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Homework Help: What force is exerted on the windscreen?

  1. May 14, 2008 #1
    1. The problem statement, all variables and given/known data
    A car is driving through a rainstorm. Each raindrop of mass [tex]0.00005kg[/tex] hits the windscreen with velocity of [tex]16 ms^{-1}[/tex]. If [tex]500[/tex] raindrops hit the glass and run down the windscreen each second without bouncing, what force is exerted on the windscreen?

    2. Relevant equations

    Force = Mass x Acceleration

    Momentum = Mass x Velocity

    Inelastic Collisions:
    [tex]m_1u_1 + m_2u_2 = (m_1+m_2)v[/tex]

    3. The attempt at a solution

    [tex]m = 5.0 \times 10^{-5}kg \ , \ u = 16ms^{-1}[/tex]

    don't know how to start, need help!
    Last edited: May 14, 2008
  2. jcsd
  3. May 14, 2008 #2
    The equations you gave are correct; but also remember that F = change in momentum / change in time.
    Make sure you keep track of your units, and a force needs to end up being kg*m/s^2.
  4. May 14, 2008 #3
    Can someone please explain. Still, I don't know how to start =(
  5. May 14, 2008 #4


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    Science Advisor
    Homework Helper

    As lzkelley said, force is change in momentum / time and momentum is mass * velocity.
    The rain drop is accelerated to the speed of the car by the windscreen and so pushes on the widnscreen with a force equal to the raindrops mass * change in velocity.

    Assuming the drop is falling vertically, you have the change in velocity, you know the total mass of raindrops falling per second so you have the force.
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