The acceleration in projectile motion can be positive, but it depends entirely on the coordinate system you choose. It's usually negative.
Here's a breakdown:
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Standard Convention (Negative Acceleration): Typically, in physics, we define the upward direction as positive and the downward direction as negative. Since gravity pulls objects downwards, the acceleration due to gravity is conventionally represented as negative (approximately -9.8 m/s²). This means the object is accelerating downwards.
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Alternative Convention (Positive Acceleration): You can define the downward direction as positive. In this case, the acceleration due to gravity would be positive (approximately +9.8 m/s²). The important thing is consistency: if down is positive, then any downward velocity would also be positive.
Key Considerations
- Gravity's Direction: Gravity always acts downwards, pulling the projectile towards the Earth.
- Coordinate System: The sign of acceleration depends on how you define your coordinate system.
- Horizontal Acceleration: The horizontal acceleration in ideal projectile motion (ignoring air resistance) is always zero (ax = 0) because there's no horizontal force acting on the projectile.
Example
Let's say a ball is thrown upwards:
- Standard Convention (Up is positive):
- Initial Velocity: Positive (upward)
- Acceleration: Negative (downward due to gravity)
- Alternative Convention (Down is positive):
- Initial Velocity: Negative (upward, but upward is now negative)
- Acceleration: Positive (downward due to gravity)
In both cases, the physics remains the same. The ball slows down as it moves upward due to the acceleration acting in the opposite direction, and then speeds up as it falls downwards. The sign is just a matter of how you choose to represent the direction.
