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: Forces and defining coordinate systems

  1. Dec 23, 2011 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    vi=vf + at

    3. The attempt at a solution
    If i was to define upward as positive y direction, would the answer be = -881 pounds (btw why is the answer in the image in Newtons?) and because i defined upward as +y would
    ƩF = T - w? where w = mg.
  2. jcsd
  3. Dec 23, 2011 #2
    I think you are right about the units. I am not too familiar with slugs, pounds, ft/s^2 but I think you're right about Newtons being a typo.

    On the other hand, you are wrong about the direction of the force. Since we set upward as positive, the initial and final velocities are negative.

    a=(Vf-Vi)/t=(-35-(160))/0.80=+156 ft/s^2

    Since the acceleration is upward, the retarding force is also upward.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook