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Homework Help: Velocity homework

  1. Mar 10, 2006 #1
    I am just getting into physics and I have met a problem, I have almost all of the information I need to make a problem for other students in my class but the problem is I do not have the time factor in v=d/t is there any other way that I am able to calculate for Velocity?
  2. jcsd
  3. Mar 10, 2006 #2

    Velocity is the time derivative of the inital position function x(t). You need time. Perhaps your time value is meant to be a variable. It would help if you descrbed your problem a little more

    If you are given the directional velocities(D.V.) i.e velocities in the x, y, and z directions, you can find the overall velocity by adding the squares of the D.V. and taking the squareroot
    Last edited: Mar 10, 2006
  4. Mar 11, 2006 #3
    Do you have accelleration?
  5. Mar 11, 2006 #4
    I don't think I was fully clear, I am trying to make a problem for fellow students in my class (I'm in 7th Grade). For my problem I don't have an acceleration because I am trying to make up the whole thing from (almost) scratch. What I have is the Drag Coefficient, Distance, Mass, Density, Volume, and Area. What I am trying to do is calculate the time it would take to fall an x amount of distance then make it into a problem using everything. And in the end it will ask them to calculate for Drag and Terminal Velocity. The problem really won't be very complicated, but the only thing I still need is velocity, so I can solve for q in Drag=mg-qv^2.

    I guess what I should be asking is if there is a different way to find time besides t=d/v because if I had the velocity wouldn't I know the time in the first place? Or maybe is there a different way to find Drag and q without knowing Velocity?

    EDIT: Problem will be somewhat like a problem with alot of unknowns but they can be found easily..

    A 20x10x5 brick with a volume of a, a density of b, and a mass of 2.27kg is falling from a height of c, the time it takes for it to fall that distance is (?) when the velocity=(?), the Drag Coefficient of this brick is 2.1. With that information solve for each of the unknowns and then solve for Drag and Terminal Velocity whereas Drag=mg-qv^2 and where q=1/2ρCdA. Vt(Terminal Velocity)=squareroot(2mg/CdρA).

    The (?)'s are the things I don't know yet.
    Last edited: Mar 11, 2006
  6. Mar 11, 2006 #5
    Is it falling from rest? If so, they can use

    x=v-initial *t +1/2*a*t^2

    And if it's falling, you do have an acceleration, g.
  7. Mar 11, 2006 #6
    You mean the acceleration due to gravity, 938m/s^2?

    And its just being dropped, to find the velocity will I have to a website that has done this with a falling brick and calculated its velocity by experiments, or is there a different way I could be able to find what t is equal to?
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