Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do springs alter velocity?

  1. Jun 22, 2011 #1

    Femme_physics

    User Avatar
    Gold Member

    For instance, if an object moves with a constant velocity on a frictionless surface and hits a spring that makes it go the other way (180 degrees), will the object continue the other way with the same exact velocity regardless of the spring coefficient? Or does the spring coefficient matter?
     
  2. jcsd
  3. Jun 22, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    An ideal spring conserves energy. So assuming it's fixed at one end, the spring will send the object back with the same speed. (The spring coefficient will tell you how much the spring must compress to stop the object's motion, but who cares?)
     
  4. Jun 22, 2011 #3

    BruceW

    User Avatar
    Homework Helper

    The spring coefficient doesn't matter (it just tells us how long the spring will take to turn the object around).
    The object will go back at the same speed as long as no energy is lost at the spring.
    In real life, the spring will have inelastic deformations (which 'loses' energy to the surroundings). In a perfect spring, there would be no energy loss, since it would deform only elastically, but real springs aren't perfect.
    Also, there may be heat loss, and friction if the spring isn't perfectly lined up, etc.
    But usually, you can say the object will come back with approximately the same speed.
     
  5. Jun 23, 2011 #4

    Femme_physics

    User Avatar
    Gold Member

    Ah, great, thanks :) I actually thought this ought to be the case, just wanted guru-confirmation! :) :approve:
     
  6. Jun 23, 2011 #5
    I think the stiffer (greater k) the spring is, the more energy the object keeps.

    The spring loses contact with the ball when the ball has velocity greater than the spring. If the spring has zero velocity, that means the ball leaves with infinitismal velocity.

    So in general the spring will have velocity and hence energy. Even if the spring is massless, with velocity the spring will stretch, so it'll have energy, and consequently the ball must leave with a different speed.
     
  7. Jun 23, 2011 #6

    Femme_physics

    User Avatar
    Gold Member

    I'm confused, you and the others are basically saying two different things. How do I know who's right?
     
  8. Jun 23, 2011 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Are you just taking a poll? I assume you've solved some spring-related problems before. So who do you think is right? Work it out for yourself.

    (Hint: RedX is wrong.)
     
  9. Jun 23, 2011 #8

    Femme_physics

    User Avatar
    Gold Member

    LOL don't get mad at me :P My confident gets really low when 2 experts argue. I thought I was misreading something or somesuch or maybe there's a caveat for what you said, so I ask. I don't know. And I solved one or two, things haven't really sunk it, which is why I'm asking basic questions.
     
  10. Jun 23, 2011 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Here's a tip: Not everyone posting is an 'expert'. :smile:
     
  11. Jun 23, 2011 #10

    Femme_physics

    User Avatar
    Gold Member

    True but I would rather you'd shun the spot-light on him not me, I'm just asking questions, he's the one who said something wrong. If anyone should be put on the spotlight is the one making false statements!

    Don't worry, I'm thinking deeply and profoundly about everything I'm being told a lot, I just haven't got to process stuff seeing contradicting replies so I had to point it out

    I did think what you said makes a whole lot more sense, but I wanted to be 100% sure so I was waiting for a confirmation... glad I got it

    Thanks! :smile:
     
  12. Jun 23, 2011 #11

    Doc Al

    User Avatar

    Staff: Mentor

    This is incorrect.

    We're talking about a situation where an object slides into an ideal massless spring, fixed at one end. (The speed of a massless spring is irrelevent as it has no kinetic energy.)
     
  13. Jun 23, 2011 #12
    Hello FP, no exams yet?

    Your spring is really an example of the 'coefficient of restitution' type problems.

    In your and Doc Al's case the coefficient is 1.

    RedX seems to be trying to take it a bit further to when the COR is less than one and getting a bit muddled in the process.
    However any energy lost ends up as deformation energy, heat and perhaps impact sound, not kinetic energy.

    http://en.wikipedia.org/wiki/Coefficient_of_restitution

    go well
     
    Last edited: Jun 23, 2011
  14. Jun 23, 2011 #13

    Femme_physics

    User Avatar
    Gold Member

    Oh, I see! Yes, that makes a whole lot of sense actually :) I shall go well. Thank you Studiot, and thank you Al Doc!
     
  15. Jun 23, 2011 #14

    Doc Al

    User Avatar

    Staff: Mentor

    I think Studiot's idea of looking at this as COR problem is an excellent one. (Assuming you've covered the topic.) You don't need to do that, but it's another way of looking at it. And the more ways you have of looking at a problem, the better your understanding becomes.
     
  16. Jun 23, 2011 #15

    Femme_physics

    User Avatar
    Gold Member


    Oh and I scored a perfect 100 on my first mechanics test. You can see all the questions I solved here:

    http://ortmechatronics.blogspot.com/2011/06/blog-post_17.html

    The finals though is at the 6/7 :smile:

    Thanks!

    I agree, this type of knowledge, though it seems a bit far fetched from the basic mechanics stuff we're learning really helps my understanding!
     
  17. Jun 23, 2011 #16
    A massless spring would give you the same velocity for the object, as then the spring could never lose contact with the object (since it moves at infinite velocity when it does) before it loses all its energy. Not exactly intuitive, but when using conservation of energy, you have to consider if the spring can keep some of the energy.
     
  18. Jun 23, 2011 #17

    Doc Al

    User Avatar

    Staff: Mentor

    What situation are you talking about? We're talking about a spring with one end fastened to a wall. A moving object hits the free end, compresses the spring, then bounces back.

    (I think you are trying to address a moving object hitting a free massless spring, but that's not what is being discussed here.)
     
  19. Jun 23, 2011 #18
    I have that situation in mind. I was just wondering if it's possible for the object to lose contact with the spring before the spring is restored to its original position. If the spring is massless, then it's not possible, since the spring moves infinitely fast.
     
  20. Jun 23, 2011 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    As you say - not intuitive. With the spring being massless, it can go as fast as you like so it keeps on pushing until it is going at the original speed; at this point it loses contact. I think the sums for a spring of finite mass would take quite a few lines.

    (The spring doesn't go "infinitely fast"- just fast enough to follow the retreating mass.)
     
    Last edited: Jun 23, 2011
  21. Jun 23, 2011 #20

    Doc Al

    User Avatar

    Staff: Mentor

    As sophiecentaur notes, the spring is not moving infinitely fast. The massless spring maintains contact until restored to its unstretched length. Once the object leaves, all the energy is in the kinetic energy of the object.

    (A massive spring is more difficult to deal with, of course.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook