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Basic kinematics problem

  1. Sep 21, 2011 #1
    1. The problem statement, all variables and given/known data

    A train moving with a constant velocity travels 135 m north in 12 s and an undetermined distance to the west. The speed of the train is 26 m/s.

    (a) Find the direction of the train's motion relative to north. (in degrees west of north)

    (b) How far west has the train traveled in this time?

    2. Relevant equations
    speed=distance/time or distance =speed*time

    v_0cos(theta)=v_x
    v_0sin(theta)=v_y

    3. The attempt at a solution
    i have no idea how to start, but im guessing i have to find the distance traveled north first in order to find the angle theta.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 22, 2011 #2

    PeterO

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    You were given the distance North - You have to find the speed North.
     
  4. Sep 22, 2011 #3
    oh!!!!!!! ok so this problem is forcing you to calculate the x component of the velocity since they pretty much give you the y component to find theta.

    Do you know how to do that?
     
  5. Sep 22, 2011 #4

    PeterO

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    Well it did travel 135m in 12 seconds.
     
  6. Sep 22, 2011 #5
    yes and that would be y. whats x?
     
  7. Sep 22, 2011 #6

    PeterO

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    Well it is logically negative, since the train went West, and the application of the Pythagorus theorem to the x and the y yields 26.
     
    Last edited: Sep 22, 2011
  8. Sep 22, 2011 #7
    thats how I see it too. The hidden part of the problem that will get you the answer lies in the pythagorean theorem.
     
  9. Sep 22, 2011 #8
    ok i think im finally understanding this
    so since it says the train traveled 135 m in 12 s, then its velocity is 135m/12s right?
    so i get 11.25m/s so thats the y component of velocity V_y
    so since its overall speed is 26 m/s then by using the pythagorean theorem we can find V_x?
    26=sqrt(V^2_x+V^2_y)=sqrt(V^2_x+(11.25)^2)
    from this i got V_x=23.44m/s
    so then tan(theta)=V_y/V_x=11.25/23.44
    theta=arctan(11.25/23.44)=25.6 degrees
    did i do this right?

    edit: so i submitted my answer (my homework is online) and i got it wrong, i still have 3 more tries i believe
    where did i go wrong?
     
    Last edited: Sep 22, 2011
  10. Sep 22, 2011 #9

    PeterO

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    If you sketch a North component of 11.25 and a west component of 23.44, with resultant 26, you will see the triangle and perhaps recognise the 25.6 degree angle you quoted. But that is not the angle "west of North" as asked.
     
  11. Sep 22, 2011 #10
    Yes you are doing it right however the angle you found is relative to the west. How would you go about finding the angle relative to the north? (hint: you are very close and 90 degree angles determine frequencies in how one should go about calling the direction north, west, east, or south). I'm sorry if that sounded a bit wordy.
     
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