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Homework Help: How can i use kinematic equations to solve this problem?

  1. May 2, 2012 #1
    Hello everyone,
    This is my first post so I am sorry if this is in the wrong section. I am currently in a rush to finish my homework. I have tried working on it for several hours but I still cannot figure it out.
    I am mainly trying to figure out how long the object will be in the air.

    A stone is thrown upward at a speed of 30m/s off the edge of a 40m tall tower.

    1. How can I find the velocity of the stone versus time. The time is suppose to start at 0 and go up increments of 0.3 sec from when the stone is thrown to the time the stone hits the ground.

    Here are the equations I used: Final Velocity = Initial Velocity + Acceleration x Time
    Distance = Initial Velocity x Time + 0.5 x Acceleration x Time^2
    Final Velocity^2 = Initial Velocity^2 + 2 x Acceleration x Distance

    I will have to graph this but I need to know how to find all the data first. Thank you for your help.
  2. jcsd
  3. May 2, 2012 #2


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    Science Advisor

    The accelaration is -g= -9.81 m/s^2 so the speed, after t seconds, is 30- 9.81t m/s. It's height will be 40+ 30t- (9.81/2)t^2 above the ground. Set that equal to 0 and solve for t. That is a quadratic equation and will have two roots, one of which is negative. That would be the time it was on the ground if you had thrown it up fast enough that it was still going at 30 m/s when it got 40 m above the ground.
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