# Is the acceleration due to gravity positive or negative

1. Sep 28, 2008

### holtvg

I know that purely the acceleration due to gravity is positive because it accelerates things not decelerates or negative acceleration. But in some cases it can be negative but i'm not sure in which cases. For example if i'm just standing here on the ground is gravity working in a positive or negative direction.

2. Sep 28, 2008

### Staff: Mentor

Gravity always is down. You can choose your coordinate system so that down is positive or so that up is positive. If up is positive then, since gravity points down, gravity would be in the negative direction.

3. Sep 28, 2008

### rcgldr

Gravity is an attractive force. The earth applies a downwards force on you, and you apply an equal and upwards force on the earth. Gravity doesn't violate Newtons 3rd law, which states that all forces (betwen objects) only exists in equal and opposing pairs.

Acceleration only occurs if there are no equal and opposing forces to gravity.

Normally the sign of acceleration due to gravity of earth on an object near the earth is negative, since the direction of altitude or height is normally upwards (outwards) relative to the earth.

4. Sep 29, 2008

### holtvg

If a ball is thrown up while it's going up gravity is negative but when it's going down it's positive, so it can be either positive or negative right? But what also confuses me as when it's is seen as negative i see it as decelerating an object when it's supposed to accelerate it.

5. Sep 29, 2008

### atyy

The important thing is whether gravity and the velocity have the same or opposite signs. Suppose you choose the up direction as negative. Then gravity is always positive. When a ball is going up, it is going negative, in the opposite direction of gravity, so it will slow down. When the ball is going down, it is going positive, in the the same direction as gravity, so it will speed up.

6. Sep 29, 2008

### Redbelly98

Staff Emeritus
If upward=positive, then gravity is negative always ... whether the ball is going up or down.

7. Sep 29, 2008

### holtvg

Guess it will be negative in my case cause up is positive and down is negative and it's pulling down. But what is really important during 2 dimensional motion is if the velocity vector is in the same direction as gravity then gravity is positive, if the velocity vector is in the opposite direction of gravity then gravity is negative on the y-axis

8. Sep 29, 2008

### Staff: Mentor

If up is positive then gravity is always negative. The direction of the velocity vector is not relevant to determining the direction of gravity.

9. Sep 30, 2008

### Redbelly98

Staff Emeritus
If the ball is falling at, for example, a speed of 10.0 m/s, then it's velocity is
v = -10.0 m/s.

Since g = -9.8 m/s2, then 1 second later we have
v = -19.8 m/s

Note that -19.8 is less than -10.0 ... so velocity has decreased, even though the speed has increased from 10.0 to 19.8 m/s.

Hope that helps clear things up.

10. Feb 6, 2011

### enap66

Here is something that may help you all discern between when to use a (-) or (+) sign in front of acceleration. Basically, I didn't quite get it 100% until I started using this. I use it as follows:

Think of velocity as directional. Acceleration is NOT directional. So if you assign a (+) to velocity when it goes up or to the right, then a (-) would be assigned to down and the left. Let's do it that way. Now, open the attachment. Ok? Let's take a look. To figure out the acceleration, use the chart and go by velocity and motion. Therefore,

If velocity is (-) (down or to the left) and x (motion) is speeding up, then acceleration will have a (-) sign in front of it.
If velocity is (+) (up or to the right) and x (motion) is slowing down, then acceleration will have a (-) sign in front of it.

Get it?

Try it on your own. It's quite simple once you've got this down.

One more... if velocity is (-) and the x (motion) is slowing down, then acceleration is (+). Nice, eh?

One more thing... I find it easier to use the vf= vo + a(t) formula with these (where a= gravity or g). If you go with the equation that holds g as a (-), it throws it off.

Good luck!

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11. Feb 6, 2011

### Naty1

Using up and down is many times not that useful: Consider the gravitational attraction between two planets, each with an inhabitant asking your question....and say one throws a ball up in the air.....he sees it deccelerating away from him while she sees it decelerating towards her on the distant planet.

Another way to get a perspective on "positive" and "negative" is to realize that gravitational potential is taken to be zero at infinite separation and negative when objects are closer...in other words, positive work must be done to separate the objects. You have to do positive work to move a balls upwards. This is in fact the convention used in physics and is why it is said that the total energy in the universe is zero...gravity offsets everything else.