When should the negative sign be included in acceleration for F=ma?

[brentd49]

Would someone explain to me when to include the negative into the acceleration a, for F=ma. I know if it is a vector you don't include the sign. But sometimes it seems almost arbitrary in textbooks when they include the negative sign and when they don't. I hope you know what I mean. I know if it is speeding up in the negative direction or slowing down in the positive direction then 'a' is negative, but sometimes it is not given a negative sign. Am I just missing something?

 

[ZapperZ]

Please check the Math and Science tutorial forum:

https://www.physicsforums.com/showthread.php?t=93670

Zz.

 

[brentd49]

Okay. I see my confusion now. One only puts the sign based on the coordinate system, but the actuall final result will be negative if the system is accelerating in negative direction or deaccelerating in positive direction.