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Homework Help: Constant Velocity should = Acceleration of 0m/s^2 right?

  1. Sep 2, 2011 #1
    (a) Suppose that a NASCAR race car is moving to the right with a constant velocity of +81 m/s. What is the average acceleration of the car? (b) Twelve seconds later, the car is halfway around the track and traveling in the opposite direction with the same speed. What is the average acceleration of the car?

    To me they should equal:
    a) A=0 m/s^2

    b) A=-0 m/s^2

    It ha been awhile since i took AP physics in high school, but to my understanding a "Constant Velocity" means neither positive or negative acceleration right? I know V=Change in Distance/ change in time and Acceleration=change in velocity/changine in time but it keeps telling me im wrong....can someone tell me what im missing?

    Thank you
  2. jcsd
  3. Sep 2, 2011 #2


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    Are you sure of the exact statement of the problem? "Velocity" is a vector quantity. If the car is moving with constant velocity, it must be moving in a straight line with constant speed. It would have 0 acceleration but that would make it impossible for it to go "around the track".

    "Speed", on the other hand, is a number. It is quite possible for a car to drive around a circular track with constant speed. In that case, its acceleration is non zero and is directed toward the center of the track.

    Please check to make certain the problem says "velocity" and not "speed".
  4. Sep 2, 2011 #3
    And thats what i honestly thought to but the problem is literally copy and paste, (i dont feel bad about doing that because im not asking for the answers to it, just trying to make sense of it)
  5. Sep 2, 2011 #4
    Hi litzkrieg,

    You're right in saying that if something has a constant velocity, then it is not experiencing any acceleration. What you need to remember is the distinction between velocity and speed; velocity has a direction (how fast are we travelling West, for example), whereas speed is just the magnitude of the velocity (e.g. speedo on a car). This means that something can have a constant speed, but change direction, but to change direction, it must experience an acceleration. So your NASCAR car might maintain speed, but it's velocity has changed a lot. Using the change in velocity, you can find the acceleration in (b).

    Hope that helps!
  6. Sep 2, 2011 #5
    Yes it did, once i understood the real difference from speed and Velocity, recalling the direction factor everything became much more clear.

    Thank you Both
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