1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Force on water inside pipe

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data

    Water flows at 7 m/s in a pipe of radius 5 cm. A plate having an area equal to the cross-sectional area of the pipe is suddenly inserted to stop the flow. Find the force exerted on the plate. Take the speed of sound in water to be 1.4 km/s. (Hint: When the plate is inserted, a pressure wave propagates through the water at the speed of sound vs. The mass of water brought to a stop in time Δt is the water in a length of tube equal to vsΔt.)

    2. Relevant equations

    P = F/A

    3. The attempt at a solution

    I am trying to attempt the problem in the following way: Consider a reference frame where the water is still. Then the insertion of the plate would actually be a plate moving at 7 m/s, causing the still water to start moving. However, I am not sure how to get the force from this. It's probably going to be from P = F/A. Can anyone help me get started on this problem?
  2. jcsd
  3. Jan 21, 2010 #2
    You can easily work it out using the hint. The amount of water stopped by the plate is v*dt*A where A=pi*r*r. Now you can find mass and momentum stopped using density and initial velocity of flowing water. Force is thus time rate of change of momentum. Since I can not think of a retarding force on water other than that by the plate, this may be the answer.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook