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Acceleration of a Falling Object from Average Velocities

  1. Feb 23, 2013 #1
    1. The problem statement, all variables and given/known data

    After gathering data via ticker tape, of a falling object, I had the following information:

    Time (s), Displacement (cm)
    0.0 - 0.1 , 6.2
    0.1 - 0.2 , 16.0
    0.2 - 0.3 , 24.5
    0.3 - 0.4 , 33.1
    0.4 - 0.5 , 43.0
    0.5 - 0.6 , 49.7

    The Question:

    Determine the average acceleration. Is the acceleration due to gravity uniform? Explain your answer using evidence from your investigation.


    2. Relevant equations

    Avg. Velocity = Displacement / time

    Avg. Acceleration = (Change in Velocity) / time


    3. The attempt at a solution

    I calculated the average velocities (cm/s) as 62, 160, 245, 331, 430, 497.

    I then calculated average velocity as 725cm/s/s :
    Avg. Accel. = (497-62) / 0.6 ​


    I was planning on proving that it is uniform acceleration by calculating the acceleration between two consecutive points however when doing so, I always get an acceleration much less than 725cm/s/s.
    Accel. = (331 - 245) / 0.2 ​
    = 430cm/s​

    I get something similar with all consecutive points and I don't understand why/ I don't know what I'm doing wrong.

    Please help!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 23, 2013 #2

    tms

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    I assume the object starts at rest. In the first interval the speed goes from 0 to 6.2 cm in 0.1 sec. In the second interval it goes from 6.2 to 16.0 cm in 0.1 sec. What is the average speed in each of those intervals?
     
  4. Feb 23, 2013 #3
    I guess I didn't set up my table properly but by displacement I meant the change for that time interval. As in between 0.1 and 0.2 seconds it travelled from 6.2 to 22.2 for a displacement of 16cm.
     
  5. Feb 24, 2013 #4

    tms

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    That's okay, you should just label it correctly.
     
  6. Feb 24, 2013 #5

    tms

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    There are only 0.5 seconds between those two measurements.
    And there is only 0.1 seconds between those two measurements.

    Try doing for acceleration what you did for velocity: At each point put the change in velocity in the table. You might also want to draw graphs of position, velocity and acceleration with respect to time.
     
  7. Feb 24, 2013 #6
    Oh! That makes sense! thank you very much! A few last questions: How would I properly label my chart to indicate the difference in displacement values at each point? Or should I just add them cumulatively to avoid the problem? Also, on a graph I'm assuming the first point would be 0.1sec and 62cm/s even though 62cm/s is the average velocity between 0s and 0.1s. Is this just for simplicity?
     
  8. Feb 24, 2013 #7

    tms

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    I'd label them t, dt, dx, x, dv, v, and a.
    Sweeping the dust under the rug is not a long-term solution.
    I was assuming it started out at t = 0 with x = 0 and v = 0. Or you could leave out the t = 0 point, if the assumption worries you.
     
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