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: Kinetics of a rigid body (conservation of energy)

  1. Jul 17, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    (1) (pink) Is it vertical displacement ?
    * 4 - 2 (unstretched lenght of the spring) ?

    (2) (blue) What does it mean ?
    * I was thinking that maybe they used The Pythagorean theorem (62+42)=7,2 ≅ 7) but I'm not sure. Besides I don't understand why they used it.

    3. Attempt

    My reasoning: In general I think they are using this formula,
    where in that case the left side is the initial state - the right side - the final state of the spring.

    Attached Files:

  2. jcsd
  3. Jul 17, 2015 #2
    I recommend setting it up carefully in terms of conservation of energy, expressing T and V in terms of all the relevant quantities (leaving as symbols), making simplifications, solving for the unknown (with symbols), and plugging in numbers at the end.
  4. Jul 17, 2015 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The 4-2 and 7-2 are both calculations of spring extension. In each case, 2 is the relaxed length. At the horizontal position, the total length is 4, so the extension is 4-2. After rotating 30 degrees, the total length is 4+6 sin(30)=7.
    I don't like the way they divide the mass by 32.2 in the KE terms. It seems more natural to me to multiply by 32.2 in the other terms - it comes to the same thing, of course. Maybe their way is standard in this non-metric system. I have no background in it.
  5. Jul 17, 2015 #4
    The English (Imperial) unit of mass is slugs. One obtains mass by dividing weight (lbs) by the acceleration of gravity (32.2 ft/s/s) to obtain the mass in slugs.

    I thought ballistics was the last holdout still using English units for real physics.

  6. Jul 17, 2015 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Ok, thanks.
  7. Jul 20, 2015 #6
    Thank you for help.

    Dr. Courtney - is this equation correct ?

  8. Jul 20, 2015 #7
    Is the only potential energy due to the spring?
  9. Jul 20, 2015 #8
    If a spring is not stretched or compressed, then there is no elastic potential energy stored in it.

    However in this case at the beggining the spring was stretched 2 metres (4-2=2), hence 1/2*k*s1^2

    At the end, the spring was stretched 5 meters (7-2=5), hence 1/2*k*s2^2

    'W' stands for: work of a weight -> WΔy= 50 * (1.5)
  10. Jul 20, 2015 #9
  11. Jul 20, 2015 #10
    I think the only potential energy is due to the spring. If the spring lies the ground then mgh=0
  12. Jul 20, 2015 #11


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The massive bar is rotating in a vertical plane. Are you saying its centre of gravity does not change in height?
  13. Jul 22, 2015 #12
    Ok, it does. Thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted