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Homework Help: Calculating Acceleration = what?

  1. Feb 25, 2013 #1
    1. The problem statement, all variables and given/known data

    For a school assignment, I have to make a choice of "appropriate methods of measurement that could be used to provide a full description of the motion". I need to calculate the acceleration of a peregrine falcon, and show my working out. What I have so far:
    - av. speed = d/t
    = 735/5
    = 147 m/s

    tada! :P Yeah, I'm really bad at physics. It's going to be really basic for you guys, and you'll probably be all like, "what? this person is actually serious? wow -.- " So sorry it's really simple, but any help would be really appreciated!

    2. Relevant equations
    I have absolutely no idea, sorry :/

    3. The attempt at a solution

    av. acceleration = d/t/t
    = 147/5/5
    = no this looks wrong, I'm going for help

    thanks in advance!
  2. jcsd
  3. Feb 25, 2013 #2

    Simon Bridge

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    Presumably the bird is in flight and you are on the ground... what can you measure about the bird's motion and how would you go about doing that? i.e. you may want to measure the bird's position... how? Remember that the bird moves in 3D. Is there anything else you can measure?

    That's the first step.
    The next step is demonstrating that the measurements could, indeed, be used to work out everything about the bird's motion.
    So what is "everything" about the birds motion? Make a list of the things that you need to be able to compute from the data.

    What I don't get is where you got those numbers you used from?
  4. Feb 25, 2013 #3
    I'm not really sure where I got the numbers from '-.- Science teacher gave them to me :P But, I have had a few more clarifications. The formula I'm supposed to use is:
    a = v-u/t
    I'm measuring the acceleration of a falcon's dive. (sorry, probably should have made that more precise in my intro thingy) I'm not sure how to calculate the velocity; I do know that the average speed is 735 m per 5 secs, or 147 m/s.
  5. Feb 25, 2013 #4

    Simon Bridge

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    How are you measuring acceleration? Did you fix an accelerometer to the falcon?
    I suspect you mean that you have to use the "appropriate measurements" to calculate the acceleration.

    So what are you measuring?

    I could be misreading the question - by "appropriate measurements" perhaps you teacher means "approriate calculations from the provided data"...
    The data was the length of the dive (did the bird plummet straight down?) and the time it took ... which we can call d and T.

    Do we assume the acceleration was constant (or we want the average acceleration?)

    Do you know about the suvat equations yet?
    v=u+aT is one of them ... that means that a=(v-u)/T - those brackets are important.

    you want acceleration in terms of distance and time.
    List what you know and find an equation with what you know, and an "a" in it.
    Solve for "a".

    Maybe that's all you need?
  6. Feb 25, 2013 #5
    As far as I'm aware we were looking for the average acceleration. The idea was that we would each be given a unique example of motion. My example was the peregrine falcon in stoop (diving straight down from cruising speed). We were then meant to write an essay style analysis describing our scenario (the falcon diving); draw and describe forces on the moving object (tick!); and use numerical data to describe the motion using numbers, calculations, and plots (d,v,a). I wanted to calculate the average acceleration because I was talking to her in class and she said that it would be a good example if I could pull it off (showing all working out, of course -.-)

    What does 'u' stand for?
    What I know is the average speed, the distance and the velocity.

    I'm really sorry, this is probably a really bad question and I'm not the smartest at physics haha :/ Thanks so much for your time and patience with me!
  7. Feb 25, 2013 #6

    Simon Bridge

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    u = initial velocity ... because writing subscripts is a pain.

    You know the average speed from the distance and the time right?

    Your force diagram should have weight and drag on it... drag points up and depends on the speed. If you want to say that the drop has a constant acceleration, then you want to talk about how the bird is streamlined. Essay questions get you like that - you have to talk about everything you do - but the advantage is that you only lose by sounding confused or leaving stuff out.

    The suvat equations come from an assumption of constant acceleration.
    They are called that after the symbols:

    u=initial velocity
    v=final velocity

    There are five of them - each missing one of the letters.
    But you only need to know two if you are good at algebra.

    For the bird u=0 is OK if the bird stops in flight and plummets - but if it turns over, then the initial speed is probably close to it's cruising speed ... see how the essay part affects the calculation?

    t and s are given.
    you don't know v and a.
    so you have two equations and two unknowns...

    Your instinct that you cannot just take the average velocity and divide by the time again is a good one - because a constant velocity should have a zero acceleration. You actually need two velocities to get an average acceleration that way.

    When you get the acceleration - don't forget to compare it with the acceleration due to gravity alone and comment on what the comparison means.
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