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Thread: a physics question...

  1. #1

    Default a physics question...

    say Car A is stationary. Car B crashes car A at a speed of 100km/h..
    john is standing 10cm infront of car A
    both car weights 1 ton.

    what is the speed equivalent of car A impact on john

    ok car A does not have handbrake on in case someone will ask....

  2. #2
    Member luntut's Avatar
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    Default Re: a physics question...

    i think more accurately, you have to provide more details:

    1. Is the collision perfectly inelastic or elastic?
    2. Is there any friction between the cars and the floor?
    3. Are there any other form of energy losses from the collision, say to sound or whatsoever?

    haha.
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  3. #3

    Default Re: a physics question...

    Quote Originally Posted by luntut View Post
    i think more accurately, you have to provide more details:

    1. Is the collision perfectly inelastic or elastic?
    2. Is there any friction between the cars and the floor?
    3. Are there any other form of energy losses from the collision, say to sound or whatsoever?

    haha.
    ok. resultant force is important too.

    the angle of impact, the force of impact, i.e. various vectors addition of all force vectors acting on car a by car b

    http://en.wikipedia.org/wiki/Resultant_force

    thanks for the knowledge sharing

    Last edited by dreaming; 11th January 2009 at 09:24 AM.

  4. #4
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    Default Re: a physics question...

    I always thought in collision problems the conservation of momentum is used to calculate the resultant velocity of the colliding bodies.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

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    Default Re: a physics question...

    Quote Originally Posted by luntut View Post
    i think more accurately, you have to provide more details:

    1. Is the collision perfectly inelastic or elastic?
    2. Is there any friction between the cars and the floor?
    3. Are there any other form of energy losses from the collision, say to sound or whatsoever?

    haha.
    ya the first point is one of the most important.
    because OBVIOUSLY the cars will crush and deform, to absorb some of the impact forces. How much is anyone's guess.
    If it's a perfectly elastic collision (e.g. something like 2 billiard balls striking), then you can use m1v1 = m2v2
    Exploring! :)

  6. #6

    Default Re: a physics question...

    Quote Originally Posted by roygoh View Post
    I always thought in collision problems the conservation of momentum is used to calculate the resultant velocity of the colliding bodies.
    http://en.wikipedia.org/wiki/Momentum

    Conservation of linear momentum
    Last edited by dreaming; 11th January 2009 at 08:59 PM.

  7. #7

    Default Re: a physics question...

    How come need to involve gravitational force of attraction F=Gmm/r^2 ???
    Quite negligible considering the numbers involved right?

  8. #8

    Default Re: a physics question...

    Dudes, it's not that simple right?

    Imagine this: initially car B has velocity 100km/h. Car A is 0km/h. We know if collision is inelastic, both cars will finally move together with a common velocity.

    So, how did car B decelerate to that final velocity? And how did car A accelerate to that final velocity? During collision when Car B is decelerating and Car A is accelerating, there will be discrepancy between the velocities of the cars, so can the cars still move as one, ie perfectly inelastic?

    So the question is, during the time Car A takes to cover the 10cm gap, to what velocity has it accelerated to?

  9. #9
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    Default Re: a physics question...

    Yeah, if we treat this like a textbook question, the conservation of momentum formula is sufficient. We can probably ignore friction since TS deliberately mentioned "no handbrake" and the "10cm" distance was probably arbitrarily assigned to mean "negligibly close", or "ignore frictional deceleration". If the question was not meant to be treated as such, then the information given is insufficient.
    Last edited by ST1100; 11th January 2009 at 02:36 PM.

  10. #10

    Default Re: a physics question...

    Quote Originally Posted by ArchRival View Post
    Dudes, it's not that simple right?

    Imagine this: initially car B has velocity 100km/h. Car A is 0km/h. We know if collision is inelastic, both cars will finally move together with a common velocity.

