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Why is the answer A? Because I thought it's C, because g acts downward and since the ball is moving downwards towards N, acceleration should be "g" and not "-g" as it was when it was being thrown up at L. Can someone please explain?

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    $\begingroup$ "...as it was when it was being thrown up at L." You should read position L as a point immediately after the ball has been released and not about the acceleration from the throw. $\endgroup$ – BowlOfRed Jan 5 '17 at 7:41
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    $\begingroup$ Speed is not a vector quantity so the sign is unchanged; acceleration is, and it is always directed downwards, it does not depend on the direction of motion $\endgroup$ – tomph Jan 5 '17 at 7:43
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It says right there in the beginning of the sentence "vectors directed upwards are taken as positive". Still you must look at the speed of the ball" it has a vertical component and an horizontal one, the vertical component is being affected negatively by g, it is slowed by g therefore the negative sign

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  • $\begingroup$ Speed has no components. It is the magnitude of the velocity $\endgroup$ – tomph Jan 5 '17 at 7:46
  • $\begingroup$ Sorry, i see, what i meant was velocity, but the word did not come to me, english isnt my first language $\endgroup$ – user141154 Jan 5 '17 at 7:48
  • $\begingroup$ Still your answer is wrong in the concept. The minus sign in the acceleration does not come from the fact that it slows things because in general it doesn't: when the ball is going down, gravity actually speeds it up! The minus sign is due to the downward direction of the vector g $\endgroup$ – tomph Jan 5 '17 at 7:51

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