- #1

Hermes Chirino

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For the x-direction, they use as premise:

$$ma_x=m\frac{dv}{dt}=-kmv_x$$

I understand the drag force $-kmv$ is equal in magnitude to the force $ma$, but opposite direction.

For the y-direction, they use as premise:

$$F_T=m\frac{dv}{dt}=-mg-kmv_y$$

I understand the total force is equal to the force of gravity $mg$, minus the drag force pointing in the opposite direction.

My question here is: Why they don't use a similar premise for the x-direction, something like:

$$F_T=ma_x-kmv_x$$

What is the diference ? What I am missing here ?