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  1. Sep 29, 2007 #1
    Can anyone please help me out with these two questions?


    1. Starting from rest, a car travels 1350m in one minute. It accelerated at 1m/s2 until it reached its cruising speed. Then it drove the remaining distance at constant velocity. What was its cruising speed?

    All I did was I divided 60s into 1350m and my result was 22.5. Is this the average velocity?

    2. A satellite is in circular orbit 600km above the Earth's surface. The acceleration of gravity is 8.21m/s2 at this altitude. The radius of the Earth 6400km. Determine the speed of the satellite and the time to complete the orbit around the Earth.
    I have no idea how to go about this problem. Is this centripetal acceleration?
     
    Last edited: Sep 30, 2007
  2. jcsd
  3. Sep 30, 2007 #2

    learningphysics

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    Give these a shot yourself... show us where you're getting stuck. We'll help you along.
     
  4. Sep 30, 2007 #3
    Do I use this equation to solve the 1st problem? [​IMG]
     
  5. Sep 30, 2007 #4

    learningphysics

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    divide the problem into two parts... while it is accelerating and while it is cruising... so let t be the time when the car reaches cruising speed...

    what is the distance travelled in the t seconds... what is the distance travelled from t seconds to 60s...
     
  6. Sep 30, 2007 #5

    lightgrav

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    the 22.5 IS the average velocity .
    If it's not obvious from the wording,
    you have to treat this first problem as 2 separate motions
    ... get the distance and time as stated.
    (no, that equation isn't a good choice, because it avoids time)
     
  7. Sep 30, 2007 #6
    [​IMG]
    V2=0+2(1)(1350)
    V=51.96m/s.

    Am i on the right path?
     
  8. Sep 30, 2007 #7

    learningphysics

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    No. because it isn't accelerating over the entire 1350m... it only accelerates until it hits the cruising speed. Then it stays at that velocity.

    you have to split the problem into 2 parts... suppose it accelerates for t seconds... what is the distance travelled in that t seconds?
     
  9. Sep 30, 2007 #8
    2. A satellite is in circular orbit 600km above the Earth's surface. The acceleration of gravity is 8.21m/s2 at this altitude. The radius of the Earth 6400km. Determine the speed of the satellite and the time to complete the orbit around the Earth.
    I have no idea how to go about this problem. Is this centripetal acceleration?


    V = [tex]\sqrt{2\mu/r - \mu/a}[/tex]

    You have to be careful when calculating r. You don't need the acceleration to solve the problem. a is the semi-major axis of the ellipse. Except you have circular orbit. This simplifies the above equation. [tex]\mu[/tex] is the gravitational parameter of the body being orbited. You can use Kepler's Third Law to solve for the orbital period. Hope this helps.
     
  10. Sep 30, 2007 #9
    a=22.5/60
    a=0.375m/s2

    vf2=vo2+2ad
    vf=31.82
     
  11. Sep 30, 2007 #10

    learningphysics

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    If we let t be the time it takes to reach cruising speed, then the distance travelled during this time is (1/2)at^2 = (1/2)(1m/s^2)t^2 = t^2/2

    So d1 = t^2/2

    The total time is 60seconds. If t is the time taken for accelerating... then 60-t is the time it was cruising.

    d2 = (60 - t)vcruising

    What is vcruising in terms of t?

    What is d1 + d2?
     
  12. Sep 30, 2007 #11

    lightgrav

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    I suggest you get the IDEAS down first ...
    then check the scenario ... approximately.
    (that usually helps guide the detailed steps)

    It ALWAYS helps to sketch these things
    . . . maybe even a graph (!)
    the average speed was about 23 m/s;
    so, about what was cruising speed?
    About how long would it take to get that fast?
    How much time would be left to actually cruise?
    What distance would've been cruised? accelerated? total?
     
  13. Sep 30, 2007 #12
    DAMN!!! I feel so retarded
     
  14. Sep 30, 2007 #13
    Can anyone give me clues to the 2nd question?
     
  15. Sep 30, 2007 #14
    Yes For The Second Question You Want To Use One Of Your Centripetal Acceleration Equations To Solve For The Speed You Have All The Information There It Is Just A Matter Of Plugging In Numbers.
     
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