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Physical

Acceleration

Acceleration is the rate at which an object's velocity changes, whether it speeds up, slows down or turns.

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The idea

Acceleration is a measure of how fast an object's velocity is changing. Velocity means speed in a certain direction, so an object accelerates whenever it speeds up, slows down or changes direction. Like velocity, acceleration has both a size and a direction, which makes it a vector quantity. Scientists measure it in meters per second squared, which tells how many meters per second the speed changes during each second.

Acceleration can point different ways. When a car speeds up along a straight road, its acceleration points the same way it is moving. When the driver brakes, the acceleration points backward, and this is called deceleration. When a car rounds a curve at a steady speed, it is still accelerating, because its direction is changing. That sideways acceleration toward the center of the curve is called centripetal acceleration.

Forces cause acceleration. Newton's second law says that an object's acceleration is proportional to the net force acting on it and inversely proportional to its mass. In plain words, a bigger push gives a bigger acceleration, and a heavier object speeds up less from the same push. The acceleration also points in the same direction as the net force. This is why a loaded truck needs a more powerful engine than a small car to pick up speed as quickly.

Check yourself

Question 1 of 2

What is acceleration that points opposite to an object's direction of motion called?

Pick one. The first pick counts.

Composition

what it's made of

  • Change in velocity — Acceleration is the rate of change of velocity: speeding up, slowing down or turning.
  • Direction — It has a size and a direction, so it is a vector quantity.
  • Net force — The overall force on an object causes its acceleration and sets its direction.
  • Mass — For the same force, an object with more mass accelerates less.
  • Unit — It is measured in meters per second squared.

Connection

how it relates

Under Motion

Related

  1. Muscle Contractionstrength 60 of 100

    Contracting muscles create the forces that accelerate the body when it starts running or jumps.

  2. Energy Formsstrength 50 of 100

    Speeding an object up takes energy, which the object carries as kinetic energy.

  3. Water Cyclestrength 30 of 100

    Falling raindrops accelerate under gravity until air resistance balances their weight.

Change

how and why it evolves

Past
Galileo Galilei showed that falling objects speed up at a steady rate, and Isaac Newton later explained acceleration as the result of forces.
Now
Today small sensors called accelerometers measure acceleration in phones, cars, game controllers and aircraft.
Next
As these sensors grow cheaper and more precise, acceleration data will likely play a growing role in vehicle safety, robotics and health tracking.

Knowledge you can walk. Read the idea, then step up, across or down.

Acceleration

Acceleration is the rate at which an object's velocity changes, whether it speeds up, slows down or turns.

Learn, discuss, create, buy or invest

The idea

Acceleration is a measure of how fast an object's velocity is changing. Velocity means speed in a certain direction, so an object accelerates whenever it speeds up, slows down or changes direction. Like velocity, acceleration has both a size and a direction, which makes it a vector quantity. Scientists measure it in meters per second squared, which tells how many meters per second the speed changes during each second.

Acceleration can point different ways. When a car speeds up along a straight road, its acceleration points the same way it is moving. When the driver brakes, the acceleration points backward, and this is called deceleration. When a car rounds a curve at a steady speed, it is still accelerating, because its direction is changing. That sideways acceleration toward the center of the curve is called centripetal acceleration.

Forces cause acceleration. Newton's second law says that an object's acceleration is proportional to the net force acting on it and inversely proportional to its mass. In plain words, a bigger push gives a bigger acceleration, and a heavier object speeds up less from the same push. The acceleration also points in the same direction as the net force. This is why a loaded truck needs a more powerful engine than a small car to pick up speed as quickly.

Check yourself

2 quick questions, instant feedback.