Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Acceleration vs. Time

  1. Sep 10, 2005 #1
    Ok here's the graph:

    http://img268.imageshack.us/img268/8762/acvt9nz.jpg [Broken]

    We are given that the particle starts from rest at the initial position 0m with the initial velocity 0m/s. I need to find what the position of the particle is at t=5s.

    Since acceleration is changing here, I didn't know what to do so I broke the problem up. First, I wanted to find the position at t=3s. I used the equation
    x = x0 + v0t + 1/2at^2 where x0 = 0m, v0 = 0m/s, a = 1m/s^2, and t = 3s.
    I came up with 4.5m.

    Next, I did the same thing for the lower part; I used the same equation but with x0 = 4.5m, v0 = 3m/s, a = -2m/s^2, and t = 2s. I came up with 6.5m and added it to first distance (4.5m) to get 11m.

    Obviously this is wrong. Any help you can provide would be much appreciated. Thank you.
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 10, 2005 #2

    wouldn't it be just the answer to your second part? you're starting at the point where the first part left off. you don't need to add that onto your answer as you've already accouned for the position up to 3 seconds with the [tex]x_0[/tex] in your second kinematic eqation.
    Last edited by a moderator: May 2, 2017
  4. Sep 10, 2005 #3
    your individual calculations are correct. However, you have to be careful of the SIGN of the acceleration in each part. What is the acceleration (as read fro mteh graph) for the first 3 seconds and then the final 2 seconds?
  5. Sep 10, 2005 #4
    You're right. I always do stupid things like that! Grr...thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook