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Homework Help: Projectile motion Javelin

  1. Sep 18, 2013 #1
    1. The problem statement, all variables and given/known data
    Jill throws a Javelin by first accelerating it from rest through 85 cm and releasing it from a height
    of 2.2 m at an angle of 35 degrees and it goes a distance of 62 m.

    A) What are the values of the vertical and horizontal components of initial velocity?

    B) Find the following:
    X-initial position, Y-initial position
    X-final pos., Y-final pos.
    X-velocity initial, X-velocity final
    Y-velocity initial, X-velocity final
    Acceleration in x and y direction
    and t-final for X and Y

    C) What is the magnitude of the impact velocity vector?

    D) What is the impact angle?

    2. Relevant equations

    I'm sure I have missed one:
    a= (Vf-Vi)/(tf-ti)
    x(t)=xi + (Vx)(t)
    y(t)=yi + (Vy)(t) - (1/2)gt^2)

    3. The attempt at a solution

    I started writing down all my known terms and have the following from the given info:
    Xi = 0
    Yi = 2.2m
    ax = 0
    ay = -g = 9.8m/s^2
    Yf = 0
    Xf + 62
    Theta = 35 degrees

    At this point I went to plug known values into the x(t) and y(t) equations to find unknowns. The needed unknowns I see now are velocity and time of impact.

    I am completely stumped on the next step to take from here to find velocity or time of impact. Any help would be appreciated.
  2. jcsd
  3. Sep 18, 2013 #2


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    Staff: Mentor

    So, does the "initial" position and velocity of the javelin correspond to when it was released by Jill, or to its position when it first begins to accelerate? The problem statement is not specific about this so it's badly posed by its author. Just thought I'd mention that.

    Assuming that the author meant the initial position and velocity correspond to the instant of release then you need to write the kinematic equations for the horizontal and vertical motions that take place from that instant until the javelin lands. Set the initial x position to zero and the initial height to the given release height for that instant. The initial velocity should be decomposed into components via the appropriate trigonometry. The javelin "lands" when its vertical height reaches zero. Two equations in two unknowns: Solve for v and time to impact.
  4. Sep 19, 2013 #3
    I have been sitting here trying to find which variable to choose, and which equation to try to work with the find t or v. There just seems to be an unknown variable too many in every one. I also wonder about the wording. I just need a hint to get started on this.
  5. Sep 19, 2013 #4


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    Science Advisor
    Homework Helper
    Gold Member

    You need to introduce an unknown for the launch velocity, v. Since you know the launch angle, you can write the horizontal and vertical launch speeds in terms of that. So for the price of one unknown you get two initial variables.
    You can do the same with time. The time taken is the same for both horizontal and vertical motions. So your two 'distance = initial distance + initial speed * time + etc.' equations share two unknowns - solve.
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