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Acceleration and Velocity Direction

  1. Jan 22, 2012 #1
    Hi all!I am studying for MCAT physics and have couple of questions (see topic title).

    Please see the pictures attached. (Pardon my art skills).

    In the first scenario, a is perpendicular to v. Does that mean that object will move same direction as a, but with constant speed, since acceleration (a) has no x component?

    In the second scenario, object will move NE, same direction as a, but its speed will increase, since acceleration a has an x component. Is that right?

    Thanks in advance!

    Attached Files:

  2. jcsd
  3. Jan 23, 2012 #2


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    Look at each of those diagrams and ask yourself "What will the velocity (resultant) be after one second, then two seconds, then two hours, assuming the acceleration vector is constant (magnitude and direction)?"
    I assume that you know how to add vectors so all that's necessary is that you apply that carefully. You will get the answer all by yourself.

    PS Powerpoint is worth while getting to know, intimately. :wink:
  4. Jan 23, 2012 #3

    Thanks for getting back to me, and yea I guess I should use PP, but I just wanted to impress everyone with my "paint" skills. JK :)

    In the first case, a=0, so v initial=v final, with the object moving NE. After 1 sec, 2 sec or two hours, object still moves at the same speed. Right?

    In the second case, a= some value (let's say it's x), so v final still points NE, but after 1 sec, 2 sec and 2 hours, object is increasing its speed respectively, as long as Delta V/Delta T ratio remains=x
  5. Jan 23, 2012 #4


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    ??? But doesn't an acceleration of a mean that the velocity in that direction increases by a m/s every second? How can the speed and direction remain the same?

    Of course, if a is zero, there will be no change.

    I think you are just trying to jump to a conclusion that you fancy without getting there rigorously.

    What does acceleration mean, for instance? Apply that in your calculation, then.
  6. Jan 23, 2012 #5
    Um maybe I am misunderstanding here

    first example acceleration is at right angles to the direction of travel

    I prefer to take as example a space craft moving at velocity
    acceleration is applied 90 degrees to the direction of motion

    I would think the space ship would move in a new direction due to the combination of its original velocity and the applied acceleration

    I use the example of a space craft here as an easy way to see

    But I probably misundertood the question
  7. Jan 23, 2012 #6


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    Some idea of Vectors is necessary here. It's not really a suitable problem for arm waving.
    The word "vector" doesn't have to bring on the pains, though.
  8. Jan 24, 2012 #7


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    At all times, there will be a velocity vector in the original direction but, as time progresses, the velocity in the direction of the acceleration vector will increase at a metres per second every second. At any time you will have two vectors to add together. Eventually, the v vector in the direction of the acceleration will be so big that it dominates.

    This is just like throwing a stone off the side of a cliff. By the time it gets to the bottom, it is travelling almost vertically.

    There, I've done it for you.
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