1. Not finding help here? Sign up for a free 30min 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!

Energy changes of a stretched string

  1. Aug 11, 2015 #1
    1. The problem statement, all variables and given/known data
    image.jpg image.jpg

    This is the answer :

    image.jpg

    For part d) of this question, I don't understand "stretching/extension reduces and velocity increases/height increases" .

    2. Relevant equations
    Hooke's Law: F=kx

    3. The attempt at a solution
    Ok, so if extension is reduce, then force reduces too. Then how does a decrease in force causes the velocity and height to increase? Does it have something to do with the principle of conservation of energy, where a gain in kinetic energy equals to a lose in gravitational potential energy?
     
  2. jcsd
  3. Aug 11, 2015 #2
    At a point lower than R , how does spring force compare to force of gravity ?
     
  4. Aug 11, 2015 #3
    Spring force is more than the force of gravity .
     
  5. Aug 11, 2015 #4
    So as long as mass is below R , acceleration is ( +ve or -ve ) ? And therefore velocity will always what ?
     
  6. Aug 11, 2015 #5
    Positive..? I'm not sure.. Therefore, velocity will increase?
     
  7. Aug 11, 2015 #6
    Yes . ( kx - mg = ma ) , kx > mg .

    Now , a = dv/dt . As a is always positive till before R , dv is always +ve , and hence velocity will increase till R .

    Hope this helps .
     
  8. Aug 11, 2015 #7
    But how do you know that a is always +ve till before R? And why height increases? Does it got to do with 1/2 mv^2=mgh (gain in kinetic energy=lost in gravitational potential energy) ? And thanks by the way. :)
     
  9. Aug 11, 2015 #8
    I thought we agreed that -
    It rises because acceleration is upwards . And that's pretty much the only reason .
     
  10. Aug 11, 2015 #9
    Yeah, I agreed to that one. But the mark scheme also states that the height increases as well. Why?
     
  11. Aug 12, 2015 #10
     
  12. Aug 13, 2015 #11
    OK thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Energy changes of a stretched string
Loading...