The amount of kinetic energy of an object depends on its mass and speed. Consider two balls of different masses rolling down an inclined plane at the same speed. The heavier ball will have more kinetic energy. Similarly, when two balls of the same mass roll down an inclined plane at different speeds, the ball that moves faster has more kinetic energy.

There are several different forms of kinetic energy, including mechanical, electrical, radiant, and thermal energy. Mechanical energy is associated with the motion of an object. The faster an object moves, the more mechanical energy it has. For example, a bullet fired from a gun or water flowing down a dam are examples of mechanical energy. Electrical energy is attributed to the flow of electric charges, as observed in the case of lightning strikes during thunderstorms or in electrical circuits and devices. Radiant energy is a form of kinetic energy that travels as electromagnetic waves, and can be experienced in the form of light and heat. Sunlight is an example of radiant energy.

Thermal energy is associated with the random motion of atoms and molecules. Temperature is a measure of the average kinetic energy of the particles in an object. When the atoms and molecules in an object move or vibrate quickly, they have a higher average kinetic energy, and the object has a higher temperature. When the atoms and molecules are moving slowly, they have lower average kinetic energy, and the object has a lower temperature. Thus, thermal energy can be observed through the temperature changes of an object. Assuming that no chemical reaction or phase change (such as melting or vaporizing) occurs, increasing the amount of thermal energy in a sample of matter will cause its temperature to increase. Likewise, decreasing the amount of thermal energy in a sample of matter will cause its temperature to decrease.

This text is adapted from Openstax, Chemistry 2e, Section 5.1: Energy Basics.