It's a matter of convention:
* Upward Direction as Positive: If you choose "up" as the positive direction for your y-axis, then the force of gravity acts downward, making it negative. This is the most common convention in physics.
* Downward Direction as Positive: If you choose "down" as the positive direction for your y-axis, then the force of gravity acts downward, making it positive.
Why it matters:
* Sign Consistency: The sign of the force of gravity must be consistent with the signs of other variables in your equations (like acceleration due to gravity, displacement, and velocity).
* Clarity in Calculations: Choosing a clear coordinate system and sticking to it throughout your calculations helps avoid confusion and errors.
Example:
Let's say you throw a ball straight up in the air.
* Upward Positive:
* Displacement: Positive when the ball is above the launch point, negative when it's below.
* Velocity: Positive when the ball is moving upwards, negative when it's moving downwards.
* Acceleration (due to gravity): Negative, always acting downwards.
* Downward Positive:
* Displacement: Positive when the ball is below the launch point, negative when it's above.
* Velocity: Positive when the ball is moving downwards, negative when it's moving upwards.
* Acceleration (due to gravity): Positive, always acting downwards.
Important Note: The direction of the force of gravity itself remains constant (always downwards), but its sign in your calculations depends on your chosen coordinate system.
