Why Does the Sign of Acceleration Affect My Physics Calculation?

[new^2^physics]

Homework Statement

A ball is tossed from an upper story window of a building. The ball is given an initial velocity of 8.00 m/s at an angle of 20.0o below the horizon. It strikest he ground at 3.00s later.

Calculate the height from which the ball was released.

The Attempt at a Solution

OKAY, I have solved this question but I AM SO CONFUSED!

y_f = y_i + y_iyt + 1/2 Ayt²

BECAUSE gravitational acceleration has a down direction regardless of whether its rising or falling. The Yaxis has an up direction, thus A=-9.80 m/s2

THUS

0=yi + (2.74)(3.00)+1/2(-9.80)(3²) = 35.88 m BUT THIS IS WRONG!
Because...
0=yi + (2.74)(3.00)+1/2(+9.80)(3²) = 52.3 m

OH my goodness gracious! Please oh please someone tell me why...

AND SPEAK To me in normal english not physics english please... I am new to physics and appreciate EASY english.

THANK YOU SO MUCH!

 

[Avathacis]

Homework Statement

A ball is tossed from an upper story window of a building. The ball is given an initial velocity of 8.00 m/s at an angle of 20.0o below the horizon. It strikest he ground at 3.00s later.

Calculate the height from which the ball was released.

The Attempt at a Solution

OKAY, I have solved this question but I AM SO CONFUSED!

y_f = y_i + y_iyt + 1/2 Ayt²

BECAUSE gravitational acceleration has a down direction regardless of whether its rising or falling. The Yaxis has an up direction, thus A=-9.80 m/s2

THUS

0=yi + (2.74)(3.00)+1/2(-9.80)(3²) = 35.88 m BUT THIS IS WRONG!
Because...
0=yi + (2.74)(3.00)+1/2(+9.80)(3²) = 52.3 m

OH my goodness gracious! Please oh please someone tell me why...

AND SPEAK To me in normal english not physics english please... I am new to physics and appreciate EASY english.

THANK YOU SO MUCH!

Is it stated that the Y axis has an upward direction? Otherwise just choose it to be downward and you've solved the problem.

 

[danago]

It doesn't matter too much whether you make it positive or negative; what matters is that you are consistent with your labeling system across all quantities. If any two quantities are in the same direction, they must have the same sign (whether positive or negative). If they are in the opposite direction, they should have opposite signs. Before you start the problem, choose either the upwards or downwards direction to be positive and stick with it.

For example, in this question the initial vertical velocity is downwards -- the same direction as gravitational acceleration -- and therefore should have the same sign as the acceleration. The solution has decided to make downwards positive (hence the positive acceleration and initial vertical velocity), but you could just as easily make them both negative if you prefer.

 

[new^2^physics]

Okay, so just to make sure I 100% understand...

in this question the initial vertical velocity is downward thus the acceleration must have the same sign.

BUT...

If this question was a hint different, and rather than throwing the ball from an upper story window down, it started from the ground and arced before hitting the ground, the initial vertical velocity would be +, and acceleration would be -

oh dear lord, I am still confused.

THANK YOU

 

[danago]

Okay, so just to make sure I 100% understand...

in this question the initial vertical velocity is downward thus the acceleration must have the same sign.

Correct

BUT...

If this question was a hint different, and rather than throwing the ball from an upper story window down, it started from the ground and arced before hitting the ground, the initial vertical velocity would be +, and acceleration would be -

oh dear lord, I am still confused.

THANK YOU

Also correct.

 

[D H]

THUS

0=yi + (2.74)(3.00)+1/2(-9.80)(3²) = 35.88 m BUT THIS IS WRONG!
Because...
0=yi + (2.74)(3.00)+1/2(+9.80)(3²) = 52.3 m

One has a consistent representation, the other does not. In the first equation you would also have obtained the correct result if you used direction consistently. Note also that the second equation yields a value of -52.3 meters, not 52.3 meters. This negative value is consistent with your designation of downward as positive in the second equation.

Bottom line: Be consistent. All bets are off if you aren't.

OH my goodness gracious! Please oh please someone tell me why...

AND SPEAK To me in normal english not physics english please... I am new to physics and appreciate EASY english.

Right back at ya: In the future, please write your posts in normal English. Don't use text speech. Don't beg and plead. Don't use all caps. Don't use ellipses (the "..." is an ellipsis). One exclamation point is all you need, and often one exclamation point is one too many.

 

[new^2^physics]

I am sorry DH, I will change the way I post...
Its just that I have shed a million tears from physics, and I am getting frusterated.
I apologize for not posting in normal English