# For projectile motion, for example if i set my coordinate system as positive so that means that the acceleration and the max height (dy) is going to be positive?

##### 1 Answer

Here's how you can think about this.

#### Explanation:

Now, let's say that you launch a projectile **straight up** and that it reaches a maximum height of **100 meters**.

You can break down its movement into two parts

*moving up towards maximum height**falling from maximum height*

If you take the **upward direction** to be **positive** and the ground level to be **sero**, then the displacement of the projectile will be

*positive on its way up*, since it goes from ground level to 100m in the assigned**positive direction**;*negative on its way down*, since now it is moving from 100m to ground level;

What about the gravitational acceleration? You know that gravity is **always** pulling objects towards the ground. If the **upward direction** remains the positive direction, then

*negative while the projectile moves towards maximum height, i.e. as it climbs*, since#g# is a vector directed**towards**the surface of the Earth and the projectile is moving in the opposite direction;*positive while the projectile is falling from maximum height*, since now the direction of gravity is the same as the direction of the projectile.

So, in this example, if you take

*on the projectile's way up*

#+"100 m" = v_0 * t + 1/2 * (-g) * t^2#

**Positive displacement** and **negative gravitational acceleration**.

*on the projectile's way down*

**Negative displacement** and **positive gravitational acceleration**.

So remember,