Acceleration
Acceleration is the rate of change of velocity. It is a vector. Uniform acceleration means constant rate of change of velocity (e.g. free fall near Earth).
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Definition of acceleration
Acceleration is the rate at which velocity changes with time. Average acceleration = change in velocity / time taken. It is a vector. The SI unit is m/s². When velocity increases, acceleration is in the direction of motion; when velocity decreases (e.g. braking), acceleration is opposite to the direction of motion.
Uniform acceleration
When the rate of change of velocity is constant, we have uniform (constant) acceleration. Free fall near the Earth's surface is approximately uniform acceleration: a = g ≈ 9.8 m/s² downward. The equations of motion (v = u + at, s = ut + ½at², etc.) apply when acceleration is constant.
Deceleration
Deceleration is acceleration that opposes the direction of motion—the object slows down. It is still described by the same vector quantity; we often say "deceleration" when the speed is decreasing. In one dimension, deceleration corresponds to acceleration with sign opposite to velocity.
When you throw a ball upward, what is the direction of its acceleration at the highest point? What about its velocity?