No, acceleration is not always negative in free fall. The sign of acceleration depends on the chosen coordinate system.
Explanation:
The key to understanding this is realizing that "negative" and "positive" are relative to a chosen coordinate system. In physics, we often define a coordinate system where upward is positive and downward is negative. However, you could just as easily define downward as positive and upward as negative.
Coordinate System Matters
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Downward as Negative: If we define the downward direction as negative, then the acceleration due to gravity (which acts downwards) is negative (-9.8 m/s²). In this case, if an object is thrown upward (positive velocity), the negative acceleration will slow it down. If an object is falling downward (negative velocity), the negative acceleration will make it go even faster in the negative direction.
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Downward as Positive: If we define the downward direction as positive, then the acceleration due to gravity becomes positive (+9.8 m/s²). If an object is thrown upward (negative velocity), the positive acceleration will still slow it down. If an object is falling downward (positive velocity), the positive acceleration will make it go even faster in the positive direction.
Key Takeaway
What's important isn't the sign of the acceleration itself, but the relationship between the sign of the acceleration and the sign of the velocity.
- If acceleration and velocity have the same sign, the object speeds up.
- If acceleration and velocity have opposite signs, the object slows down.
Example:
Imagine dropping a ball. Let's use both coordinate systems:
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Downward Negative: Velocity is negative (getting more negative). Acceleration is negative. Same sign; the ball speeds up in the negative (downward) direction.
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Downward Positive: Velocity is positive (getting more positive). Acceleration is positive. Same sign; the ball speeds up in the positive (downward) direction.
In both scenarios, the physical outcome is the same: the ball falls faster and faster.
In summary, the acceleration due to gravity is always directed downwards. Whether you represent that with a positive or negative number depends solely on the convention you choose for your coordinate system.
