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Speed-time graph What is the highest speed?

  1. Jun 18, 2013 #1
    1. The problem statement, all variables and given/known data
    The problem is telling to write the interval of time in which a car has the highest speed.
    We see in a graph (time-speed) negative and positive values.




    2. Relevant equations
    Definition of speed.
    Logic.

    3. The attempt at a solution

    So, I wonder whether they are asking the absolute value of speed, or the highest postive one. It's a semantics matter. What are your two cents? I feel like we want to know, in our system of reference, the highest speed, so the biggest positive one? But, I'm still in Grade 11 and this is my first Physics class...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 18, 2013 #2

    PeterO

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    If you have really been given a speed - time graph showing negative speed, then the graph is incorrect.
    It would have to be a velocity-time graph to have negative values.
    The descriptors positive and negative do not apply to speed - that's why you don't see any negative values on the speedometer of a car.
     
  4. Jun 18, 2013 #3
    Sorry, in French, « vitesse » actually means « velocity » and is sometimes wrongly translated to « speed ». So, I should have said velocity, which can have negative values.
     
  5. Jun 18, 2013 #4

    PeterO

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    OK imagine you are replicating your velocity-time graph in a car using 1st gear and reverse gear (1st for positive, reverse for negative)
    When will your speed have been the highest? Were you in 1st gear or reverse gear at the time? I know which one will have been most frightening!
     
  6. Jun 18, 2013 #5
    Well, if we are faster in reverse, the ''velocity'' would be higher. So, you're telling me we should consider the absolute value?
     
  7. Jun 18, 2013 #6

    PeterO

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    Interesting concept.

    Just as positive and negative do not relate to speed, higher and lower do not relate to velocity.
    Per chance, un probleme de la langue?

    Velocity can be positive or negative, its value can be large or small.
    Speed can be high/low - leading to you travelling fast or slow.

    Speed is certainly the absolute value of velocity.
     
  8. Jun 18, 2013 #7
    You're right, language problem hihi. Then, I recapitulate. What's the largest velocity? The one with the highest absolute value!
     
  9. Jun 18, 2013 #8

    PeterO

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    Toujours la lange.

    The highest speed is when the velocity has the greatest value - whether that be positive or negative.

    That is also the largest velocity. Largest implies greatest in size, which is what "converts" velocity to speed.

    "La vitesse le plus grand" means the largest absolute value of the velocity - or in English simply "the highest speed". [did I mix genders there?]
     
  10. Jun 18, 2013 #9
    Perfect haha. It's "la" because vitesse is feminine!
     
  11. Jun 18, 2013 #10

    PeterO

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    I know it is la vitesse, but can you have "la vitesse" and then "le plus grand" ?
     
  12. Jun 18, 2013 #11
    Oh no! La plus grande vitesse or la vitesse la plus grande. "La" always refers to vitesse. Hehe learning French in physics forum :)
     
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