Ball thrown upward

  1. Hi everyone,

    I was just reading a physics textbook when I noticed something. The motion diagram shows that if a ball is thrown upward into the air, the acceleration is always the opposite of the velocity. Wouldn't the acceleration initially have to be in the same direction as the velocity? How else could the ball go upward if it's never accelerating?
     
  2. jcsd
  3. LURCH

    LURCH 2,512
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    The key here is the phrase, "...if a ball is thrown upward...," in which I have emphasized the past tense. If the ball is being thrown, then yes, it is accelerating upward. However, once the act of "throwing" is complete, only gravity accelerates the ball (ignoring wind resistance, of course).
     
  4. What do you mean "act of throwing is complete"? If you mean when the ball is on its way back down, then the acceleration is still positive, it's just in another direction. In a motion diagram the arrows for velocity and acceleraton will all point in the same direction for the ball on its way down.

    What I am referring to is the few moments just after the ball has left the persons hand. At this point, the acceleration arrows for the motion diagram would HAVE to be in the same direction as the velocity arrows, indicating positive acceleration. Then, while the ball is still traveling upward, deceleration would kick in and then the motion diagram arrows would have to point in the opposite direction of the velocity arrows.

    It's not really the concept I'm worried about, it's the diagram they made. All the arrows for velocity on the balls upward movement are pointing in opposing directions relative to the acceleration arrows. This seems incorrect.
     
  5. cepheid

    cepheid 5,194
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    Just a nitpick. "Is thrown" can hardly be argued to be the past tense. That having been said, LURCH is exactly right about how we are meant to interpret what has been said. I suppose that saying, "Once a ball *has been thrown* upward..." might be slightly clearer. I'm not sure exactly what your book said, since you have only paraphrased it.
     
  6. cepheid

    cepheid 5,194
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    NO. The upward force (and therefore acceleration) occurs only when the ball is in contact with the thrower's hand. As soon as he or she releases it, the ball's motion becomes projectile motion (motion under the influence of gravity only). Its initial velocity (upon being released) is upward, and its initial acceleration (upon being released) is downward.

    Yes, the ball really does begin decelerating as soon as the thrower lets go of it. That is what you seem to be having trouble with.
     
    Last edited: May 26, 2009
  7. The instant the ball is no longer in contact with the hand, it is no longer being accelerated upwards and at that point the only force is the gravitational acceleration downward. Even though the ball is still moving upwards due to its initial positive velocity, it is slowing down to zero velocity at its maximum height, and then negative as it falls back down.
     
  8. Pengwuino

    Pengwuino 7,118
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    It is correct. Remember, something can be decelerating due to gravity but still have a positive velocity. Once the ball leaves the persons hand, the only force on it is due to gravity which ALWAYS is downward. Think of a car. When you slam on the breaks, you're immediately decelerating but you're stlil going forward with a positive velocity; it's just that the magnitude of that positive velocity is decreasing. In gravity's case, unlike a car, it doesn't stop at 0 but continues to go into the negative, that is to say it starts falling.

    It's best to think of acceleration as the change in velocity per unit time . Even though you have a positive velocity, that change in velocity can be positive or negative. In gravity's case, always negative if your positive direction is away from the earth.
     
    Last edited: May 26, 2009
  9. cepheid

    cepheid 5,194
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    change in velocity *per unit time*


    (I think this is an important addition, even if we are trying to keep things simple)
     
  10. Pengwuino

    Pengwuino 7,118
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    Yes that is quite important, thank you, I made the correction.
     
  11. ZapperZ

    ZapperZ 29,828
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    Acceleration is the rate of change of velocity. If the velocity is decreasing, you have a negative acceleration in that direction. But a negative vector in a particular direction is equal to a positive vector in the opposite direction!

    Zz.
     

  12. That's the explanation I was looking for. Thanks a lot for clearing that up. It's first year physics, so I wanted to make sure that I was getting the fundamentals right.
     
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