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  1. #1
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    physics question (basic)

    I am having one hell of a time figuring out this question and hoping someone on BG can help me. I won't give specific numbers because I don't want you to solve the problem for me but, in general:


    let's say an object is traveling with constant acceleration in one dimension. let's also say the object begins by traveling in a negative direction but the acceleration eventually will make the velocity positive. so, the equation might look something like:


    position = (initial velocity [negative]) * (time) + (acceleration [positive]) * (time squared)


    here's the question I can't wrap my head around: How do you find the average speed between two points in time if the velocity is negative at the first point in time but positive at the second point in time?


    note that I'm not asking for the average velocity - that's ez pz, simply calculate the difference in displacement and divide by time, yadda yadda. but in order to find the average speed, I need to find the distance traveled, which is NOT the same as the displacement in this case. how do I do that?


    for example (and these are not the same numbers as in my problem) what is the average speed of the object if the equation is:
    position = -1.5 * (time) + .5 * (time squared)


    from between time = 1 and time = 10.


    mods please move this if you feel the need; I wasn't sure if it would fit better in the general forum or the tech forum.

  2. #2
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    It's been a couple years since I've done any physics, but here's how I would do it.

    Speed is velocity without regard to direction. So if we can find the distance traveled before and after the turnaround (where velocity becomes 0), we can add up the displacement and that's our total distance traveled.

    Using your example x = -1.5t + .5t^2

    The point at which v=0 is where you turnaround. So, get the velocity formula by deriving your position formula.

    v = -1.5 + t

    Solve for v=0

    t=1.5

    You now know that at t=1.5 you turn around. Therefore, from t=1 to t=1.5 you will have an average negative velocity, and from t=1.5 to t=10 an overall positive.

    Using these values you can get displacement from 1 to 1.5, and also from 1.5 to 10.
    x = -1.5(1) + .5(1)^2 = -1
    x = -1.5(1.5) + .5(1.5)^2 = -1.125
    -1.125 - (-1) = .125

    So from t=1 to t=1.5 you moved .125.

    Do the same thing for t=1.5 to t=10

    x = -1.5(1.5) + .5(1.5)^2 = -1.125
    x = -1.5(10) + .5(10)^2 = -15 + 50 = 35
    35 - (-1.125) = 36.125

    So total you moved 36.125 + .125 = 36.25m over 10 sec, therefore your average speed is 3.625m/s.


    I think I did that right, please double check.



    TL;DR

    Determine displacement when velocity hits 0. Determine displacement at the end of the interval since velocity hit 0. Add them together, divide by the total travel time.

  3. #3
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    physics question (basic)

    Speed is magnitude of velocity as mentioned above. |dx/dt|

    10m/s in -x dir + 10m/s in +xdir has vavg=10m/s

    Above is clear walkthrough step by step but to make it exceedingly plain so you get the concept:

    Velocity is a vector, it has a direction. Speed is a scalar, it does not.

    You want your average speed between two trips. You leave your house and go to the store. You stop at the store. You leave the store and drive back to the house. What would you say your average speed was? Without any hesitation you would answer distance to store / time it took to get to store + distance / time it took to get home divided by 2. Easy peasy as you said. Speed is a scalar quantity so it doesnt care whether you were going to the store or coming back.

    If I asked about your average velocity relative to the position of your house as the origin of a coordinate system, that would be a different story.

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    Neither of you are going to point out that the initial equation he wrote down is incorrect?

    d =vi*t+(1/2)*a*t^2, that 1/2 is pretty important.

    or am I misreading something.

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    Yeah he wrote the equation down wrong originally.

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    ok thanks guys. that helps a lot. and yes I wrote it down wrong. I'll go back and try the problem again (it's already graded, I missed .25 out of 10 points... meh) just to make sure I got it. :D


    and yeah I'm definitely not a physics man. this class is intro-level physics for dummies and I'm still not very good with it lol. not that it's hard, I just don't like it at all and it's difficult to get engaged in subject matter you find mind-numbingly boring. but it's required for all computer science majors so I have to get through it. then we can get back to sweet sweet assembly code, cache management, algorithms... all the good stuff


    in any case, I appreciate the help!

  7. #7
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    Quote Originally Posted by Zzsark View Post
    Neither of you are going to point out that the initial equation he wrote down is incorrect?

    d =vi*t+(1/2)*a*t^2, that 1/2 is pretty important.

    or am I misreading something.
    I didn't take a single glance at any equations lol. Fuck equations in plaintext entry format, that's AIDS for the eyes. But tbh I probably wouldn't have caught it if I had cuz constants.

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    Quote Originally Posted by Yuri-G View Post
    mind-numbingly boring....... assembly code, cache management, algorithms.
    FTFY, but good luck

    And I 100% agree Sath. Can you input equations on BG the correct way though? Is there code for that?

  9. #9
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    lol Zzsark, exactly. if I find enjoyment in something as mind-numbingly boring as cache management then I must be on the right path


    kudos to you physicists and rocket scientists out there. I couldn't do it. but a lot of people say that about machine architecture soooooo I guess we all have our weirdness lol

  10. #10
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    Quote Originally Posted by Zzsark View Post
    FTFY, but good luck

    And I 100% agree Sath. Can you input equations on BG the correct way though? Is there code for that?
    https://www.sharelatex.com/ is how I would do it if I really needed to post something on BG. Like codepad.org but for latex entry.

  11. #11
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    alright, I am starting to (grudgingly) enjoy this. also learning that I need to trust my instincts with physics problems. example:


    two blocks on a pulley system, at rest, equal height. they are released and the larger block travels X distance.


    - first question: what is the change in gravitational potential energy? ez pz. mgh of the smaller block minus mgh of the larger block.
    - second question: what is kinetic energy of the system at this point?
    - third question: what is the velocity of the second block at this point?


    I'm thinking, ok, find the velocities, using the acceleration of gravity, 1/2mv^2, you got this. *runs a bunch of numbers* ok, that wasn't right... wait a second, if I have to find the velocity to find the kinetic energy then why is velocity in the third question and not the second? that seems odd...


    take a break, come back. look at the question again. LIGHT. BULB. um, obviously the kinetic energy of the system is the opposite of the loss in potential energy? duh? and obviously from that you can find the velocity given the masses of both blocks, which explains why the questions were posed in that order.


    feels good. REALLY GOOD. starting to get a handle on this... I still hate physics and have no intention of continuing on this path, but at least it's starting to make sense :D

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