    So, how did car B decelerate to that final velocity? And how did car A accelerate to that final velocity? During collision when Car B is decelerating and Car A is accelerating, there will be discrepancy between the velocities of the cars, so can the cars still move as one, ie perfectly inelastic?

    So the question is, during the time Car A takes to cover the 10cm gap, to what velocity has it accelerated to?
    yes yes yes... that's my question.. in short i just wanna know the 'speed' of the car that'll hit john

  11. #11

    Default Re: a physics question...

    Quote Originally Posted by tionghan View Post
    yes yes yes... that's my question.. in short i just wanna know the 'speed' of the car that'll hit john
    well....watever...but did the police come in time

  12. #12

    Default Re: a physics question...

    Quote Originally Posted by tionghan View Post
    yes yes yes... that's my question.. in short i just wanna know the 'speed' of the car that'll hit john
    Dude if you're interested in acceleration you cannot just use the momentum and energy equations. Those have no time derivatives.

    You could simplify the problem by adding a damper to car A. When car B hits car A the damper compresses. This allows car B to decelerate and car A to accelerate while both cars stay in contact. The rationale for using a damper is to model the crumple zones in the cars during collision. And we all know how to incorporate the equation for a damper.

    The equations are now easy to obtain and solve. Most likely the problem becomes simply one of solving eigenvalues. This is my suggestion, but since i'm only a poly dropout, check with someone more knowledgeable.

  13. #13

    Default Re: a physics question...

    The question is really lacking in information and the simplest answer is that Car A will also hit John at 100km/h.

    Assuming no frictional force, no lost of energy on impact, *Insert your own assumptions.

  14. #14
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    Default Re: a physics question...

    is the 2nd car are on its brakes?

  15. #15

    Default Re: a physics question...

    Quote Originally Posted by ahbian View Post
    The question is really lacking in information and the simplest answer is that Car A will also hit John at 100km/h.

    Assuming no frictional force, no lost of energy on impact, *Insert your own assumptions.
    ...yes assume car A is on very smooth ice....
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  16. #16
    Member luntut's Avatar
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    Default Re: a physics question...

    Quote Originally Posted by ahbian View Post
    The question is really lacking in information and the simplest answer is that Car A will also hit John at 100km/h.

    Assuming no frictional force, no lost of energy on impact, *Insert your own assumptions.
    i doubt car A will hit john at 100kmph.

    car B will hit A, assuming perfectly inelastic collision (no possible from the way cars are designed unless you got front crash guard), Car B and A will move together at the same speed, reduced speed.

    Before Collision:
    Momentum = Mass of Car V x Velocity of Car B.

    After Collision:
    Momentum = (Mass of Car B + Mass of Car A) x Velocity of Movement of both cars.
    Since assuming mass of both cars are the same, and knowing momentum is constant, we will get half the velocity of the original car B, which is at 50kmph.

    So at what speed does A hit John? If there is no friction, resulting in any deceleration or whatsoever.. you have your answer. But in real life, this is not possible also....
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  17. #17

    Default Re: a physics question...

    TS - what type of cars are you thinking of crashing?
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  18. #18

    Default Re: a physics question...

    This is screwing with my limited grasp of physics,if B stops immediately after hitting A, will that mean that A velocity can be theoretically B velocity with all the assumptions thrown in?

  19. #19

    Default Re: a physics question...

    Quote Originally Posted by tionghan View Post
    say Car A is stationary. Car B crashes car A at a speed of 100km/h..
    john is standing 10cm infront of car A
    both car weights 1 ton.

    what is the speed equivalent of car A impact on john

    ok car A does not have handbrake on in case someone will ask....
    Hmmm...u know, Albert Einstein once say " Education is what is left after u forget everything u learn in school"

    what i am educated in is, you better not be john...

  20. #20
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    Default Re: a physics question...

    Quote Originally Posted by tionghan View Post
    both car weights 1 ton.
    The question is inherently nonsensical flawed. A ton is a measure of mass not weight.

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