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

Which attains more height and why?

  1. Feb 2, 2010 #1
    Hi..A disk A and disk b are rolling without slipping with same speed on a horizonal surface..Disc a rolls up an incline and disk b rolls up an incline except that it is fricitionless! Which disk attains more height nw why?
     
  2. jcsd
  3. Feb 2, 2010 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Sounds like homework. What do you think?
     
  4. Feb 2, 2010 #3
    Try relating your Physics homework problems to your real life. Maybe you like to bowl. Imagine a bowling lane with an incline at the end instead of pins. Case A - You bowl your ball on a normal bowling lane at normal strength. Case B - You bowl your ball on asphalt at normal strength.

    Once you've pictured a real world situation ask yourself simple questions:

    (1) Which lane has more friction, Case A or Case B?
    (2) Which ball will make it higher up the incline, Case A or Case B?

    Put the two together and WahhhhLA. You understand a little bit of Mr. Newton
     
  5. Feb 2, 2010 #4
    Cute problem... obviously the "without slipping" part is limited.
     
  6. Feb 3, 2010 #5
    Thanks for replying guys.Are you saying that in Case of disk A Both translational and rotational kinetic energies will be converted into potential energy..But in case of disk b,only translation kinetic energy will be converted to potential energy as there is no friction to support rotation!! But where is that rotational kinetic energy go? Will it be wasted? If yes,how? There is no dissipative force present!
     
  7. Feb 3, 2010 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Without friction, the rotational energy doesn't go anywhere. The disk keeps rotating at whatever angular velocity it had when it entered the frictionless surface.
     
  8. Feb 3, 2010 #7
    theoretically speaking based from the situation disc B would certainly the one that would gain the greater height because according to newton's 1st law of motion, "a body that stays at rest remains at rest and if it is moving, it will move at a constant speed." in this case disc B will experience no external forces that may cause it to accelerate nor to deccelerate like frictional force which is present on disc A. thus it will move at a constant speed and it will never stop until reaches the highest point.
     
  9. Feb 3, 2010 #8

    Doc Al

    User Avatar

    Staff: Mentor

    You forgot about gravity. And don't forget that in some situations--like this one--friction can actually help you go higher.
     
  10. Feb 3, 2010 #9
    Under ideal situations both will attain the same height considering the fact that the disks are not accelerating/de accelerating by virtue of their rotating motion.

    When A goes up that slope, it rolls and so there's no friction stopping it's motion...only gravity is doing that. Same is the case with disk B.
     
  11. Feb 3, 2010 #10

    Doc Al

    User Avatar

    Staff: Mentor

    When they hit the slope, A experiences friction but B does not. That makes a difference.
     
  12. Feb 3, 2010 #11
    thanks for replying all...doc is right ...friction helps in disc A in attatining more height as rotational kinetic energy is utilised and in case of disk B only translation kinetic energy is utilised....doc Al ..can u please correct me if i am wrong!!
    when disk A of mass M goes up an incline,equations of motion are (assuming friction is enough for rolling without slipping)
    translation:Mg-friction(f)=ma
    rotation:fR=mR^2/2 * a/r
     
  13. Feb 3, 2010 #12
    and friction is acting upwards..
     
  14. Feb 3, 2010 #13

    Doc Al

    User Avatar

    Staff: Mentor

    You are correct.
    The only change I'd make would be to account for the angle of the incline: Use mg sinθ, instead of mg.
    Absolutely.
     
  15. Feb 3, 2010 #14
    oh yea ..mg sinx ..m silly
     
  16. Feb 3, 2010 #15
    thank you so much...
     
  17. Feb 3, 2010 #16
    doc al..sir just in case you are not super busy...it will be great if you could reply to my angular momentum thread..
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook