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# Why is acceleration due to gravity negative?

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Acceleration due to gravity is generally considered to be negative because the force of gravity pulls in the downward direction. The frame of reference from Earth's surface most often used in physics has the sky in the upward direction and the ground in the downward direction.

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The acceleration due to gravity on Earth rounded to the nearest hundredth is -9.81 meters per second squared. This means that if no forces other than that of gravity are present, then for every second an object is falling, its velocity towards Earth increases by 9.81 meters per second. Negative acceleration does not necessarily mean that the object is slowing down. Speed is the magnitude of how fast an object is moving, while velocity is the magnitude and the direction of the movement. If the acceleration and velocity are in the same direction, then the speed is increasing, meaning that the object is speeding up. If acceleration is in the opposite direction of velocity, then the speed is decreasing and the object is slowing down. In any frame of reference in physics, mathematics or other uses, down and left are generally considered the negative directions, while up and right are generally considered to be positive. Frames of reference are most often used in physics; they keep positive and negative directions universal during the analysis of a problem.

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G-force refers to the force of gravity, equal to a value of 1 for a body at rest at sea level, but which can be increased significantly through vertical or horizontal acceleration. An example of g-force increasing through acceleration is the effect felt by riders on a roller coaster during its downward plunge. The amount of accelerated g-force that a human body can endure is limited, and at levels beginning between 9 and 18 g, it can cause a loss of consciousness.

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A pulley works by changing the direction that force must be applied to lift an object against gravity. Other, more complex, systems use multiple pulleys to reduce the amount of force that must be applied.

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An object's acceleration is constant and downward due to the force of gravity In projectile motion, but the object's velocity reverses direction during the object's motion. This is because of the inertia described in Newton's first law of motion and because acceleration and velocity are two different physical quantities. Velocity is the speed and direction of an object, while acceleration is the rate that the velocity changes.