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Homework Help: Projectile in Motion

  1. Feb 5, 2007 #1
    1. The problem statement, all variables and given/known data
    A rocket powered hockey puck on frictionless ice slides along the y-axis. The front of the rocket is tilted at an angle from the x-axis. The rocket motor ignities as the puck crosses the origin, exerting a Force(thrust) on the puck.
    A. Find algebraic equation y(x) for the pucks trajectory
    B. Thrust= 2N.
    Mass of rocket/puck togther =1kg
    Intial speed is 2.0m/s^2 along the y-axis. Make graph of f(x) at 45 deg. and -45 deg. fromx=0 to 20m

    2. Relevant equations
    y=y(nought)+vt+.5at^2
    x=x(nought)+vt+.5at^2
    a=F(net)/m

    3. The attempt at a solution
    A. I get: y(x)=V*(sqrt(2x/a*cos))+xtan
    But it is incorrect.
    B. at -45 deg, y(8.5) is suppose to equal 0 but my equation doesnt follow this.
     
    Last edited: Feb 6, 2007
  2. jcsd
  3. Feb 5, 2007 #2
    Anyone?...
     
  4. Feb 5, 2007 #3
    Its always difficult to answer these questions without the work shown. What would help is to show the x and y accelerations as a function of angle. In one case the puck will try to fly opposed by gravity, in the -45, the rocket thrust will add to gravity. In both cases the x acceleration is some fraction of the rocket force.
     
  5. Feb 6, 2007 #4
    actually, the puck does not leave the ground; the puck is just in a x/y plane on ice taking on a projectile motion.
    For X: x= .5a(x)cost*t^2, a(x) is acceleration in the x direction.
    Y: y=vt+.5a(y)sin*t^2, a(y) is the acceleration in the y direction.
    I solved for t in my x equation and got t=sqrt(x/a*cos)
    Substituting t into Y I get y(x)=V*(sqrt(2x/a*cos))+xtan.
    a=2m/s^2 so the equation can simplify to
    y(x)=V*(sqrt(x/cos))+xtan.
     
    Last edited: Feb 6, 2007
  6. Feb 6, 2007 #5
    lets call a(x)=a1. So x=.5*a1*cos(theta)*t^2 as initial Vx=0;

    then t=sqrt(2x/(a1*cos(theta))) as in somewhere the .5 got dropped?
     
  7. Feb 6, 2007 #6
    ok, my prof. gave us the solution to this problem today. I did get the generic equation right but still the wrong value. He said:
    y=sqrt((2*1kg*x)\(2N*cos45))+xtan45; this simplyfies to :
    y=2.38*sqrt(x)+x, but how? where does this 2.38 come from? xtan45=1x but what happens to the other terms?
     
  8. Feb 6, 2007 #7
    Scratch that last note, I got it
     
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