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Momentum/Impulse- Calculating Average Force of Raindrops

  1. Jan 19, 2015 #1
    1. The problem statement, all variables and given/known data
    The problem says:
    "A record rainstorm produced 304.8 mm (approximately 1 ft)
    of rain in 42 min. Estimate the average force that the rain exerted
    on the roof of a house that measures 10 m * 16 m. Indicate
    any assumptions you made. (Note: density of water is 1000 kg/m3"

    2. Relevant equations
    Volume= lwh
    ΔP= mΔV

    3. The attempt at a solution
    I determined the mass of the rain that fell in the 42 minute/2520 second interval to be 48,768kg and the mass of rain to fall in one second to be 19.35kg.
    From there I am absolutely at a loss of what to do to figure out the average force. Assuming that the final velocity of the rain is 0, I'm thinking that I need the initial velocity of the rain in order to figure out the change in momentum.
    The answer itself isn't a problem, as I was given a key, but I'm not sure if I should use the mass of all of the rain and 2520 seconds as Δt or use 19.35kg and 1 second as Δt. I'm just really not sure where to go after calculating the masses of the amounts of rain.
    The key says the answer is 2x10-4 N, but I can't figure out how to get to that answer.
  2. jcsd
  3. Jan 19, 2015 #2


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    Which is absolutely correct. Large raindrops in a torrential downpour might be moving 10 m/s. That and the mass rate you've calculated eventually give you a number that's for force ~ 2, but the order of magnitude is off by a factor of 6. Whoever solved it for the answer key probably used 10 m/s for the fall rate, but mixed grams and tons.
  4. Jan 19, 2015 #3
    Whew! Glad to know it wasn't a lack of knowledge on my part that was stopping me from getting to that answer. Thank you very much for your help!
  5. Jan 19, 2015 #4


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