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Thread: Newton's Laws - Examples

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    Newton's Laws - Examples

    1) Say there are two cars; one with a force of 400N and the other one at 800N. Their masses are the same, therefore they have different acceleration. If they were to collide head on and the drivers weren't wearing seatbelts, would the driver from Car A (400N) fly backwards with the car or would it fly through the windshield? I think it's the latter. And if so, would they travel at the same velocity as the car before impact or would they travel at a much slower speed?

    The reason I ask that is because Newton's First Law states that "an object will remain at rest or travel at constant velocity unless acted upon by an unbalanced external force". Now the unbalanced external force in this case is the 700N force of the other car colliding, so would the person inside still travel at the constant velocity of the car when hit?

    In the same scenario, would the driver of the 700N force car still fly forwards? Or would the driver just remain in the car?

    There's also another question I have:

    2) Explain why higher velocities require greater force to stop. For example, say you fall from a 10 storey floor onto a trampoline, the trampoline would require more force to stop you (therefore, you're most likely to rip through the trampoline), but if you jumped from 4metres up onto a trampoline, you'd be fine.

    How does this work since F =Ma and gravitational acceleration and mass is constant?

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    Senior Member jazz519's Avatar
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    Re: Newton's Laws - Examples

    Quote Originally Posted by _Anonymous View Post
    1) Say there are two cars; one with a force of 400N and the other one at 800N. Their masses are the same, therefore they have different acceleration. If they were to collide head on and the drivers weren't wearing seatbelts, would the driver from Car A (400N) fly backwards with the car or would it fly through the windshield? I think it's the latter. And if so, would they travel at the same velocity as the car before impact or would they travel at a much slower speed?

    The reason I ask that is because Newton's First Law states that "an object will remain at rest or travel at constant velocity unless acted upon by an unbalanced external force". Now the unbalanced external force in this case is the 700N force of the other car colliding, so would the person inside still travel at the constant velocity of the car when hit?
    https://imgur.com/a/PIBrI

    img_1016-min.jpg

    https://imgur.com/a/7IPo0

    img_1018-min.jpg

    Quote Originally Posted by _Anonymous View Post
    In the same scenario, would the driver of the 700N force car still fly forwards? Or would the driver just remain in the car?
    This part is all to do with Newton's third law that can be used to derive the momentum expression (which will describe the velocities of the cars involved in the collision), since the rate of change of momentum of an object is directly proportional to the resultant force applied and is in the direction of the resultant force. The resultant force is equal to the rate of change of momentum.

    There's also another question I have:

    Quote Originally Posted by _Anonymous View Post
    2) Explain why higher velocities require greater force to stop. For example, say you fall from a 10 storey floor onto a trampoline, the trampoline would require more force to stop you (therefore, you're most likely to rip through the trampoline), but if you jumped from 4metres up onto a trampoline, you'd be fine.

    How does this work since F =Ma and gravitational acceleration and mass is constant?
    The trampoline spring provides a force against you and because your going in the opposite direction to the force being applied i.e. head on it provides a retarding force (so an acceleration that acts to slow down the moving object), but remember back to Newton's first law - inertia stating that the object wants to continue moving in a straight line and as you get faster your inertia increases and therefore the force required to stop you is much greater and the trampoline is unable to support that
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    Re: Newton's Laws - Examples

    Quote Originally Posted by jazz519 View Post
    https://imgur.com/a/PIBrI

    img_1016-min.jpg

    https://imgur.com/a/7IPo0

    img_1018-min.jpg



    This part is all to do with Newton's third law that can be used to derive the momentum expression (which will describe the velocities of the cars involved in the collision), since the rate of change of momentum of an object is directly proportional to the resultant force applied and is in the direction of the resultant force. The resultant force is equal to the rate of change of momentum.
    As we haven't learnt about Momentum yet, I am a bit confused with your explanation - hopefully I'll understand when we learn about Momentum.

    Quote Originally Posted by jazz519 View Post
    There's also another question I have:



    The trampoline spring provides a force against you and because your going in the opposite direction to the force being applied i.e. head on it provides a retarding force (so an acceleration that acts to slow down the moving object), but remember back to Newton's first law - inertia stating that the object wants to continue moving in a straight line and as you get faster your inertia increases and therefore the force required to stop you is much greater and the trampoline is unable to support that
    Why does an object's Inertia increase if the acceleration is constant? I understand the Velocity is increasing, but wouldn't you only care about Acceleration and Mass since F = ma and hence think about how much Force you would need to have to support the Trampoline?

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    Re: Newton's Laws - Examples

    By the way, Anon:

    momentum = mass x velocity

    force is proportional to the rate of change of momentum

    Also there is the well-known Law of conservation of momentum: if 2 cars smash into each other, the sum of their momenta before and after the collision remain unchanged. Car-1 may end up with more, and car-2 with less, but their total stays equal. But this is a simplistic presentation. Since momentum is a vector quantity (has magnitude and direction), the sum itself is a vector sum.
    Last edited by Drongoski; 27 Nov 2017 at 9:48 PM.
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    Re: Newton's Laws - Examples

    Quote Originally Posted by _Anonymous View Post
    As we haven't learnt about Momentum yet, I am a bit confused with your explanation - hopefully I'll understand when we learn about Momentum.



    Why does an object's Inertia increase if the acceleration is constant? I understand the Velocity is increasing, but wouldn't you only care about Acceleration and Mass since F = ma and hence think about how much Force you would need to have to support the Trampoline?
    Inertia is just how much force we have to exert on something to stop it moving or make it move (if something has got a higher velocity than we need a greater acceleration to stop it in the same distance or we need to have an acceleration that acts for a longer time). The decceleration (negative acceleration) will be dependent on the velocity as acceleration is the rate of change of velocity (i.e. a=vf-vi/t)
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