Acceleration

Acceleration is the change in how fast something is moving, or the change in the direction it is moving. In science, anytime an object speeds up, slows down, or turns, it is accelerating. A car leaving a stoplight accelerates. So does a runner crossing the finish line and slowing to a stop. A ball rolling around a curved track is accelerating too, even if its speed stays the same, because its direction is changing.

This is different from how people use the word in everyday life. Most people think acceleration just means speeding up. In physics, slowing down is also acceleration. Scientists sometimes call it negative acceleration or deceleration, but it follows the same rules.

Acceleration is measured in units of speed per unit of time. One common unit is "miles per hour per second." If a car speeds up by 10 miles per hour each second, its acceleration is 10 miles per hour per second. Scientists more often use meters per second squared. Either way, the idea is the same: how much the speed changes, and how quickly.

Acceleration happens because of force. A force is a push or a pull. Isaac Newton figured this out in the 1600s. His second law of motion says that a bigger force makes a bigger acceleration, and a heavier object is harder to accelerate. Pushing a shopping cart is easy. Pushing a car with the same strength barely moves it at all.

Gravity is one of the most common causes of acceleration. When you drop a ball, gravity pulls it down, and it falls faster and faster. Near Earth's surface, a falling object speeds up by about 22 miles per hour every second it falls. That is why jumping off a high place is so dangerous. After just a few seconds, you are moving very fast.

You can feel acceleration in your body. When a car speeds up, you feel pressed back into the seat. When it brakes hard, you feel thrown forward. On a roller coaster, the sudden changes in speed and direction are what make your stomach flip. Astronauts in a rocket feel crushed into their seats during launch because the rocket is accelerating so hard.

A steady speed is boring to your body. You cannot feel it, which is why you do not notice Earth spinning under your feet. Acceleration is what you notice, because acceleration is what change feels like.