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Two balls thrown from bridge one upwards, other downwards?

  1. Dec 27, 2015 #1
    I would have thought the force of a persons arm being thrown downwards (with gravity) would essentially increase a balls initial downward acceleration immediately following the balls release. As compared to an upwards thrown balls initial acceleration; in which the persons upwards arm movement is working against gravity. Of course, in my mind, the question is metaphorically suggesting two people who could hypothetically both throw a ball (simultaneously - in the same direction) at exactly the same speed as each other....which I'm sure the odds are pretty low. For my question I have answered B and wanted to check on the question and my answer.

    Q) Two people are standing on a bridge with a tennis ball each. They throw balls at the same time and the same speed. But one person throws the ball upwards and the other throws it downwards. Which ball has the greatest acceleration at the instant of release (just after they are let go)

    a. The ball thrown upwards.

    b. The ball thrown downwards.

    c. The acceleration for both is the same.

    Regards,

    Jesse
     
  2. jcsd
  3. Dec 27, 2015 #2
    What is acceleration definition?
     
  4. Dec 27, 2015 #3
    Well in both cases the acceleration will be due to gravity, so it's really just a matter of looking at the kinematic equations for the two situations and looking at it more in-depth.
    This is kind of tricky to think about because we're dealing with this in general terms, but the key part is that it's asking about the acceleration immediately at the moment of release, or t = 0. Your intuition is not wrong in the case where after the initial second, the ball being thrown down will experience a greater speed than the ball thrown up earlier, but the question is asking about acceleration. Think about what acceleration is going to be for the two balls in this scenario, and if it is dependent on time.
     
  5. Dec 27, 2015 #4

    CWatters

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    After release how does the arm apply a force on the ball?
     
  6. Dec 27, 2015 #5

    A.T.

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    That's the only one of the 3 answers which doesn't make sense to me.
     
    Last edited: Dec 27, 2015
  7. Dec 27, 2015 #6
    Which ball has the greatest acceleration at the instant of release (just after they are let go)

    Just after they let go, there is no interaction between people and ball, there is only g.
     
  8. Dec 27, 2015 #7

    sophiecentaur

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    Newton tells us that the acceleration is only due to forces being applied. When it's been launched, the only force / acceleration is due to gravity. But the direction of its initial velocity is important. When you throw it down, it's already got a downwards velocity (you were accelerating it with the force of your arm) but, when you throw it up, the velocity has the other sign, g will slow the speed to zero (at the top if its flight) and then its downward velocity will increase all the way to the ground.
    What I have described is, of course, 'obvious' and it used to be explained in the terms you use - till Newton spotted what is actually going on. There are a set of "Equations of Motion" which describe motion under constant acceleration. They do make sense and will give you the right answer to all these problems. They are based on Newton's laws of motion. You really have to do some example questions yourself to get familiar with how it works.
     
  9. Dec 27, 2015 #8

    HallsofIvy

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    Then you would be wrong. Throwing downward increases the initial velocity, not the acceleration.
     
  10. Dec 27, 2015 #9

    A.T.

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    As compared to just dropping. But the two cases to consider here are both assumed to have the same initial speed.
     
    Last edited: Dec 27, 2015
  11. Dec 27, 2015 #10
    Highlighted key words...
    F=ma
    The mass of the balls is the same.
    What is the F on each ball after release?
    Once you know about F and m, you can say something definite about a.
     
  12. Dec 27, 2015 #11
    Don't confuse acceleration with velocity.
     
  13. Dec 27, 2015 #12

    HallsofIvy

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    Same initial speed, yes, but opposite directions.
     
  14. Dec 27, 2015 #13

    CWatters

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    How does this relevant to the question?
     
  15. Dec 27, 2015 #14

    jbriggs444

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    It could be relevant if we wanted to consider some velocity-dependent force. There is one handy.
     
  16. Dec 28, 2015 #15
    Thankyou all very much. I have a lot to learn, I need to increase the velocity of my studies acceleration.
     
  17. Dec 28, 2015 #16
    once the ball is released downward, it is accelerating at 1g. the ball released upward is not accelerating once it leaves your hand. like a bullet fired from a level gun. once it leaves muzzle, it instantly is decelerating
     
  18. Dec 28, 2015 #17
    yes, so relaltive to the person throwing the ball.. the downward ball is accelerating toward earth. the tossed up ball is decelerating relative to the person at a with a force of 1g. after it slows to 0 velocity relative to the person, it will start accelerating toward the earth with a force equal to 1g.
     
  19. Dec 28, 2015 #18

    CWatters

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    Both balls are accelerating downwards due to gravity. The only difference is the initial velocity.
     
  20. Dec 28, 2015 #19
    Acceleration is defined as the change in velocity per unit time, for example, meter/second/second or velocity/second. When the ball is thrown regardless of its direction and velocity, they are both affected by a single force at the time of release; the acceleration due to gravity g (9.8m/s2). In other words, the ball thrown downward is accelerating by 9.8m/s every second while the ball thrown upwards is decelerating -9.8m/s every second until it reaches its highest altitude and descends.
     
  21. Dec 29, 2015 #20

    A.T.

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    Using "accelerating" and "decelerating", together with opposite signs is just confusing. The acceleration vector has the same direction and magnitude in both cases.
     
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