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!

Another Mechanics problem

  1. Nov 9, 2005 #1
    A piano is being raised to the third floor of a building using a rope and pulley. The piano is 6.0 m above the ground, moving upwards at 2.9 m s-1, when the rope snaps. Calculate how much time (in s) elapses before the piano hits the ground.

    i need to know how to approach the problem
    thanks in advance.
     
  2. jcsd
  3. Nov 9, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Treat the piano like any other falling object. You are given its initial speed and height. (Hint: What's the general kinematic relationship that gives position as a function of time for uniformly accelerated motion?)
     
  4. Nov 9, 2005 #3
    If we use s = ut + 1/2at^2
    for the piano to reach the top
    s=6
    u=2.9
    v=0
    a=Don't know
    t=?
    or
    s=0
    u=2.9
    v=?
    a=What would be the a?
    t=?
     
  5. Nov 9, 2005 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Of course you know a! Once the rope snaps, the piano is a freely falling object.

    A more complete kinematic expression would be:
    [tex]y = y_0 + v_0 t + 1/2 a t^2[/tex]
     
  6. Nov 9, 2005 #5
    yes but then the initial velocity can't be 2.9 but will be 0 so that we could use a=g
     
  7. Nov 9, 2005 #6
    is Vo = initial velocity
    and Y = vertical displacement
    what does Yo = ? initial displacement.
     
  8. Nov 9, 2005 #7

    Doc Al

    User Avatar

    Staff: Mentor

    Why can't the initial velocity be 2.9 m/s? a = -g.

    Yes, [itex]y_0[/itex] is the initial displacement.
     
  9. Nov 9, 2005 #8
    if we consider these two motions
    one the piano being pulled up at 2.9ms-1 but the final velocity will become
    0ms-1
    two the piano falling down from the 0ms-1 to final velocity with -a therefore
    -displacement taking vectors upwards to be positive.

    or are we considering it as one motion where the displacement is 0
    initial velocity is 2.9 but then how does a = -g.
     
  10. Nov 9, 2005 #9

    Doc Al

    User Avatar

    Staff: Mentor

    You could consider the motion as one continuous segment (that's how I'd do it) or break it up into two segments (up and down); done correctly, both methods will give the same answer.

    At the moment the rope breaks, the piano has an initial speed (2.9 m/s) and an initial height (6.0 m). Once the rope breaks the only force acting on the piano is gravity, so a = -g. The final height is zero.

    You could break the motion up into two segments (up and down), but if you know how to use that kinematic equation there is no need to do that.
     
  11. Nov 9, 2005 #10
    0 = 6 + 2.9t - 4.9t^2
    how do get t?
     
  12. Nov 9, 2005 #11
    if i use the quadratic formula "and should i be using it in the first place for such basic type of problem" but anyway i get 1.441 sec as +ve value
     
  13. Nov 9, 2005 #12

    Doc Al

    User Avatar

    Staff: Mentor

    Looks good to me.
     
  14. Nov 9, 2005 #13
    thank you__________
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Another Mechanics problem
  1. Another problem! (Replies: 1)

  2. Another problem (Replies: 1)

Loading...