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Homework Help: Initial Velocity lab

  1. Sep 23, 2008 #1
    1. The problem statement, all variables and given/known data
    We are doing a lab where the teacher took an chalkboard eraser and he put it between to rulers in a hallway. He used some sort of slingshot to project that eraser.( we did not see how far the eraser went) We have to find how far the eraser will travel in meters. All we were given was an initial velocity of 13. We have to find the distance using that number.

    2. Relevant equations

    3. The attempt at a solution
    I know velocity is m/s but all we know is the initial velocity. I don't think its possible to find the distance with this much information. Please help. (We are learning derivative and integrals that might help I'm not quite sure)
  2. jcsd
  3. Sep 23, 2008 #2


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    If the angle was horizontal, then you can figure it out. The horizontal velocity times the time to drop from rest will tell you how far it went.
  4. Sep 23, 2008 #3
    The instructor is shooting the eraser on the floor in the hallway. So the angle is 180 degrees right? but he shoots the eraser forward horizontally he does not drop it. Also we don't know how long the eraser travels on the floor. Is their a formula to solve this problem with just the initial velocity
  5. Sep 23, 2008 #4


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    Oh. I see not through the air. Well then you need to know what the coefficient of friction is with the floor. That is the retarding force.

    So distance is given by Vf2 = Vo2 + 2*a*x

    But a is unknown.

    Determine a is what you need for that

    Otherwise time it until it stops. Then you can figure the distance. Or the a and just use the velocity formula.
  6. Sep 23, 2008 #5
    I am really sorry for not being clear enough and wasting your time. The teacher did the lab outside so we could not see how long it took the eraser to move or how far the eraser went. When he came inside he said the initial velocity is 13 and he told us to find a formula and predict how far the eraser went. We do not know seconds and we have to solve for meters but my guess is it traveled around 14 meters? ( I have no clue on how to solve it with a formula that i have to create on the second week of first year physics with only one number initial velocity of 13)
  7. Sep 23, 2008 #6


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    Get an eraser (the same eraser?) and slide it along the floor. Time it until it stops. Measure the distance.

    Then apply what you find to the initial condition he gave you.
  8. Sep 23, 2008 #7
    One last question. He used a sling shot so i assume the eraser is f;lying down the hallway. I did not see how fast it was going so how do i know how fast i should slide it along the floor.
  9. Sep 23, 2008 #8


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    Doesn't matter. x = 1/2 a t2

    Time and distance yields a.

    Then you can use the first equation where he told you it was 13m/s

    132 = 02 + 2*a*x

    The best he could have done is launch it at a 45 degree angle. And you can calculate that distance.
    Last edited: Sep 23, 2008
  10. Sep 23, 2008 #9
    Thank You
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