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Car and wall

  1. Jan 16, 2017 #1
    I've been trying to study for my final and I can't seem to figure out how is this suppose to work.

    1. The problem statement, all variables and given/known data

    A wall (m infinite, v=0) hits a car (m=2600 kg; v=142 km/h). The car becomes deformed and the crush zone (0.7 m) is compressed. Calculate the corresponding acceleration (assuming a constant value). Within which time interval does that compression happen? Try to find out, how fast each part of the airbag system therefore has to operate. Imagine the crush-zone is replaced by a spring. Calculate the required spring constant.

    2. Relevant equations


    3. The attempt at a solution
    I think it has to do with newtons second law but I dont know what to do with that velocity.
     
  2. jcsd
  3. Jan 16, 2017 #2

    PeroK

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    What else do you know about kinematics? Netwon's second law is a start.
     
  4. Jan 16, 2017 #3
    I know the Kinematic Equations and how they work.
     
  5. Jan 16, 2017 #4

    PeroK

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    Then you can apply them here!
     
  6. Jan 16, 2017 #5
    Can you give me a hint? I know that I'm over looking something very basic but I can't seem to find what?
     
  7. Jan 16, 2017 #6

    PeroK

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    Car - initial velocity given - uniform acceleration assumed - stopping distance given - find force and stopping time.

    That's the problem in a nutshell, isn't it?
     
  8. Jan 16, 2017 #7
    So that 0.7 can be taken as the distance?
     
  9. Jan 16, 2017 #8

    PeroK

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    Yes.
     
  10. Jan 16, 2017 #9
    Thanks a lot!

    So I should assume that the final velocity is 0 and use the kinematic equation to get acceleration?
     
  11. Jan 16, 2017 #10

    PeroK

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    You need to think on your feet more. If the car is still moving, then you know too little to solve the problem.
     
  12. Jan 16, 2017 #11
    I'm at a loss again then, is average velocity the same as a constant velocity?
     
  13. Jan 16, 2017 #12

    PeroK

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    That question doesn't make a lot of sense.
     
  14. Jan 16, 2017 #13
    Can you please show me the formulas I'm suppose to be taking into account?
     
  15. Jan 16, 2017 #14

    PeroK

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    We can only help so much. We can't teach you physics from the beginning. One formula you might like to consider, for constant acceleration is: ##a = \frac{\Delta v}{\Delta t}##.
     
  16. Jan 16, 2017 #15
    V = d/t

    then

    a = v/t

    I think I got it?
     
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