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!

Finding normal force with momentum

  1. Oct 29, 2006 #1
    Water falls without splashing at a X kg/s from a height H into a bucket of mass M. The bucket sits on a sacle. Determine the reading of the sacle as a function of time.

    I know the the sum of all forces is equal to the derivative of momentum with respect to time.

    Mg is the force at time = 0.

    The correct answer is mg + xtg + x*SQRT(2gH)

    I know

    F = dp / dt
    dp = F dt

    but I don't know what to do.
     
  2. jcsd
  3. Oct 29, 2006 #2
    Since p = mv, the expression F = dp/dt can be expanded for situations involving changing masses: F = m dv/dt + v dm/dt.
     
  4. Oct 29, 2006 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The scale is going to measure the downward force. There are two things that contribute the downard force. What are they?

    AM
     
  5. Oct 29, 2006 #4
    The total force measured by the bucket is the weight of the bucket + weight of water + force of collision.

    So, I understand mg + xtg which gives me force. But where did the x * SQRT(2gH) come from. SQRT(2gH) is equivalent to time, meaing that x * SQRT(2gH) is a mass. Why is a mass included in an equation for force?
     
    Last edited: Oct 29, 2006
  6. Oct 29, 2006 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The rate of change of momentum is the rate of mass flow x the speed of the water when it hits the bucket.

    [tex]dp/dt = vdm/dt[/tex]

    In order to determine the speed, use the fact that potential energy is converted to kinetic energy. So, for an element of mass, [itex]\Delta m[/itex]:

    [tex]\Delta mgh = xtgh = \frac{1}{2}\Delta mv^2 = \frac{1}{2}xtv^2[/tex]

    AM
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Finding normal force with momentum
  1. Finding Normal Force? (Replies: 3)

Loading...