Acceleration

Speed tells you how fast an object moves; acceleration tells you how fast its velocity changes. Velocity is speed with a direction, so acceleration covers three events: speeding up, slowing down, and changing direction. A car braking hard and a car rounding a bend at steady speed are both accelerating, because their velocity is changing in both cases. Only steady motion in a straight line has zero acceleration.

Slowing has its own everyday word, deceleration, but physicists call it negative acceleration, since the change in velocity points backward. Turning counts as acceleration even when the speedometer never moves, because the direction of the velocity has changed. Test any claim about acceleration with one question: is the velocity changing? If yes, the object is accelerating. If no, it is not.

The formula is small: acceleration equals the change in velocity divided by the time it took. A bike that goes from rest to 8 m/s in 4 seconds accelerates at 8 divided by 4, or 2 meters per second squared. Read it aloud: two meters per second, every second. Its speed climbs by two meters per second each second it keeps pushing. Always find the change in velocity first, then divide by the time.

And watch the sign. If a car goes from 20 m/s to 10 m/s in a few seconds, the change is negative, a back-pointing change, so the acceleration points backward too. Small accelerations build fast: 2 m/s^2 sounds gentle, but after 10 seconds a bicycle is moving at 20 m/s. Big accelerations, like a jet at takeoff, are what press you into the seat.