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Homework Help: Acceleration,velocity and time of a car

  1. Dec 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A 20 kg car slides 5 m down the smooth(frictionless) ramp with an initial celocity 0.

    a) Determine the acceleration when the car moves from A to B.
    b)Determine the velocity of the car in point B.
    c)Determine the point (distance L) where it strikes the ground at C.
    d) How long does it take from A to C ?

    Image of the car

    http://img205.imageshack.us/img205/1/carcx.jpg [Broken]
    2. Relevant equations

    3. The attempt at a solution

    Can I find the acceleration doing this.

    G=mgsin(30) = 10 N

    F=ma -> a=F/m -> a=2 m/s

    If this is wrong , can anybody tell what i am doing wrong.I haven't solved problems like this before and I want to know if i am doing it right or no
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 22, 2009 #2
    You've defined you x axis parallel to the plane and your y axis perpendicular to the plane so the way you've solved for the acceleration seems right to me.

    See if you can solve b. You know the acceleration, the intial velocity and the distance he travels... If we could solve for the time it takes him to get to point b, maybe we could calculate his final velocity ;)

    Start with that and if you can't solve c, and d let us know where you're having troubles.
  4. Dec 22, 2009 #3
    Thank you. Now b -> we know that a=vdv/ds -> integrate and we find v=2[tex]\sqrt{5}[/tex].
    Now i have problems finding the distance L. If the angle between L and C is also 30 degree i can find it but i don't think it is 30 degree. And i also don't think that BC is a straight line. So can you help me finding this distance L ?
  5. Dec 22, 2009 #4
    The velocity we already defined was parallel to the plane, so we need to break this down into horizontal and vertical vectors and look at that way in order to solve the distance L. See the figure attached. You can solve for the horizontal component of velocity and the vertical component of velocity right? How can you use this new information with the information we know from the drawing ( h = 1 ) to solve the distance L.

    Note that the horizontal velocity will have no acceleration throughout its free fall only the vertical velocity is changing ;)

    Attached Files:

  6. Dec 22, 2009 #5
    No, I can't. Sorry but I need more help. I am trying to understand it but i can't.

    Now we need to find vy - right ? We have v=2[tex]\sqrt{5}[/tex] , we have the angle -> vy = vsin(30) -> vy = [tex]\sqrt{5}[/tex] .
  7. Dec 22, 2009 #6
    DISREGARD THIS POST, I misread your post above.

    Attached Files:

  8. Dec 22, 2009 #7
    Okay so you've established the intial velocity of that segment of its motion. We know the distance it travels vertically so what formula can we use to solve for the time it will take to reach the ground?
  9. Dec 22, 2009 #8
    Ok, but first we need to find this distance L. And I can't find it
  10. Dec 22, 2009 #9
    You need to find what I mentioned above in order to solve for L :p
  11. Dec 22, 2009 #10
    OK. So t=S/v -> t=5 / 2[tex]\sqrt{5}[/tex] -> t=[tex] \sqrt{5}[/tex]/2 . But this is the time from A to B. Now what should i do ?
  12. Dec 22, 2009 #11
    You're not looking for the time from A to B, you're looking for the time from B to the C.
  13. Dec 22, 2009 #12
    OK.So . Can I use this formila t=s/v ? If yes -> t=[tex]\sqrt{5}[/tex] / 10 . And not lets find distance L . S=vt -> 2[tex]\sqrt{5}[/tex] * [tex]\sqrt{5}[/tex]/10 = 1. Therefore distance L= 1
    Last edited: Dec 22, 2009
  14. Dec 22, 2009 #13
    You are using the Vy to help you find t, you know you pass through a vertical distance of 1m so the formula you're looking to use is as follows:

    h = Voyt + 1/2at^2, solve for t

    Hint: Quadratic formula
  15. Dec 22, 2009 #14
    OK, but where this formula come from. Whats the origin formula. Sorry for my questions , but i just can't get it.
    Last edited: Dec 22, 2009
  16. Dec 22, 2009 #15
    D= Vot + 1/2at^2 is a standard formula for constant acceleration problems.
  17. Dec 22, 2009 #16
    Ok. I think i don't know this formula . Thank you very much
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