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Homework Help: Theoretical description about free fall and acceleration

  1. Jun 24, 2014 #1
    Hey everyone... I have a question about free fall and acceleration.

    I recently had a question on an exam that went something like this. "A ball is thrown directly up from a building 59.4 meters high. It barely misses the building on its way down and hits the ground 4 seconds after being thrown. What is the final velocity of the ball?"

    I got the answer correct through a plug and chug method of using the formulas. However, I don't understand.

    How does one simple formula account for both the ball moving up and then coming back down? Why don't you have to use two formulas... The first showing the final height after the ball was thrown from a height of 59.4m, and the next formula taking that maximum height to calculate the balls final velocity when it hits the ground.

    I'm confused how the formula incorporates both the going up and going down of the ball.

    Please help! I'd like a conceptual description if possible.
  2. jcsd
  3. Jun 24, 2014 #2
    The kinematic equations provide a description of the behavior of an object when a constant force is applied, in this case the force of gravity.
    From this the velocity of the object and as an extension the position of the object can be determined.

    Why does it work for both "up" and "down". Well to confuse the issue the ball is only ever travelling in one direction. When the ball is thrown it is travelling "up" with a positive velocity. Just before it hits the ground the ball is still travelling "up" just this time it has a negative velocity.
  4. Jun 24, 2014 #3


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    You get the relation between height of the ball and the time of flight by assuming constant acceleration. It is just a second order function of the time, like a parabola. You can describe both branches - rising and descending- of a parabola, with the same formula y=ax2+bx+c, don't you? In case of a vertically thrown ball, y =g/2 t2 + v0t+ h, (v0 being the initial velocity and h is the initial height). Plotted on the t-y plane, it is a parabola.

  5. Jun 24, 2014 #4
    In that specific example you have a 1D vertical motion and you are wondering why same formula works for both moving up and down.
    A similar problem can be a car moving forward and then backward.
    In these problems it doesn't matter whether a particle is moving forward - backward or up - down, what matters is the total displacement.
    Once you set up your coordinate system and defined positive and negative directions, you just substitute the initial position from the final position of the particle to get the displacement. And in all those 1D kinematic equations (like y = v0t + 0.5t2) you just use this displacement (here y). Its not important what the particle is doing between its initial and final positions.
    Going up or going down, a formula that uses displacement is enough.
    Last edited: Jun 24, 2014
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