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Homework Help: Acceleration of a ball

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data
    You throw a ball straight up with a speed of 14m/s. What is the acceleration of the ball 1.5s after you threw it?

    Alright, the dilemma im having is the answer is -9.8m/s^2.

    It doesn't make sense to me how the acceleration can be that at 1.5s. The -9.8m/s^2 (gravity of earth) is the force pulling down on the ball not the acceleration after 1.5S as it is still moving up rather then down. What is the correct acceleration, or is -9.8m/s^2 correct?
  2. jcsd
  3. Oct 30, 2008 #2
    -9.8 is correct because the time doesn't matter. Gravity will have a constant acceleration on the object. 9.8 is the acceleration, NOT FORCE, because the unit for acceleration is m/s^2
  4. Oct 30, 2008 #3
    -9.8ms-2 is correct. It looks to me like you have velocity and acceleration confused. Velocity is the speed and direction that the ball is moving, acceleration is the speed and direction in which the velocity is changing. This can be a tough concept to grasp (I found it a little difficult at least).

    On the ball's way up, every second its velocity will be becoming 9.8ms-1 slower. When it's at its peak, its velocity will be zero, however it can still have an acceleration (9.8ms-2). Then on its way down, it will be speeding up by 9.8ms-1 every second.

    From this we can see that the velocity of the ball has completely changed direction, whilst the acceleration hasn't changed at all!!

    I am aware that I'm not great at explaining things like this, but I hope it's helped a bit until someone else can come and explain the concept better!

    Two important things to note are:
    acceleration does not have to be in the same direction as velocity, if it's in the opposite direction, it merely means that it's slowing down.
    acceleration due to gravity (at levels we're dealing with) does not change.
  5. Oct 30, 2008 #4
    arlight thanks, that clears it up!
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