Acceleration due to gravity (positive or negative)

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Lambert87
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Hi,

Can anyone help? The teacher said today that acceleration due to gravity was -9.81, and because it's gravity it's negative.

But some of the questions gravity (g) is positive?

Confused. Can someone shed a light on this?
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Deranging
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The acceleration due to gravity is really a vector quantity, so basically comes with a magnitude and a direction. So essentially what it boils down to is how you are measuring it. If you are measuring the acceleration due to gravity in an upwards direction then it is roughly  -9.81 \text{ms}^{-1} , while if you are measuring the magnitude of the acceleration due to gravity or measuring it in a downwards direction it is roughly  9.81 \text{ms}^{-1} on the surface of the earth.

Often it is convenient to take the 'up' direction as being positive, but this is only a choice any other choice is equally valid as long as you are careful and consistent in your approach.
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Physics Enemy
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(Original post by Lambert87)
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Gravitational forces (F = GMm/r^2) are +ve by convention, thus g is too as F and 'a' it causes point in the same direction (F = ma).

Note: g is gravitational acceleration on earth's surface; g = 9.81 ms^-2 (3 s.f).

But if g opposes motion (deceleration or reducing acceleration), it's preferable to define g as -ve and direction of motion as +ve.

So with vectors e.g) F and 'a', we can define +ve and -ve directions as it suits, as long as we are consistent. Scalars are always +ve tho.
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Anonymouspsych
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(Original post by Lambert87)
Hi,

Can anyone help? The teacher said today that acceleration due to gravity was -9.81, and because it's gravity it's negative.

But some of the questions gravity (g) is positive?

Confused. Can someone shed a light on this?
Right, so it is important to be aware of the fact that this is a quantity denoting acceleration and acceleration is a vector quantity; that is it has both magnitude and a specified direction. The reason your teacher said it is negative is because she has taken upwards from the surface to be positive and since g causes things to accelerate towards the centre of the earth, this is the opposite direction making it negative. However if you defined the downwards direction to be positive, you could just as easily say g is 9.81 instead.

This normally isn't an issue with simple calculations but when dealing with problems such as projectile motion it is important which direction (upwards and downwards) you define as being positive and negative.
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