the sign of "g"

[inaccensa]

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I am really getting confused with the signs. This is what i think is correct

g= up = neg
g= down =positive

y=up= positive
y=down= negative

v=up=positive
v=down =negative

When i try to solve questions with constant acceleration (the 4 equations), sometimes I get the correct answer and at times I get them wrong. What is the correct sign convention?

 

[VanBrown]

just think about whether or not something is accelerating or declerating. Is it going faster or slower?

when you throw an object up its speed slows down until it reaches v=0 so it is deccelerating... g=-9.8m/s2. After this it starts to speed up as it falls.. it is accelerating and g=9.8m/s2 up to terminal velocity.

In short... think of whether the object is slowing down or speeding up.

 

[FIREitUP]

are you being consistent with your signs? if g is negative, then down would be negative and the displacement downward would be negative also. the sign you make g is arbitrary, but you must be consistent.

 

[bored]

signs dont matter, just say up is positive and make gravity negative, but if there are any velocities going upwards, then they wil be positive.

 

[vickpick]

velocity is relative to the starting or ending point(+ = going away from start point, "-" = going towards the start point)

acceleration (g) is relative to the change in V (+ if speed increasing due to gravity, - if speed decreases due to gravity.)

 

[inaccensa]

so I feel like I'm getting different answers here. What I understand from one post is that if

displacement UP is taken Positive, then g is also positive & vice-versa.

and someone else confirmed the signs that I had posted, but the problem is that i don't get consistent results with them

 

[kentavr]

There is no ambiguity.
There is one simple principle to get it right.

1. First choose your coordinate axis direction.
(let say for certainty you choose it "UP")

2. Any vector value (velocity, force, accellaration etc) has a sign. If it is in the same direction as your axis then it is PLUS.
(In our example if value directed UP it is Plus assuming your axis is up)

 

[inaccensa]

There is no ambiguity.
There is one simple principle to get it right.

1. First choose your coordinate axis direction.
(let say for certainty you choose it "UP")

2. Any vector value (velocity, force, accellaration etc) has a sign. If it is in the same direction as your axis then it is PLUS.
(In our example if value directed UP it is Plus assuming your axis is up)

so if choose up as positive, then the g, v,y should always be positive?

 

[matth87]

I am really getting confused with the signs. This is what i think is correct

g= up = neg
g= down =positive

y=up= positive
y=down= negative

v=up=positive
v=down =negative

When i try to solve questions with constant acceleration (the 4 equations), sometimes I get the correct answer and at times I get them wrong. What is the correct sign convention?

The correct sign convention is which ever way you denote your coordinate system. There is no one correct positive direction. If you create a x/y coordinate systems and label a direction positive or negative and make sure all you vectors have the correct sign you will always arrive at the correct answer.

Sign is relative to your coordinate system.

 

[Chuck's Right Foot]

so if choose up as positive, then the g, v,y should always be positive?

No.

Let's take your example, that things moving "UP" have a positive velocity.

Because of gravity, they will be "decelerating" so your acceleration due to gravity is approximated at -9.8m/s^2.

Things moving DOWN will still experience a g of -9.8m/s^2 but since they are moving down, their initial velocity is already less than 0.

 

[kentavr]

so if choose up as positive, then the g, v,y should always be positive?

No...

1. If you choose that you axis directed UP. what is the G direction? To the center of the earth. That means it opposite to your axis direction. And that means G has a minus sign.
2. Velocity v - when the ball goes up it is the same direction as your axis and the sign is plus. However, when the ball goes down it is the opposite and the sign is negative.
3. Y - it is your axis (Isn't it?) and if the distance goes up it is positive. If it is going down (underground) it is negative.

 

[inaccensa]

No...

1. If you choose that you axis directed UP. what is the G direction? To the center of the earth. That means it opposite to your axis direction. And that means G has a minus sign.
2. Velocity v - when the ball goes up it is the same direction as your axis and the sign is plus. However, when the ball goes down it is the opposite and the sign is negative.
3. Y - it is your axis (Isn't it?) and if the distance goes up it is positive. If it is going down (underground) it is negative.

That is exactly what I had said the first time. So my initial directions and signs were correct? However, when I solve the problems I don't always end up with the correct answers.

 

[inaccensa]

I am really getting confused with the signs. This is what i think is correct

g= up = neg
g= down =positive

y=up= positive
y=down= negative

v=up=positive
v=down =negative

When i try to solve questions with constant acceleration (the 4 equations), sometimes I get the correct answer and at times I get them wrong. What is the correct sign convention?

This is what i had asked the first time around

 

[Chuck's Right Foot]

That is exactly what I had said the first time. So my initial directions and signs were correct? However, when I solve the problems I don't always end up with the correct answers.

That is not what you said at first. Look at your first post.

Try this:

A ball is thrown at 10 m/s directly upwards from a window 20 meters off the ground. What is its displacement after 2s. What is its velocity after 3s? What value should you use for a? What value is V0? How long does it take the ball to hit the ground?

 

[inaccensa]

That is not what you said at first. Look at your first post.

Try this:

A ball is thrown at 10 m/s directly upwards from a window 20 meters off the ground. What is its displacement after 2s. What is its velocity after 3s? What value should you use for a? What value is V0? How long does it take the ball to hit the ground?

So taking up for v and Y as positive and G is neg.
dY= vt+.5at^2
= 10x2 +.5 (-10)x4
= 0
vf=vo+at
= 10 - 10*2
=-10m/s

 

[Chuck's Right Foot]

So taking up for v and Y as positive and G is neg.
dY= vt+.5at^2
= 10x2 +.5 (-10)x4
= 0
vf=vo+at
= 10 - 10*2
=-10m/s

So far so good. How long does it take to hit the ground.

 

[inaccensa]

So far so good. How long does it take to hit the ground.

So the V0 =10m/s, at max height, Vf =0m/s

Vf =Vo +at
0 = 10-10*t
-10= -10 *t
t =1sec
it takes 2 secs

 

[Chuck's Right Foot]

It was thrown from above ground level. Try again.