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Super easy driving question - instantaneous acceleration

  1. Sep 25, 2011 #1
    super easy driving question -- instantaneous acceleration

    1. The problem statement, all variables and given/known data
    a Person in a car is driving down a straight road. The instantaneous acceleration is decreasing with time but is directed in the same direction as the cars motion the speed of the car is constant or increasing with time. explain

    3. The attempt at a solution
    i know its not decreasing with time.
     
    Last edited: Sep 25, 2011
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  3. Sep 25, 2011 #2

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    Re: super easy driving questioninstantaneous acceleration

    Hi again physicsgurl12! :smile:

    For this you need to understand a little bit about acceleration and speed.

    The definition of acceleration is: the change in speed per unit of time.
    (Similarly the definition of speed is: the change in position per unit of time.)

    So if acceleration is positive, this means that the change in speed is also positive, meaning that speed increases.

    Can you tell whether the acceleration is positive or negative?
     
  4. Sep 25, 2011 #3
    Re: super easy driving questioninstantaneous acceleration

    If the direction of acceleration is directed in the same direction as the car's velocity than it must be positive.

    If the car's acceleration is positive (greater than zero) than the velocity must be increasing per unit time, regardless if the acceleration is decreasing in terms of magnitude.

    Example : Velocity starts at 10 m/s forward at t=1
    Acceleration is 10 m/s^2 (same direction as velocity) at t=1s forward but decreases to 2 m/s^2 at t=2s. Speed at t=2s (new speed) will be 14 m/s instead of being 20 m/s if the acceleration was constant.

    The car is still going forward and it's speed is increasing because you still have an acceleration in the same direction.
     
  5. Sep 25, 2011 #4
    Re: super easy driving questioninstantaneous acceleration

    negative. because its decreasing?
     
  6. Sep 25, 2011 #5
    Re: super easy driving questioninstantaneous acceleration

    No decreasing means going from a big value to a lesser value. Going from 10 to 3 is decreasing.

    Negative means less than zero. A negative acceleration would be an acceleration against the car.

    If you drive a car into a building the car will stop and go backwards because the building is doing a negative acceleration relative to the car's initial trajectory.
     
  7. Sep 25, 2011 #6

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    Re: super easy driving questioninstantaneous acceleration

    Consider the speed of a skateboard.
    Let's assume that the speed of the skateboard is decreasing.
    Does that mean the speed is negative, or in other words that your skateboard is moving backward?
     
  8. Sep 25, 2011 #7
    Re: super easy driving questioninstantaneous acceleration

    moving backwards? because its distance and if the speed decreases it doesnt mean it has to go behind the orgin.
     
  9. Sep 25, 2011 #8

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    Re: super easy driving questioninstantaneous acceleration

    Uhh, right.
    So when you're moving forward with a decreasing speed, you're still going forward, meaning you have a positive speed.
    That is, until the speed becomes zero, after which you would be going backward.

    Now back to the problem.
    So your acceleration is decreasing.
    Does the problem statement say in which direction the acceleration is (even though it is decreasing)?
     
  10. Sep 25, 2011 #9
    Re: super easy driving questioninstantaneous acceleration

    nope it just its decraseing and opposite of the car.
     
  11. Sep 25, 2011 #10

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    Re: super easy driving questioninstantaneous acceleration

    Hmm, I read: "The instantaneous acceleration (is decreasing with time but) is directed in the same direction as the cars motion".
     
  12. Sep 25, 2011 #11
    Re: super easy driving questioninstantaneous acceleration

    oh. just kidding. thats what i get for trying to do two at once
     
  13. Sep 25, 2011 #12

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    Re: super easy driving questioninstantaneous acceleration

    Sooooooo?
     
  14. Sep 25, 2011 #13
    Re: super easy driving questioninstantaneous acceleration

    so then its moving the same direction as the car
     
  15. Sep 25, 2011 #14

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    Re: super easy driving questioninstantaneous acceleration

    Uhh? :confused:

    What is moving in the same direction as the car?
     
  16. Sep 25, 2011 #15
    Re: super easy driving questioninstantaneous acceleration

    the acceleration. isnt that what we were talking about?
     
  17. Sep 25, 2011 #16

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    Re: super easy driving questioninstantaneous acceleration

    Ahhh, well, "acceleration" isn't "moving"...

    So the acceleration is in the same direction as the car.
    What does that tell you about the speed of the car?
     
  18. Sep 25, 2011 #17
    Re: super easy driving questioninstantaneous acceleration

    that its accelerating
     
  19. Sep 25, 2011 #18

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    Re: super easy driving questioninstantaneous acceleration

    Good! :shy:

    Do you have the answer to your problem now?
     
  20. Sep 25, 2011 #19
    Re: super easy driving question -- instantaneous acceleration

    um i feel like its constant because its decreasing acceleration even though its going the same direction as the car
     
  21. Sep 25, 2011 #20

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    Re: super easy driving question -- instantaneous acceleration

    Uhh... exactly what is constant?
     
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