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Distance traveled when Accel=0

  1. Feb 17, 2013 #1
    1. The problem statement, all variables and given/known data

    The gravitational force on a baseball is -Fg. A pitcher throws the baseball with velocity v by uniformly accelerating it straight forward horizontally for a time interval Δt = t − 0 = t. If the ball starts from rest, determine the following: (Use any variable or symbol stated above along with the following as necessary: g for the acceleration due to gravity.)

    a.Through what distance does it accelerate before its release?
    b. What force does the pitcher exert on the ball?

    2. Relevant equations
    displacement =vi+at^2 for (a.) And random variations that have not worked out as of yet. But i feel i am making it harder than it really is.
    i thought -1/2 at^
     
  2. jcsd
  3. Feb 18, 2013 #2

    tms

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    You must have left out some information.
     
  4. Feb 18, 2013 #3
    (a) displacement =vi+at^2 .... yes, but it starts from rest.
    (b) a bit trikier: force = mass x acceleration. Your teacher might want you to include Fg, so the force have two components, one up and one forward, if not then just the forward force that the pitcher exerts to accelerate the ball.
     
  5. Feb 18, 2013 #4

    HallsofIvy

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    Your post and its title is difficult to understand. The title says "Distance traveled when Accel= 0" but then you mention both gravitational acceleration, g, and a "uniform acceleration straight forward". So there is no "Accel= 0" in the problem?

    You ask "Through what distance does it accelerate before its release?" If this is only "before its release" it is still in the pitcher's hand? That would imply that the acceleration due to gravity is offset by the pitcher's hand holding it and has no part in this problem.
     
  6. Feb 18, 2013 #5
    Since the pitcher is throwing the ball exactly horizontally, during the pitch there is no net force in the y direction, but there is an acceleration (and hence a force) in the x direction.

    The problem is giving you the final v in the x direction after the acceleration is 0 (ie the pitcher has already released the ball and there is no force in the x direction, only gravity pulling down).

    You have to imagine the process and ask which 'phase' is being asked about. It reads to me like its asking about the phase prior to release of the ball.

    Ball from rest (0 net force everywhere)

    accelerates exactly horizontally (net force only in x direction) to a final velocity v, in time t. Phase 1.

    the ball is released leading to the second phase where there is 0 force and 0 acceleration in the x direction but a non zero force in the y direction from gravity.

    You have enough information to find the displacement (find acceleration first because we know v, vi, and t then you can find displacement knowing v, vi, a, t).

    The question about the force on the ball I'm not exactly sure as it seems you need the mass unless, as stated above you need some kind of trig trick to find an expression. I doubt the latter because in each phase there is net force only in 1 direction so no trig necessary.
     
  7. Feb 19, 2013 #6
    ..."Use any variable or symbol stated above along with the following as necessary: g for the acceleration due to gravity."
    No need to solve, just state formulas with symbols for the variables.
     
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