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: How to find total force?

  1. Feb 24, 2012 #1
    An athlete whose mass is 86.5 kg is performing weight-lifting exercises. Starting from the rest position, he lifts, with constant acceleration, a barbell that weighs 470 N. He lifts the barbell a distance of 0.50 m in a time of 2.0 s.

    I cannot seem to find the answer to this. This is what i did:

    - I found the total force exerted on the feet by doing: 86.5*9.8 + 470N = 1317.7N
    - Then I used d = a*t^2 and got that the acceleration is 25m/s^2
    - Then i found the mass of the barbell by doing 470N/9.8 and got 47.96kg.
    - After using F=ma i added the two forces and got the answer 2516.7N.

    This isn't the right answer. What did i do wrong? Any help is appreciated.

  2. jcsd
  3. Feb 24, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    hi treyh! :smile:
    i don't understand the question :redface:

    is his body remaining stationary (apart from his arms)?

    or is his centre of mass also moving up 0.50 m ? :confused:
  4. Feb 24, 2012 #3


    User Avatar
    Homework Helper

    I just read the original post very carefully, but the only question mark I found was at the end of " What did I do wrong?"

    I am tempted to reply: " You didn't ask a question?"

    The line I highlighted red above is interesting. If an object falls under the influence of gravity - with an acceleration a little under 10 m/s^2 - it covers just under 20m in 2 seconds. This barbell covered only 0.5 m in 2 seconds yet you said the acceleration was 25 m/s^2 - more than double the acceleration due to gravity ??
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook