1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Two D kinematics

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data
    A particle moves in the xy-plane with constant acceleration. The particle is located at r= 2i + 4j at t = 0 s. At t = 3s it is at r = 8i - 2j and has velocity v = 5i - 5j

    What is the particle's acceleration vector

    2. Relevant equations

    X_f = x_i + v_ix(delta t) + 1/2 a_x (delta t )^2 (1)
    Y_f = y_i + v_iy(delta t) + 1/2 a_y (delta t )^2 (2)

    3. The attempt at a solution

    So i try to solve for a_x and a_y individually.

    since t = 3, xi = 2, xf = 8

    i use formula #1. Now i come across a problem, the question does not give me the initial velocity, and i tried to assume that it was 0, but when i did that it doesnt' match the answer in the book. So now i am really confused as to how to solve this problem. The same occured when i tried to solve for the acceleration in the y direction.

    How do i solve for v initial in both x and y direction?

    the answer is a = 2i - 2j
    Last edited: Oct 21, 2007
  2. jcsd
  3. Oct 21, 2007 #2


    User Avatar
    Homework Helper

    Use the fact that velocity at t = 3 is v = 5i - 5j

    X_f = x_i + v_ix(delta t) + 1/2 a_x (delta t )^2 (1)

    8 = 2 + v_ix(3) + 1/2a_x(3)^2 equation 1

    You also know that

    V_f = v_ix + a_x*t

    5 = v_ix + a_x(3) equation 2

    use equations 1 and 2 to solve for a_x.

    You can also more directly for a_x solve using

    X_f = x_i + v_f(delta t) - 1/2 a_x (delta t)^2

    this formula is less commonly presented in texts...
  4. Oct 21, 2007 #3
    Ah i forgot about that equation.

    Thank you :P
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook