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Motion in physics

  1. Oct 10, 2015 #1
    1. The problem statement, all variables and given/known data
    I have a picture of the question

    2. Relevant equations
    I dont know the equation realted to this question

    3. The attempt at a solution
    I didnt understand the question
     

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  3. Oct 10, 2015 #2

    vela

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    Why don't you start by identifying what the problem is asking for? What are the definitions for those quantities?
     
  4. Oct 10, 2015 #3
    For a ) I used this ##s_a=(d_1/t_1+d_2/t_2)/2## for b)I did ##s_a=(2d/(t_1+t_2))##
    It worked but I dont know why ??
    For c I am stucked I know the answer of d its zero
     
  5. Oct 10, 2015 #4

    vela

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    You might find it helpful to explain your reasoning when writing out your solutions. You should definitely do it when posting here rather than expecting us to read your mind. If you're just plugging in numbers to random formulas with no real understanding why, you're setting yourself up for failure in physics.

    Your method for (a) looks wrong. Your method for (b) is wrong. The units don't work out. You can't add d/t1 to t2.
     
  6. Oct 10, 2015 #5
    Lets look at then please wait me
     
  7. Oct 10, 2015 #6
    For a) the average speed is ##s_a=Δx/Δt## ok.So half of the time the car goes 55 km/h lets call total trip t and total distance d so the equation will be ##55km/h.t_1+90km/h.t_1=d## , ##t_1=t/2## average speed is total distance divided by time so ##d/2t_1= 145t_1/2t_1=72.5 km/h##

    For b) So half of the distance is ##d/2## so ;

    ##d/2=55km/h.t_1##
    ##d/2=90km/h.t_2##

    so ##t_1/t_2=1,636## so ##t_1=t_2.1,636##

    ##s_a=Δx/Δt##

    ##s_a=d/(t_1+t_2)=180t_2/(t_2+t_2.1.636)##
     
    Last edited: Oct 10, 2015
  8. Oct 10, 2015 #7
    Is that wrong ?
     
  9. Oct 10, 2015 #8

    vela

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    Did you mix up ##t_1## and ##t_2## in the denominator in (b)? (a) looks right.
     
  10. Oct 10, 2015 #9
    I made a wrong in b but I dont know why I check the answer
    total time=t1+t2
     
  11. Oct 10, 2015 #10
    Now I fixed
     
  12. Oct 10, 2015 #11
    lets come to see c) which its (savg1+savg2)/2
     
  13. Oct 10, 2015 #12

    vela

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    The average speed is not the average of the speeds.
     
  14. Oct 10, 2015 #13
    then what it is ?
     
  15. Oct 10, 2015 #14

    vela

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    Go back to the definition. You used it already.
     
  16. Oct 10, 2015 #15
    ##s_a=Δx/Δt##
    Δx=2d
    Δt=2t
    Can you show me how to do that please
     
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