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Help with overall distance S

  1. Nov 10, 2009 #1
    1. The problem statement, all variables and given/known data

    A spots car reaches the speed V1 = 100 km/h within 5 sec. At that speed, its breaking distance
    is S2 = 40 m.
    a) What is the overall distance S covered by the car?


    2. Relevant equations

    V=V0 + at

    S=S0 + V0t + 1/2a1t2

    Overall distance = S1 + S2


    3. The attempt at a solution

    I'm having trouble calculating the time and calculating S1. We know that S2 = 40. How to calculate S1
     
    Last edited: Nov 10, 2009
  2. jcsd
  3. Nov 10, 2009 #2

    Doc Al

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    Staff: Mentor

    I presume S1 is the distance covered as it accelerates to the given speed? The equations you have are just what you need. Find the acceleration, then use it to find the distance. (Be sure to convert the speed to standard units--m/s.)
     
  4. Nov 10, 2009 #3
    Yes - S1 is the distance covered as it accelerates to the given speed .

    OK the acceleration is a1 = V1 / t1 = 100/5 = 20

    And now we have S1=1/2a1t2

    How to find the time - t ? And by the way velocity is uniformly accelerated
     
    Last edited: Nov 10, 2009
  5. Nov 10, 2009 #4

    Doc Al

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    Staff: Mentor

    No. You first need to convert the speed to standard units. Then your equation will work fine.

    The time is given. (You just used it to find the acceleration!)
     
  6. Nov 10, 2009 #5
    Ok . Can you tell me how to convert the speed to standard units.
     
  7. Nov 10, 2009 #6

    Doc Al

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    Staff: Mentor

    100 km = ? meters
    1 hr = ? seconds

    Divide!

    (Or use Google to do the conversion for you. :wink:)
     
  8. Nov 10, 2009 #7
    Ohh yeah. So a1 = V1 / t1 = 100/3.6 * 1/5 = 5.5 .
    Correct ?
     
  9. Nov 10, 2009 #8

    Doc Al

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    Staff: Mentor

    Not yet.

    [tex]\frac{100 km}{hr} \times \frac{? m}{km} \times \frac{hr}{? s} = [/tex]

    Fill in the quesion marks so that each of the two "conversion factors" equals 1. How many meters in one km? How many seconds in an hour?
     
  10. Nov 10, 2009 #9
    OK. 1km=1000m and 1hr = 3600s.

    So 100 km/h = 27.7 m/s
     
  11. Nov 10, 2009 #10

    ideasrule

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    Homework Helper

    Yeah. Notice that this is equivalent to dividing 100 km/h by 3.6.
     
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