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Simple question on newtons third law of motion

  1. Jan 31, 2008 #1
    Hey guys, I was just wondering if you could help me with a question that has been bugging me for a while now. Ive thought of many possibilities to the answer yet i still cant find the right one. The question is that, when a fly strikes the windshield of a truck moving at lets say 60km/h, why doesn't the windscreen shatter. The fly would experience a huge force in order to make it travel at the same speed of the truck and according to newtons third law, every action has an equal and opposite reaction. Therefor, wouldnt that huge force on the fly cause a huge reaction on the glass, hence shattering you?

    Any assistance to this question would be most appreciated.
    Thanks in advance
  2. jcsd
  3. Jan 31, 2008 #2

    Doc Al

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    Staff: Mentor

    While it's certainly true that the contact force exerted on the fly equals the force exerted on the windscreen (thanks to Newton's 3rd), why assume that a force that would splatter a fly is sufficient to shatter a windscreen?
  4. Jan 31, 2008 #3
    I think that you are confusing the force on the fly (tiny) with the effects of the force (huge acceleration, thus squish). The windscreen area the fly hits depresses a tiny bit to provide this tiny force.
  5. Jan 31, 2008 #4


    Staff: Mentor

    This is actually a 2nd law question, not 3rd law. The fly experiences a very large acceleration, but since its mass is very small by F = ma you get a moderate force. The reaction force is equal and opposite, so it also is moderate and therefore not strong enough to shatter the glass. Also, since the mass of the car is so large the moderate force doesn't decelerate it very much at all.
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