# Question about acceleration for rocket launch and return (1 Viewer)

#### Karldahemster

##### Student
So when a rocket is launched;

a=T-mg/m

You sub in g=+9.8ms^-2

However, when the rocket is returning to earth, do you still use +9.8ms^-2 or -9.8ms^-2?

Thanks.

-9.8ms^2

#### Drdusk

##### π
Moderator
That was a year ago....

In any case I think it's better to write it as:

$\bg_white \sum F = T + mg \Rightarrow a = \frac{T + mg}{m}$

because Newtons second law states that the SUM of forces add to ma. It is therefore better to write it like that as then when the rocket is going up, gravity acts against the motion which means g = -9.7, and when it's coming back gravity acts with the rocket, giving g = 9.7. This reduces confusion as all you need to look at is if gravity is acting against or for the projectile/rocket...

It is kinda weird and confusing to write g as -9.8 when it's not acting against the rocket when it's coming back, it's much easier to make a silly mistake doing it like that.