does that look right???

    Attached Files:

  5. Sep 25, 2008 #4
    Yes, your equations look good. Great job.

    Also, notice that the initial velocity is independent of mass ("m" cancels out of your equations).
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