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Which of the Following is false?

  1. Sep 4, 2013 #1
    1. The problem statement, all variables and given/known data

    Which of the following statements is false?

    A.) The velocity of an object can change while maintaining a constant speed.
    B.)The speed of an object can change without the direction of its velocity vector changing.
    C.)An object that has a non-zero acceleration must have a changing velocity.
    D.) An object that maintains a constant speed must have zero acceleration.



    2. Relevant equations

    Speed = |Velocity|

    Acceleration = ΔV/Δt

    3. The attempt at a solution

    I tried to go through these one at a time and treat each one as a separate true or false question. Here is my thought process.

    A.) The velocity of an object can change while maintaining a constant speed.
    - I thought this would be true because velocity takes direction into account, whereas speed does not. Therefore, you could change direction without changing the magnitude of your velocity, which would maintain a constant speed but change the overall velocity.

    B.) The speed of an object can change without the direction of its velocity vector changing.
    - I also thought this was true, since you can accelerate or decelerate linearly, which would change the magnitude of the velocity (and therefore the speed) without changing the direction.

    C.) An object that has a non-zero acceleration must have a changing velocity.
    - I think this is true. Acceleration, by definition, is a change in velocity with respect to time. Therefore, if acceleration ≠ 0, it seems as if velocity would have to change somehow.

    D.) An object that maintains a constant speed must have zero acceleration.
    - This gave me the most trouble. I think it is false, but I do not know why it technically would be, I am mostly just using the process of elimination. One thing I was not sure of was whether or not changing the direction but not changing the magnitude of velocity could be considered "acceleration." If it is, then this could be false. Otherwise, I am not sure what else would apply here.

    So in the end, I personally think D is the culprit. All I am looking for is any corrections to be made in my above reasoning (if I reasoned correctly could you confirm it, I think I understand most of it but I'm at that uncertain point in the learning process where confirmation is helpful) and then, if D is the correct answer, it would be very helpful to have a more formal reason as to why D is false. Thank you very much in advance, and let me know if I can clarify any of the above statements.
     
  2. jcsd
  3. Sep 4, 2013 #2

    phinds

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    Think about the characteristics of a ball on the end of a string that is spun around your head at a constant RPM
    to see why you are right
     
  4. Sep 4, 2013 #3
    Circular motion actually makes that a whole lot clearer in my head. Since it is constantly accelerating towards the center right? But at the same time speed is constant.
     
    Last edited: Sep 4, 2013
  5. Sep 5, 2013 #4

    phinds

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    exactly
     
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