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Analytical kinematics

  1. Mar 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A ball is fired vertically upward from the top outside ledge of a 256ft high building with an initial velocity of 160ft/sec. Assue lgl=32.0 ft/sec^2.

    2. Relevant equations
    What is the speed of the ball at t=3 seconds?
    What is the velocity of the ball just before it hits the ground at the base of the building?
    What was the total time the ball was in the air?

    3. The attempt at a solution
    it takes the ball 5secs to hit the max point of trajectory, the ball goes 400ft upward,

    to find speed you have take the change in distance/ change in time. the time would be 3-0 but the distance is where im stuck.

    to find the velocity of the ball just before it hits ground. you take the change in displacement/change in time, but what is the time

    the totall time it is in the air, i am totally lost on
  2. jcsd
  3. Mar 3, 2009 #2
    V = distance/time is nice for calculating the AVERAGE velocity of an object over some distance. However, the average velocity is not what the question asks for. The question asks for the instantaneous velocity of the object just before it hits the ground. You need to find a different equation that will help you calculate the instantaneous velocity of an object undergoing a constant acceleration.
  4. Mar 3, 2009 #3


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    Homework Helper

    Welcome to PF.

    It's best to focus on answering the questions they ask, but it's good that you know how to find those things.

    For a) you know how to find the time to max height, so why can't you figure what happens after 3 seconds with whatever equation you used to find 5 sec?

    For b, Figure the max height. You did determine it goes up 400 ft. So how long does it take to drop to the ground below - 400 + 256 below? And how fast will it be going when it does? You know distance and acceleration.

    Here are some formulas you might ind useful.

    Your observations on distance and time are going to be that useful in answering the questions they ask because they want to know instantaneous v.

    For c) time up + time down = total time. You have half of it already.
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