# Acceleration

**Acceleration**is change of the velocity of an object.

*Velocity*is the speed and direction of the object. When an object changes speed or direction, it accelerates. When this change occurs quickly, the object accelerates quickly.

Acceleration is important in physics. Mathematically, acceleration is the derivative, or rate of change, of velocity. The force something needs to accelerate an object depends on the object's mass: the amount of "stuff" the object is made from.

Sometimes, people think of an object, like a car, as moving mostly in one direction. Then, they may only say the car accelerates if its speed increases. If the speed decreases, they may say it *decelerates*. Physically, speeding up and slowing down are both acceleration.

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