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ZingPath: Mechanical Energy

Kinetic Energy: How It Changes with Mass and Speed

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Kinetic Energy: How It Changes with Mass and Speed

Physical Science

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Learners investigate factors that influence the kinetic energy of an object.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that kinetic energy is the energy of an object due to its motion.
  • Explain that the kinetic energy of an object is directly proportional to its speed.
  • Explain that the kinetic energy of an object is directly proportional to its mass.

Everything You'll Have Covered

Energy can be described as the ability to move objects or cause change. The SI unit for energy is the joule (J), named for James Joule, who showed that work can be converted to heat. The joule is the work done by a force of one newton (N) acting through a distance of one meter. Examples of types of energy include chemical, electrical, kinetic, light, magnetic, nuclear, potential, thermal, sound, and wind. Three types of potential energy are elastic, chemical, and gravitational. The total amount of potential and kinetic energy in a system is called mechanical energy. Kinetic energy is the energy an object possesses due to its motion and can be represented mathematically with the following equation: KE = (mv2), where KE is kinetic energy, m is mass, and v is speed.

The first law of thermodynamics or the law of conservation of energy states that energy cannot be created or destroyed. Energy simply transforms from one form to another. For example, a book on a shelf has gravitational potential energy. If it falls off the shelf, it gains kinetic energy. As it gets closer to the ground, the potential energy decreases, and it moves faster, so the kinetic energy increases. Though the form of energy transforms, the amount of energy remains the same. Even if energy seems to disappear, it does not. If, for example, a skateboard rider is coasting on a flat road, he will eventually come to a stop. It may seem that the mechanical (kinetic and potential) energy of the rider has been destroyed, but that would violate the law of conservation of energy. Instead, friction and air resistance cause some of the mechanical energy to convert to thermal energy. As the wheels roll along the pavement, the temperature of the ball bearings inside the wheels and the temperature of the surface of the wheels increase slightly. The total amount of energy stays the same as mechanical energy is transformed to thermal energy. On a larger scale, the law of conservation of energy indicates that the all energy in the universe remains constant. It should be noted, however, Albert Einstein's theory of relativity demonstrates that energy can be converted to rest mass, and mass can be converted to energy. In his classic equation, Einstein explained that E = mc2, which means that energy equals the mass of an object multiplied by the speed of light squared. Thus, neither mass nor pure energy are conserved separately, as it was understood in pre-relativistic physics. Scientists currently refer to the conservation of total mass-energy, which includes energy of the rest mass.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts acceleration, mass, potential energy
Course Physical Science
Type of Tutorial Experiment
Key Vocabulary energy, experiment, height