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Homework Help: A little help on some physics questions

  1. Oct 21, 2012 #1
    are velocity and displacement vector quantities or scalar quantities?

    if an object is going up is that when its acceleration is negative, and when that same object is coming back down does its acceleration become positive?

    if a car is going downhill is its acceleration/velocity negative?
  2. jcsd
  3. Oct 21, 2012 #2
    They are definitely vector quantities, even though a lot of times in simple problems such as an object falling they might appear to be scalar since there's only one dimension in question.

    No, your confusing acceleration with velocity and then some. If you're talking about throwing a an object in the air, the acceleration is a constant. The acceleration is the rate of change of velocity, and is dependent only on Force. The force of gravity is constant, thus acceleration due to gravity is constant.
    You can define the positive and negative directions whichever way you want, but when the object is going up the velocity is (positive or negative) and when it's falling back down, the velocity is the opposite sign.

    This is a similar problem in which it totally depends on whether you define up or down to be positive. It really doesn't matter, as long as it's consistent throughout the problem.
  4. Oct 21, 2012 #3
    Velocity is a vector. Displacement is a scalar, since it measures the distance from some origin.

    You can define your coordinate system in any way you choose. But acceleration is constantly pointed in whatever direction you consider "down", so long as no other force but gravity acts.

    Again, you are a (budding) physicist here. The best thing about being a physicist is that you get to create the framework to solve the problem in whatever way you deem fit. You can define whatever coordinate system you want, so long as it is right handed and each direction is perpendicular to the other two directions. If you think it makes the math easier to say that a car going downhill has a negative y component, then that is your freedom. You can even define a coordinate system such that the car is not traveling downhill at all, but is instead moving along the axis you've defined. It may not necessarily be ideal to do so, but sometimes it can be. Point being, you have total freedom here, so both yes and no are correct answers.
  5. Oct 21, 2012 #4


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    Staff: Mentor

    Welcome to the PF, venture.

    Please re-read the Rules link at the top of the page. You are supposed to show your own attempt at solving the questions befure you get tutorial help. Unfortunately, your homework has been done by a couple of our helpers this time...
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