# M1 Projectile Motion (Basic Question)

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Thread starter 1 month ago
#1
if a ball is thrown vertically downwards from a point X which is h metres above the ground and we are asked to find the time it takes for it reach the reach ground.
should we take take the displacement (height) and the final velocity with which it hits the ground as negative, if the ball is thrown vertically downwards?
Last edited by Aleksander Krol; 1 month ago
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1 month ago
#2
(Original post by Aleksander Krol)
if a ball is thrown vertically downwards from a point X which is h metres above the ground and we are asked to find the time it takes for it reach the reach ground.
should we take take the displacement (height) and the final velocity with which it hits the ground as negative, if the ball is thrown vertically downwards?
You can define the positive direction as you want. However, you have to be consistent. A sketch generally helps where the positive direction is clearly marked.

If downwards is chosen to be positive, then the initial (and final) velocity is positive as is the displacement of the ground from the point of being thrown and also the acceleration due to gravity.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#3
(Original post by mqb2766)
You can define the positive direction as you want. However, you have to be consistent. A sketch generally helps where the positive direction is clearly marked.

If downwards is chosen to be positive, then the initial (and final) velocity is positive as is the displacement of the ground from the point of being thrown and also the acceleration due to gravity.
so displacement, final velocity and acceleration due to gravity, all of them are positive. thanks for clearing my doubt about this.
And if the ball was thrown vertically upwards, is the final velocity (with which it hits the ground) and the displacement and acceleration due to gravity will be negative?
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1 month ago
#4
(Original post by Aleksander Krol)
so displacement, final velocity and acceleration due to gravity, all of them are positive. thanks for clearing my doubt about this.
And if the ball was thrown vertically upwards, is the final velocity (with which it hits the ground) and the displacement and acceleration due to gravity will be negative?
It depends on which way you define as positive. Hence the comment about making it clear and recommending you sketch the scenario with the positive direction clearly marked on. For the previous scenario, all suvat terms will have the same sign, and its easier to choose them as positive as it makes the working clearer.

If the ball is thrown vertically upwards, the initial velocity will have the opposite sign to gravity (acceleration), the final velocity and the displacement of the ground from the initial position, Which set is positive is up to you. Geneally, its easier to choose the "larger" set as the positive direction.
Last edited by mqb2766; 1 month ago
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Thread starter 1 month ago
#5
(Original post by mqb2766)
It depends on which way you define as positive. Hence the comment about making it clear and recommending you sketch the scenario with the positive direction clearly marked on. For the previous scenario, all suvat terms will have the same sign, and its easier to choose them as positive as it makes the working clearer.

If the ball is thrown vertically upwards, the initial velocity will have the opposite sign to gravity (acceleration), the final velocity and the displacement of the ground from the initial position, Which set is positive is up to you. Geneally, its easier to choose the "larger" set as the positive direction.
thanks got it now!
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