Results 1 to 3 of 3

Math Help - Rectilinear Motion Help

  1. #1
    Junior Member
    Joined
    Jan 2010
    Posts
    25

    Rectilinear Motion Help

    I am just learning how to do this for my calculus class so forgive me for being so ignorant, but I am trying.

    For the problems I am doing, these two formulas are being used:

    v(t)= v(sub zero) + at
    s(t) = s(sub zero) + v(sub zero)t + .5at^2

    I'm having a lot of trouble with applying these. For example, the current problem I am on says:

    "Spotting a police car, you hit the brakes on your new Porsche to reduce your speed from 90mi/h to 60mi/h at a constant rate over a distance of 200 feet. (88ft/s=60mi/h)

    Find acceleration in ft/s^2"

    I need to understand which formula is appropriate for various situations. I realize:
    initial velocity= 132ft/s
    final velocity= 88ft/s
    initial displacement=0ft
    final displacement= 200ft

    I have no idea how to find time to solve for acceleration, so both formulas seem useless to me.

    If someone could explain these formulas and help me on this current problem, I would really appreciate it. I've been on this same problem for 3 hours and have gotten nowhere.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Opalg's Avatar
    Joined
    Aug 2007
    From
    Leeds, UK
    Posts
    4,041
    Thanks
    7
    Quote Originally Posted by Simon777 View Post
    For the problems I am doing, these two formulas are being used:

    v(t)= v(sub zero) + at
    s(t) = s(sub zero) + v(sub zero)t + .5at^2

    I'm having a lot of trouble with applying these. For example, the current problem I am on says:

    "Spotting a police car, you hit the brakes on your new Porsche to reduce your speed from 90mi/h to 60mi/h at a constant rate over a distance of 200 feet. (88ft/s=60mi/h)

    Find acceleration in ft/s^2"

    I need to understand which formula is appropriate for various situations. I realize:
    initial velocity= 132ft/s
    final velocity= 88ft/s
    initial displacement=0ft
    final displacement= 200ft

    I have no idea how to find time to solve for acceleration, so both formulas seem useless to me.
    You need to put the given information into both those formulas.

    For the formula v(t)= v_0 + at, you have v(t) = 88 and v_0 = 132, which tells you that at = -44 (it's negative because the car is decelerating, so a is negative).

    For the formula s(t) = s_0 + v_0t + \tfrac12at^2, you have s(t) = 200, s_0=0 and v_0=132. Plug those into the formula and you get 200 = 132t + \tfrac12(at)t. I have written the last term \tfrac12at^2 as \tfrac12(at)t, because you can now substitute in the value for at that you got from the previous equation. You then have a simple equation for t, and once you know t you can put it into the formula at = -44 to find a.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Jan 2010
    Posts
    25
    Quote Originally Posted by Opalg View Post
    You need to put the given information into both those formulas.

    For the formula v(t)= v_0 + at, you have v(t) = 88 and v_0 = 132, which tells you that at = -44 (it's negative because the car is decelerating, so a is negative).

    For the formula s(t) = s_0 + v_0t + \tfrac12at^2, you have s(t) = 200, s_0=0 and v_0=132. Plug those into the formula and you get 200 = 132t + \tfrac12(at)t. I have written the last term \tfrac12at^2 as \tfrac12(at)t, because you can now substitute in the value for at that you got from the previous equation. You then have a simple equation for t, and once you know t you can put it into the formula at = -44 to find a.
    I didn't even think about substituting, thank you.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. rectilinear motion/arc length
    Posted in the Calculus Forum
    Replies: 6
    Last Post: March 23rd 2011, 02:46 PM
  2. Rectilinear Motion
    Posted in the Calculus Forum
    Replies: 3
    Last Post: August 11th 2010, 06:48 PM
  3. Rectilinear Motion
    Posted in the Calculus Forum
    Replies: 2
    Last Post: December 25th 2008, 05:18 PM
  4. Rectilinear motion using Antiderivatives
    Posted in the Calculus Forum
    Replies: 3
    Last Post: March 25th 2007, 02:24 PM
  5. Rectilinear Motion
    Posted in the Calculus Forum
    Replies: 2
    Last Post: February 20th 2007, 02:49 PM

Search Tags


/mathhelpforum @mathhelpforum