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Math Help - Physics, vertical kinematics, gravity

  1. #1
    Member realintegerz's Avatar
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    Physics, vertical kinematics, gravity

    I'm having trouble with a few problems... =(, they involve vertical movement

    And these are equations that are supposed to help and use in each problem:

    For the following eq.'s, Gravity = 9.8 m/s^2 or 32.2 ft/s^2


    A) Final velocity = initial velocity + gravity x (time)
    B) Displacement = 1/2[inital + final velocity]time
    C) Displacement = [Initial velocity x time] + 1/2(gravity)(time^2)
    D) [Final velocity]^2 = [Initial velocity]^2 + 2(gravity)(displacement)


    1) 2 identical pellet guns are fired simultaneously from the edge of a cliff. These guns impart an initial speed of 30m/s to each pellet. Gun A is fired straight upward, with the pellet going up and then falling back down, eventually hitting the ground beneath the cliff. Gun B is fired straight downward. In the absence of air resistance, how long after pellet B is hits the ground does pellet A hit the ground?

    When A reaches back to where it started and falls down, it has the same speed as B (30m/s) but like the problem states, it takes the trip up as well..

    The answer is 6.12 seconds, how do I get that answer...

    2) A wrecking hall is hanging at rest from a crane when suddenly the cable breaks. The time it takes for the ball to fall 1/2 way to the ground is 1.2 seconds. Find the time it takes for the ball to fall from the rest all the way to the ground

    The answer is 1.7 seconds, and im totally lost on this one
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  2. #2
    Member realintegerz's Avatar
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    And for the displacement, it is actually y, but y = displacement, or represents it
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  3. #3
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    skeeter's Avatar
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    1. v_f = v_0  + at

    for gun A, let t_1 be the time for the pellet to hit he ground ...

    v_f = 30 - 9.8t_1

    for gun B, let t_2 be the time for the pellet to hit he ground ...

    v_f = -30 - 9.8t_2

    since v_f is the same for both guns ...

    30 - 9.8t_1 = -30 - 9.8t_2

    30 + 30 = 9.8t_1 - 9.8t_2

    60 = 9.8(t_1 - t_2)

    6.12 = t_1 - t_2

    2. y = \frac{1}{2}at^2

    falling halfway ...

    \frac{y}{2} = \frac{1}{2}a(1.2)^2

    falling the entire way ...

    y = \frac{1}{2}at^2

    double the halfway equation ...

    y = a(1.2)^2

    since y = y ...

    \frac{1}{2}at^2 = a(1.2)^2

    solve for t.
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  4. #4
    Member realintegerz's Avatar
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    so i can substitute 9.8 m/s^2 for a ?
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  5. #5
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    so i can substitute 9.8 m/s^2 for a ?
    Yes, you can, but you won't need to. a will disappear when you solve for t
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  6. #6
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by realintegerz View Post
    Gravity = 9.8 m/s^2 or 32.2 ft/s^2
    Pet peeve alert! Pet peeve alert! Danger Will Robinson!

    Gravity is a force, not an acceleration. What you mean to say is that the acceleration due to gravity is 9.8 m/s^2, or simply g = 9.8 m/s^2. I have seen students get VERY confused on this point.

    And I have seen a few instructors doing this too. They should be lightly coated in cornmeal (season to taste) baked at 350 F for 10 minutes and consumed by rabid hyenas.

    End of pet peeve. You may now return to your regularly scheduled homework.

    -Dan
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