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# why we write g as negative in projectile motion?

why we take g as negative in projectile motion because it is coming downward so g must be positive?

### 11 Answers

- 9 years agoFavorite Answer
'g' is the acceleration due to gravity.

gravitational force acts towards the centre of the earth i.e downwards we consider.

thus by cartesian coordinate graph,

forces going upwards are along +ve y-axis,

downwards along -ve y-axis,

forward are along +ve x-axis,

backwards along -ve x-axis.

Formulae are derived using this graphical methods so 'g' acting downwards is considered along -ve y-axis thus we put -ve sign in front of 'g'.

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- Bob BLv 79 years ago
The negative sign refers to the direction of the force, which is downward. By convention, negative velocity and negative acceleration are downward.

If you set g as positive, then a projectile launched upwards would have negative velocity, which doesn't make intuitive sense.

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- TomLv 59 years ago
YOU DONT HAVE TO!

Its YOUR convention that YOU choose to answer your question,

e.g. let vertical velocity be v = u + at

If I choose a = -g then v = u - gt where +ve motion is upward, however,

Where I to choose +ve motion as downwards (why not) then

v = u + gt

(remembering that a falling body here would have +ve velocity)

You may encounter both conventions in your studies, and if it is not stated,

it is assumed that down is negative, but you could choose either to suit your problem.

Markers usually are aware of this and you should never be penalized as long as you assert the convention you use.

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- 7 years ago
In the two dimensional is directed in the upward direction and the accelaration due to gravity is always directed towards the centre of earth in downward diretion. As both have opposite direction so we use negative sign for g.

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- Anonymous9 years ago
G is the constant gravitational force put upon everything on earth. The reason its negative is to decelerate the object in motion till it comes to a stop without it toy rockets would never come down and sky diving would be impossible.

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- Anonymous9 years ago
G is the force of gravity. Gravity goes "down" as we perceive it. So, launching something "up," always encounters the difficulty of it being pulled down, and we assign this downward force a negative value.

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- 6 years ago
Its because when projectile motion is in upward direction ,it is opposite to gravitational force therefore it is take as g negative and when it comes down back, it is along -y axis therefore it remains -g

Thanks-

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- 9 years ago
the negative and positive signs are just directions. In physics, it is a norm that the direction "upward" or toward the atmosphere is positive, and the direction "downward" or toward the ground is negative

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- 9 years ago
The signs are only due to your choice in coordinates. You can chose different coordinates and make your gravity positive or negative.

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- Anonymous9 years ago
In most Cartesion graphs, going up is an increase in the value of y and going down is a decrease in the value of y.

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