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

Homework Help: Ball rolls down ramp

  1. Aug 12, 2005 #1
    A 110 g marble rolls down a 39.0 deg. incline. At the bottom, just after it exits onto a horizontal table, it collides with a 290 g steel ball at rest.

    How high above the table should the marble be released to give the steel ball a speed of 200 m/s?

    well first i tried to use the formula for perfectly elastic collisions
    (Vf)steel= (2m(marble)/ m(marble)+m(steel))*v_f(ball1)

    and i solved for v_f and then i plugged that into mgh(initial)= 0.5m*v^2

    and none that didnt work so now im confused again
  2. jcsd
  3. Aug 12, 2005 #2
    I just want to be sure. Do you mean final velocity of 20 m/s? Or do you really mean 200 m/s?

    In an elastic collision you can assume that the Kinetic Energy of the balls will be the same before and after.

    KE = 1/2 mv^2

    1) Find the final KE of the steel ball after impact (v=20 or v=200).
    2) Use this number to calculate the velocity of the smaller ball just prior to impact.
    3) Calculate the initial height necessary in order to accelerate the ball to the velocity found in step 2.

    Furthermore if I'm not mistaken, use the sin(39) to help calculate the acceleration of the ball down the ramp. After all if the ball were on a table, the angle would be zero and sin(0) = 0. If the ball were up against a wall the angle would be 90 (straight down), and sin (90) = 1.
    Last edited: Aug 12, 2005
  4. Aug 12, 2005 #3
    o whoops.. i meant cm/s
  5. Aug 12, 2005 #4
    ok this is what i got so far

    0.5*m*v^2= 0.5*.290kg*200cm/s^2= 5800

    5800= (0.5)(.110kg)(v^2)
    i solved for v and i got 324.7cm/s^2

    now i know this is the velocity at the bottom of the ramp but i dont know hwo to get the height from here.
  6. Aug 13, 2005 #5


    User Avatar
    Homework Helper

    Use conservation of energy.

    gain in KE = loss in PE.
  7. Aug 13, 2005 #6


    User Avatar
    Homework Helper

    what is the unit of energy you got???
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook