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Suburu WRX and a Honda Accord have collided at an intesection

  1. Sep 12, 2006 #1
    A suburu WRX and a Honda Accord have collided at an intesection. The suburu was initially travelling north. The honda was travelling west, about to initiate a right hand turn.

    Obsercation of the scene of the accident and interviews with witnesses yeilds the following information:
    1. The driver of the honda was a 52 yr old man of average build
    2. The driver of the suburu was a 24yr old man of average build
    3. After the impact the two vehicles traveled as one object
    4. A witness states that neither driver appeared to break prior to the collision
    5. The length of the skid was 12.8m in a direction N 7degrees W
    6. The mass of the honda is 1400kg
    7. the mass of the wrx is 1200kg
    8. it was a dry day and the coefficient of friction between the tyres and the road is 0.6
    9. the road is straight and flat
    10. a witness states that the honda was stationary at a stop sign prior to entering the intersection
    11. The beginning of the skid mark was 6m from the stop sign


    Determine the following:
    a) was the suburu exceeding the 80km/h speed limit?
    b) was the suburu visible to the driver of the honda before they initiated the turn?

    *make sure u state all assumptions you have made
    **the visibility of a car for a person with 20/20 vision is 1375feet and for a person with 20/50 vision is 550 feet.
     
    Last edited by a moderator: Sep 14, 2015
  2. jcsd
  3. Sep 12, 2006 #2

    J77

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    You need to show some of your working for help...

    Although, I can say that (1) and (2) probably won't add too much to the answer :biggrin:
     
  4. Sep 12, 2006 #3

    andrevdh

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    What I would do is use the work kinetic energy theorem to calculate their combined velocity just before they started skidding. Then use momentum conservation for the collision to calculate the velocity of the suburu just before the collision.
     
  5. Sep 12, 2006 #4
    how would i use the theorem before they started skidding though?
     
  6. Sep 12, 2006 #5

    andrevdh

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    You use the theorem for the skidding part only.
     
  7. Sep 12, 2006 #6
    ok. and what abot the care that entered the intersection (6m away at the stop sign) does that information seem relevant?
     
  8. Sep 12, 2006 #7

    J77

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    That's one of the cars involved in the crash - you could probably take it to be stationary, or 10kph...
     
  9. Sep 12, 2006 #8

    andrevdh

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    To be honest I myself will just ignore it since I have no idea what to do with it (only God and maybe high speed photography will enable one to know how to take that into account) and use momentum conservation for the rest. The skid marks were produced probably (a little bit after the collision) due to someone who had the sense to slam on his brakes (delayed reaction time) or the wheels dragging due to failure?
     
    Last edited: Sep 12, 2006
  10. Sep 12, 2006 #9

    J77

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    I wouldn't ignore it - if you do, the Suburu will have nothing to crash into :biggrin:
     
  11. Sep 12, 2006 #10
    hmm i see my homework as a bit ambuguous haha damn homework
     
  12. Sep 12, 2006 #11
    i meant ambiguous
     
  13. Sep 12, 2006 #12
    i still seem to be lost on this question.
     
  14. Sep 12, 2006 #13
    can anyone give me some extra help?
     
  15. Sep 13, 2006 #14

    J77

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    Well, you have two objects which you know the mass and velocities of; the impact at right angles; they travel as a single object with which you know the mass and can work out the velocity; you know the distance traveled to zero velocity after the collision; this should give an average deceleration which you can compare to frictional forces...
     
  16. Sep 13, 2006 #15

    andrevdh

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    Apply the theorem to the skidding part of the collision - Assumption: The wrecks came to a halt at the end of it. Show us your results in your next post.

    My thoughts on the inclusion of the 6 meters is that since the distance is so large one can safely assume that there were no interaction forces with the road present during the collision. This means that momentum will be conserved for the collision part of the incident.
     
  17. Sep 13, 2006 #16
    thing is i dont know the velocities of the two objects all im given is the mass of them both, the angle which they skid as one and how many metres they skid
     
  18. Sep 13, 2006 #17
    im only given the skid mark and the distasnce of it i dont have any velocities
     
  19. Sep 13, 2006 #18

    J77

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    ...but you need to work out the velocity of the Suburu.

    The Honda has just set-off from being stationary, so you could assum it to have zero velocity or, say, 10kpm - didn't we do this yesterday :smile:
     
  20. Sep 13, 2006 #19

    andrevdh

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    Have you done the work-kinetic energy theorem yet?
     
  21. Sep 13, 2006 #20
    KE = 1/2 mv2 right?
    we never got taught the work-kinetic energy theorem which is annoying (as u can see iv only started physics and im not very good at it)
     
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