Mechanics Kinematic General Concept question, not a numerical?

[T.S.M]

Mechanics Kinematic General Concept question, not a numerical!?

I'm really confused concerning the positive direction and the acceleration taken while concerning the motion of the ball!
Well, the main thing that i wanted to ask is that if 2 balls are been projected at the same time in two opposite directions in the same 'scene'. How many positive directions we will take, as in only one positive direction in one scene whatsoever are the directions of the to ball or every individual has it's own positive direction!
Please clarify that how many positive direction we will take for two balls thrown at same time in one scene!

Please explain one more thing, My teacher told me that if the positive direction is downwards and the ball is going upwards from the reference point, it's common sense that if ball moves upwards it has NEGATIVE acceleration, that is a= - g but as it is going in the -ve direction i.e; upwards so - (- g) will give us + g and thus we will take acceleration positive and in the same scene if the ball is going down it has defo + a and as it is also going in the positive direction so + (+ g) will give us +g, I was just wondering how can A be positive for both balls whereas they are traveling in opp. direction. So do u guys agree with this concept?
P.s: Whereas one of my teacher rejected this idea, LOL , :S

Please make these both concepts clear to me!
Thank-you! :)

 

[tiny-tim]

Welcome to PF!

Hi T.S.M! Welcome to PF!

… if the positive direction is downwards and the ball is going upwards from the reference point, it's common sense that if ball moves upwards it has NEGATIVE acceleration, that is a= - g but as it is going in the -ve direction i.e; upwards so - (- g) will give us + g and thus we will take acceleration positive and in the same scene if the ball is going down it has defo + a and as it is also going in the positive direction so + (+ g) will give us +g, I was just wondering how can A be positive for both balls whereas they are traveling in opp. direction.

A (the acceleration) has nothing to do with the velocity (not even with its direction) …

A is the same whether the ball is going up down or at an angle.

Imagine a train moving in a strong wind blowing South along the track …

if the train is going South anyway, the wind will make it go faster, so v will become more southerly …

if the train is going North, the wind will make it go slower, so v will become less northerly, ie more southerly …

either way, the acceleration is South. ​

 

[T.S.M]

Hey thanks!
Yes yes i tried the way you told, that is keeping the acceleration the same, i got the answer right! :D Thank-you :)

Errmm btw can you please tell me whether the theory, my teacher told is right or wrong?

 

[tiny-tim]

Let's see …

My teacher told me that if the positive direction is downwards and the ball is going upwards from the reference point, it's common sense that if ball moves upwards it has NEGATIVE acceleration, that is a= - g but as it is going in the -ve direction i.e; upwards so - (- g) will give us + g and thus we will take acceleration positive and in the same scene if the ball is going down it has defo + a and as it is also going in the positive direction so + (+ g) will give us +g,

hmm … I don't really understand that …

are you sure you have it right?​

 

[T.S.M]

Yep, i'm! :S
That's what is confusing me! :O