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1d Kinematics, bridge jumping.

  1. Oct 10, 2006 #1
    Bond is standing on a bridge, 10 m above the road below, and his pursuers are getting too close for comfort. He spots an approaching flatbed truck loaded with mattresses and decides to jump from the bridge onto the truck to make his getaway. He notes that the telephone poles the truck is passing are 20 m apart in this country and the time for the truck to cover this distance is 0.714 s. Estimating that the bed of the truck is 1.8 m above the road, Bond quickly calculates how many poles away the truck should be when he needs to jump. Assuming he jumps off just before the truck's front reaches a pole, how many poles are between Bond and the truck when Bond jumps off?

    I have no idea how to get started on this problem.. Advice please.
     
  2. jcsd
  3. Oct 10, 2006 #2
    Well, you will have two equations describing the two motions. There is one particular variable, one physical quanitity that must happen for Bond to make his getaway safely. This is all I will help you for now. Do you have any thoughts now? How do you think you can do this?
     
  4. Oct 10, 2006 #3

    Hurkyl

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    I often start with two opposite questions:

    (1) What sorts of things can I figure out from the given information?

    (2) What sort of information would allow me to figure out the answer?



    A different thing I might try is to translate the problem from words into arithmetic. Then, maybe the equations will suggest things to do.
     
    Last edited: Oct 10, 2006
  5. Oct 10, 2006 #4
    The truck has to be under him when he lands 1.8M from the highway, correct? Otherwise he hits the ground. So the distance he falls is 18.2M
     
  6. Oct 10, 2006 #5
    Good, the truck must be under him. So, I am a little unsure where you are getting 18.2m from. Are you sure it isn't 8.2m? Okay, so you are off to a good start. About how long do you suppose it will take for Bond to travel this distance?
     
  7. Oct 10, 2006 #6
    No because 20 - 1.8m is 18.2.. not 8.2.

    Oh nvm, I'm wrong I mistaked the distance of the poles and how high the truck is. So it is 8.2
     
    Last edited: Oct 10, 2006
  8. Oct 10, 2006 #7
    So I did 1.8 = 10 + 0t + (1/2)(-9.8)t^2

    -8.2 = -4.9t^2
    .597 = t^2
    .357 = t
     
  9. Oct 10, 2006 #8
    Sure, that would be right for how long it takes for him to fall. Now, what do you know about how long it takes for him to fall and how long the truck takes to get to that point?
     
  10. Oct 10, 2006 #9
    I know that it takes the truck .714s to go 20m
     
  11. Oct 10, 2006 #10
    Okay, that is a different associative time, which will be important later. What is important about the time Bond spends in the air? Where do we want him to land?
     
  12. Oct 10, 2006 #11
    We want him to land on the truck. He has to time it pefectly so he doesnt undershoot or overshoot the truck. We do not know how long the truck is though.
     
  13. Oct 10, 2006 #12

    OlderDan

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    That is a weakness in the problem. It also fails to say where Bond is in relation to the poles. Just calculate how many "pole separations" the truck has to move during the time he is in the air. I think they just want you to count poles, not find some fractional part of a pole separation. Allow for some reasonable truck length and express your answer as a whole number of poles. Alternatively, calculate the distance (in pole separations) his landing point has to move while he is in the air. The answer book probably takes one of these two approaches.
     
  14. Oct 11, 2006 #13
    So i just typed in 2 poles and it worked, for the correct answer haha.
     
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