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Problem on Mechanics (Newton's law of motion)

  1. Feb 5, 2016 #1
    1. The problem statement, all variables and given/known data
    This question goes like this:
    In an accident involving a big lorry and small car, the drivers of both vehicles were seriously injured. A student made a remark about the accident as follows: "A big lorry and a small car moved in the opposite directions and made a head-on collision. By Newton's third law of motion, the forces acting on both drivers are the same. As a result, both drivers were seriously injured."
    Comment on the student's remark.

    2. Relevant equations
    Third law: Whenever an object A exerts a force on another object B, B exerts a force of equal magnitude and opposite direction on A at the same time.

    3. The attempt at a solution
    I think the student is correct. As the two vehicles collide head-on, they form an action-and-reaction pair. Therefore, both the lorry and the car receive the same magnitude of force exerted on them by each other.
    What I'm not so sure about is which force caused the drivers' injury?
    Is it because: the cars receive forces from opposite directions and this force causes the cars to stop abruptly, while the drivers still tend to move forward due to inertia. As a result, they hit the windscreens and get injured?
    Like the teacher said this question is not that easy so I'm quite uncertain about my answer.
  2. jcsd
  3. Feb 5, 2016 #2


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    Heavy lorry hits light car - imagine the situation and describe in words what you think would happen .
  4. Feb 5, 2016 #3
    I think the lorry with a larger mass would drive the light car backwards during the collision and the passenger in the light car would be more seriously injured. But this wont explain why the driver in the lorry is also seriously injured.
  5. Feb 5, 2016 #4


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    In reality anything can happen as the result of a head on crash . Roughly speaking though the bigger and stronger the vehicle you are in relative to the one crashing in to you the less injuries you are are likely to receive .

    It's a nonsense question really but think about the different accelerations experienced by the two drivers during the crash .
  6. Feb 5, 2016 #5
    I know right my teacher likes raising silly questions lol
    Thanks anyways
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