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Homework Help: Free Fall Lab Question: Calculating Average Acceleration and Velocity

  1. Oct 11, 2012 #1
    1. The problem statement, all variables and given/known data
    Within this lab I was supposed to perform a virtual experiment and find the rate a ball dropped on the moon, mars, and earth. I have recorded all of my results in the table below. My problem is that my average velocity on earth ended up as 8.9 m/s, and I thought that the constant was supposed to be 9.8 m/s? I don't know where I went wrong in my calculations.

    My other question comes with acceleration, according to my teacher I will only have 4 values for acceleration for each table of data however why couldn't I have five, in the second column could acceleration be calculated?

    Finally when my average acceleration stayed steady at 10.25 and then decreased to 9 this seems incorrect to me because I thought that an object in free fall accelerates constantly?

    Time (s) 0 0.2 0.4 0.6 0.8 1
    Distance (m) 0 0.2 0.8 1.81 3.23 5.01
    Average Velocity (m/s) 0 1 3 5.05 7.1 8.9
    Average Acceleration 0 0 10 10.25 10.25 9

    Time (s) 0 0.4 0.8 1.2 1.6 2
    Distance (m) 0 0.13 0.52 1.17 2.09 3.27
    Average Velocity (m/s) 0 0.325 0.975 1.625 2.3 2.95
    Average Acceleration

    Time (s) 0 0.3 0.6 0.9 1.2 1.5
    Distance (m) 0 0.17 0.68 1.51 2.68 4.17
    Average Velocity (m/s) 0 0.566666667 1.7 2.766666667 3.9 4.966666667
    Average Acceleration

    2. Relevant equations

    Average Velocity= (final distance-initial distance)/ (end time-beginning time)
    When I attempted to solve this for example I did (5.01-3.23)/(.2) and came up with an average velocity of 8.9. I don't see how this is correct and am hesitant to turn in my answer. I am now worried that I have calculated all of my initial velocities incorrectly.

    3. The attempt at a solution

    My attempts at a solution are listed above.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 11, 2012 #2
    The velocity won't be constant throughout the drop, it should be increasing which your data shows. You're thinking of acceleration which should be a constant 9.8m/s^2. Your acceleration isn't 'decreasing', it's just there are most likely measurement errors in recording the data which will affect the outcome. I'd say it's pretty close to 9.8 and since you don't have a large amount of trials, that should be fine.

    You're calculating everything right, but I'm confused what you're trying to get at with this statement:
    Just go about calculating acceleration and velocity as you did for the other drops.

    EDIT: be sure to report that there were 'possible sources of error' throughout this experiment which is why you don't have an exact acceleration of 9.8m/s^2 and what not. Make sure to expand on that and really think what caused the errors. Don't just say 'human error' as that's part of pretty much every experiment. Look at how you did the experiment, the tools you used. If you used a ruler to measure the distance, to what accuracy can you read the tick marks (1/8in, 1/4in, 1/32in, ect)? The more tick marks the more accurate the ruler is and the less error that is introduced. Also look at the timer used to measure times. How accurate is that? Was there noticeable wind during the experiment? Just think about everything that went on during the drop.
  4. Oct 11, 2012 #3
    About 9 m/s is correct average velocity between 0.8 s and 1 s of free fall.

    The fluctuations in average acceleration are due to errors in measurements. For example, the distance at 1 s must be less than 5 m.

    As to why you should not have acceleration in the second column, yes, you can compute and write it there. But observe it will be wildly different from the other values. Why? Because the velocity in the first column - zero - is not average velocity in free fall.
  5. Oct 11, 2012 #4
    I am confused though because the lab was a virtual one, so how could their be errors in data collection? My teacher asks us to calculate our percent error but when there is a virtual lab involved I fail to see how there can be any percent error. Where would the measurement errors for my acceleration come?
  6. Oct 11, 2012 #5
    Look up what the values of acceleration are for earth (9.8), mars, and the moon. Compare your calculated results to those that are recorded. And there are always errors haha. Since this one is virtual, there aren't many, but maybe comment on the number of significant figures reported and if you know at all how the experiment was 'done' then talk about that.
    Last edited: Oct 11, 2012
  7. Oct 11, 2012 #6
    This is not how experimental errors are determined.

    In this particular case, it is known that acceleration must be constant. So the first step in error analysis is to find the mean acceleration. Based on that, the errors in all the values may be estimated.
  8. Oct 11, 2012 #7
    He's asking for percent error though.

    Wouldn't it just be (calculated-known)/known * 100?
    Last edited: Oct 11, 2012
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