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!

Sliding Mass

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data

    A 15 kg mass is sliding at 30 m/s when it encounters a slope (smooth=no friction) and descends 60 m. It encounters a horizontal rough stretch (friction = 80N in opp. direction). Calculate the velocity at x=0 (rough stretch starts here), x=50, x=100, x=200. Calculate where it comes to a stop.

    MASS
    _______
    h =60m \
    ............\friction = 80N for horizontal line only (indicated by ~)
    .............\~~~~~~~~~~
    ..............x=0 x= 50.....
     
  2. jcsd
  3. Dec 16, 2008 #2

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What are your thoughts on the problem?
     
  4. Dec 16, 2008 #3
    i do not know how to do the sliding part to get the initial speed, but i think from x=0 i would use w=K final- K initial, which would be (friction x m x g)x= 1/2 x mass x final velocity -1/2 x mass x initial velocity

    also, when i calculate the velocity for x=50, do i use the final velocity for x=0 for the initial velocity for x=50, or do i use the velocity from when it came down the slope for initial velocity for x=50
     
    Last edited: Dec 16, 2008
  5. Dec 16, 2008 #4

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Let's stick to the first question initially. Can you think of some quantity that will be conserved whilst the block is sliding down the slope?
     
  6. Dec 16, 2008 #5
    i guess energy would be conserved

    this would be easrier but i have a lack of formulas
     
  7. Dec 16, 2008 #6

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That would be correct. Hopefully, the formulas will come to you as we talk though it.

    What two forms of energy are involved in the block sliding down the slope?
     
  8. Dec 16, 2008 #7
    kinetic and potential
     
  9. Dec 16, 2008 #8

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Good. So you know that the sum of the potential and kinetic energy at the top and the bottom of the slope must be equal.

    What is the value of the total energy at the top of the slope?
     
  10. Dec 16, 2008 #9
    Total energy = PE + KE
    TE = mgh+1/2 x m x v squared
    TE= 9000+6750
    TE=15750

    i used g=10
     
  11. Dec 16, 2008 #10
    now wat?
     
  12. Dec 16, 2008 #11
    can anybody help with this?
     
  13. Dec 16, 2008 #12
    As you know energy is conserved.

    [tex] E_{total\ before}=E_{total\ after} [/tex]

    [tex] Eg1+Ek1+Et=Eg2+Ek2 [/tex]

    [tex] mgh_{1}+\frac{1}{2}mv_{1}^2-F_{f} \triangle d = mgh_{2} +\frac{1}{2}mv_{2}^2 [/tex]

    always make the lowest point have a gravitional energy of zero
     
  14. Dec 16, 2008 #13
    wat is Ff change in d?
     
  15. Dec 16, 2008 #14
    Ff (80N) is the force in friction and Change in d is x
     
  16. Dec 16, 2008 #15
    thank you so much bright wang
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Sliding Mass
Loading...