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Math Help - what force is exerted on the windscreen?

  1. #1
    Member looi76's Avatar
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    what force is exerted on the windscreen?

    Question:
    A car is driving through a rainstorm. Each raindrop of mass 0.00005kg hits the windscreen with velocity of 16 ms^{-1}. If 500 raindrops hit the glass and run down the windscreen each second without bouncing, what force is exerted on the windscreen?

    Attempt:

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

    don't know how to start, need help!
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by looi76 View Post
    Question:
    A car is driving through a rainstorm. Each raindrop of mass 0.00005kg hits the windscreen with velocity of 16 ms^{-1}. If 500 raindrops hit the glass and run down the windscreen each second without bouncing, what force is exerted on the windscreen?

    Attempt:

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

    don't know how to start, need help!
    Let's "ramble" this one out step by step.

    500 raindrops per second means that you have 500 times 0.00005 kg striking the windshield per second.

    So in one second 0.0250 kg of mass at 16 m/s strikes the windshield. This creates a change in momentum of 0.4 kg m /s in that one second. This is, in turn, equal to the impulse exerted on the windshield in one second: 0.4 Ns. Thus the force on the windshield is 0.4 N s / 1 s = 0.4 N.

    -Dan
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  3. #3
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    mr fantastic's Avatar
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    Quote Originally Posted by topsquark View Post
    Let's "ramble" this one out step by step.

    500 raindrops per second means that you have 500 times 0.00005 kg striking the windshield per second.

    So in one second 0.0250 kg of mass at 16 m/s strikes the windshield. This creates a change in momentum of 0.4 kg m /s in that one second. This is, in turn, equal to the impulse exerted on the windshield in one second: 0.4 Ns. Thus the force on the windshield is 0.4 N s / 1 s = 0.4 N.

    -Dan
    I think that the velocity of the car and the angle at which the raindrops hit the windscreen are important pieces of missing infromation ..... Assumptions can be made about the latter - nevertheless the velocity of the car will still be a relevant factor .....
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by mr fantastic View Post
    I think that the velocity of the car and the angle at which the raindrops hit the windscreen are important pieces of missing infromation ..... Assumptions can be made about the latter - nevertheless the velocity of the car will still be a relevant factor .....
    Good points on both comments.

    -Dan
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