acceleration

When something changes speed, e.g. a car on a roller coaster, it is said to be accelerating. When the speed of a car increases on a downhill section, it accelerates. When its speed decreases on an uphill section, the car is said to have negative acceleration, called deceleration.

The ground accelerates when it shakes during an earthquake as it speeds up and slows down. The movement of the ground can cause buildings to sway. A building will accelerate as it changes its speed while moving from side to side.