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Homework Help: Speed Problem

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
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    2. Relevant equations

    3. The attempt at a solution
    I tried using the acceleration, ax, and the distance, 9m, to find the time. THen, since speed = d/(delta)t, I got 2.12 m/s. However, the book says that the answer is 3.00m/s. This is for part A.

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  3. Jan 18, 2015 #2


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    It's not clear what calculations you have done. Please post your work in detail.
  4. Jan 18, 2015 #3
    The problem talks about a ball, at rest, going down an inclined plane with an acceleration of 0.5 m/s^2. The inclined plane is 9 meters long. Once the ball gets to the bottom it goes back up another inclined plane. which is 15 meters long, it goes to rest. The question wants to know the speed when the ball is at the bottom of the first inclined plane. So, basically, the question stated that we have an acceleration of 0.5 m/s^2 but didn't tell us the time. So, to fin the time, I divided the acceleration by 9 meters, the distance. The answer would be a decimal with a unit of 1/s^2. Then, I inverted the decimal and got 18 s^2. Finally, I sqaure rooted the answer to get 4.24 seconds. My assumption was that this was the (delta)t. Now, to find the speed at the bottom of the inclined plane, I used d/(delta)t, where d is the total distance and (delta)t is the change in time. Thus, I did 9m/4.24s and got 2.12 m/s for the speed. However, the books answer was 3.00 m/s.
  5. Jan 18, 2015 #4


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    Yeah, I read the problem statement and such.

    What I wanted to see was your calculations, not a blow-by-blow account of "I did this, and I got that, but you know, I'm thinking I needed to do this as well ..."

    This is Physics Forums, and we deal with equations and calculations here.
  6. Jan 18, 2015 #5
    Well, I dont have a solid camera and cant take a solid picture. I explained what I did and wanted help to see what I did wrong and understand why it is this way conceptually. I'll go to a math forum. Hoefully I can get a better explanation there.

    Moderator note: Removed insult.
    Last edited by a moderator: Jan 18, 2015
  7. Jan 18, 2015 #6
    Im pretty sure my ca;lculations are correct, but my set-up is probably wrong. I just don't see why I cant get any help in setting the problem up. I can do the math. If I had trouble with that, I wouldn't be in Differential Equations.
  8. Jan 18, 2015 #7


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    Although you made it so the dimensions are correct, this is not a valid way of finding the time.

    Call the unknown final speed "V"
    In terms of V, (and the given acceleration) how long will it take to reach the bottom?
    In terms of V, what is the average velocity of the ball?
    If you multiply the average velocity and the time it took, what should the answer come out to be?
  9. Jan 18, 2015 #8


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    Writing out a verbal description of calculations almost always involves more work than just typing the equations in, even as a last resort.

    Good luck with trying this approach on a math forum. They'll probably tell you the same thing I did.

    And I'd watch that kind of snarky attitude of yours. It'll get you banned from most help forums, including this one.
    Last edited by a moderator: Jan 18, 2015
  10. Jan 18, 2015 #9
    @Nathanael Thanks for the response
    I figured the problem out on the math forums, so I get how to do it now.
    Thanks for the advice. At least I got actual help from other who can actually explain the conceptual portion of it and how to set it up. They made perfect sense.
  11. Jan 18, 2015 #10


    Staff: Mentor

    Often the hardest part is setting up the equations. I gather that you posted the problem on a different site, and got some help there. We would have been more than happy to help if you had shown the work you did, rather than provide a brief summary of it. When you post a homework problem here at Physics Forums, we require that you show the work you have done. Your first post in this thread just barely qualifies for "showing the work."

    Most of the homework helpers and mentors on this forum would have responded exactly the same as SteamKing did, so I am in complete agreement with him. Also, if you wish to last very long here you need to dial back your attitude... Fair warning.
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