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Homework Help: Identify the following motions as positive or negative accelerations

  1. Oct 21, 2014 #1
    1. The problem statement, all variables and given/known data
    An object changes its velocity from 10 m/s [W] to 20 m/s [W] in 4.0s.
    An object changes its velocity from 20 m/s [W] to 10 m/s [W] in 4.0s.

    2. Relevant equations

    3. The attempt at a solution
    I'm thinking the first one is positive acceleration and the other one is negative acceleration (people also say deceleration but my teacher told us to use the term negative acceleration) because if you're moving west, west is negative so isn't -10 greater than -20? I'm awful at physics so I don't know if I'm thinking this through correctly.
  2. jcsd
  3. Oct 22, 2014 #2
    West is only negative if you define it to be so.
    It would be good practise to see how you get the same answers for this problem whether you take west to be negative or positive.

    You put N/A for relevant equations, but you could have put an equation relating acceleration to change in velocity which would really help with the problem.
    Acceleration is proportional to change in velocity, and remember than when you see "change" in basic physics you can usually replace that with "final minus initial"
    So if your initial velocity is -20m/s and your final velocity is -10m/s what was the change in velocity? Take the sign from that result and that should tell you the sign of the acceleration
  4. Oct 22, 2014 #3
    Actually on second thought you should probably just treat it as positive.
    Getting a positive acceleration on an object travelling in the negative direction is still causing the object to slow down and I think the point of this problem is for you to call that negative acceleration.

    So for this sort of problem just keep velocity positive and indicate direction like they did in the question: [W] or [E]
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