CogKnoSys

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Physical

Kinetic Energy

Kinetic energy is the energy an object has because it is moving, and it depends on the object's mass and speed.

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The idea

Kinetic energy is the energy an object has because it is moving. The word kinetic comes from the Greek word kinesis, which means motion. A rolling ball, a flowing river and a flying bird all have kinetic energy. Scientists group the many forms of energy into two main classes: potential energy and kinetic energy.

The amount of kinetic energy depends on an object's mass as well as its speed. It equals half the mass multiplied by the square of the speed. Because speed is squared, an object that doubles its speed has four times as much kinetic energy. This is why a car traveling twice as fast as another needs four times as much distance to stop, assuming the same braking force.

Kinetic energy can pass from one object to another and change into other kinds of energy. In billiards, the cue ball slows down sharply when it strikes another ball, and the ball it hits speeds away. A cyclist who coasts up a hill turns most of the bike's kinetic energy into gravitational potential energy, which returns as speed on the way down. If the cyclist brakes instead, friction changes the kinetic energy into heat.

Check yourself

Question 1 of 3

If an object doubles its speed, how much kinetic energy does it have?

Pick one. The first pick counts.

Composition

what it's made of

  • Mass — At the same speed, a heavier object carries more kinetic energy.
  • Speed — Kinetic energy grows with the square of an object's speed.
  • Work — The kinetic energy of a moving object equals the work needed to bring it from rest to that speed.
  • Rotation — Spinning objects, such as flywheels, also store kinetic energy.
  • Joule — The SI unit used to measure kinetic energy and every other form of energy.

Connection

how it relates

Under Energy Forms

Related

  1. Frictionstrength 70 of 100

    Friction turns kinetic energy into heat, which is why a sliding object slows down.

  2. Gravitystrength 60 of 100

    Gravity speeds up a falling object, so its kinetic energy grows the longer it falls.

  3. Accelerationstrength 60 of 100

    Accelerating an object to a higher speed takes work, and that work becomes kinetic energy.

  4. Muscle Contractionstrength 40 of 100

    Contracting muscles use chemical energy from food to produce the kinetic energy of a moving body.

Change

how and why it evolves

Past
In 1722, Willem 's Gravesande tested the idea by dropping weights from different heights into a block of clay. In the mid-19th century, William Thomson, later Lord Kelvin, was credited with coining the term kinetic energy.
Now
Today, flywheels have been developed as a way to store energy in spinning motion.
Next
Engineers will likely keep looking for better ways to capture the energy of moving things that would otherwise be wasted.

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

Kinetic Energy

Kinetic energy is the energy an object has because it is moving, and it depends on the object's mass and speed.

Learn, discuss, create, buy or invest

The idea

Kinetic energy is the energy an object has because it is moving. The word kinetic comes from the Greek word kinesis, which means motion. A rolling ball, a flowing river and a flying bird all have kinetic energy. Scientists group the many forms of energy into two main classes: potential energy and kinetic energy.

The amount of kinetic energy depends on an object's mass as well as its speed. It equals half the mass multiplied by the square of the speed. Because speed is squared, an object that doubles its speed has four times as much kinetic energy. This is why a car traveling twice as fast as another needs four times as much distance to stop, assuming the same braking force.

Kinetic energy can pass from one object to another and change into other kinds of energy. In billiards, the cue ball slows down sharply when it strikes another ball, and the ball it hits speeds away. A cyclist who coasts up a hill turns most of the bike's kinetic energy into gravitational potential energy, which returns as speed on the way down. If the cyclist brakes instead, friction changes the kinetic energy into heat.

Check yourself

3 quick questions, instant feedback.