You've probably had days when you felt tired. You may even
have said, "I don't have much energy today." But what exactly is
energy? Simply defined, energy is the ability to do work.
Energy can be categorized as either potential energy or kinetic
energy. Potential energy is stored energy, like the energy
stored in a battery, or the energy of position, like a boulder
about to roll down a mountain. Kinetic energy is the energy of
motion.
To understand how potential and kinetic energy are related,
consider a car. The gasoline in the car's tank has potential
energy. For the car to move, the car's engine
transforms the potential energy of the gasoline to
kinetic energy: the motion of the car.
What are some other examples of motion in everyday
life? Here are a few: We walk or run from place to
place. Water in a river flows downstream. Sound
waves travel to our ears. All these examples of motion
are examples of kinetic energy. Kinetic energy can
occur on a large scale, like a jet lifting into the air.
Kinetic energy can also occur on a microscopic scale,
like the movement of molecules in a glass of water.
There are different forms of kinetic energy. One form of kinetic
energy is radiant energy, also called light energy. Radiant
energy is energy that travels in waves of light. Some of this light
we see, like the light that travels from the sun to Earth. Other
forms of radiant energy, like radio waves, are invisible to human
eyes.
Another form of kinetic energy is thermal energy. Thermal
energy is also known as heat. Heat is produced when molecules
vibrate and collide with each other or with the sides of a
container. Heat is often a by-product of motion. For example,
think about a car moving along a road. The motion of the car
tires along the surface of the road produces heat that can be felt
in the tires.
A third form of kinetic energy is electrical
energy. Electrical energy comes from the
movement of charged particles. Electrical energy
can be produced in nature, like lightning strikes
during a thunderstorm. Humans can also produce
electrical energy, using machines. We use
electricity in our homes, where wires carry
electrical charges to lightbulbs and appliances.
Electrical energy can be transformed into other
forms of energy, too. For example, when you flip a
light switch, electrical energy is transformed into
radiant energy that lights the room. After a short
time, the lightbulb becomes very hot as the
vibrating molecules in the bulb result in thermal
energy.
A fourth form of kinetic energy is sound energy. Like radiant
energy, sound energy also travels in waves that consist of
particles in motion. When a sound is produced, the waves travel
through air (or another type of matter) until they reach our ears.
Our ears convert the motion of the sound waves into electrical
impulses that then pass on to our brain. The nature of the
sounds we hear depends on the characteristics of the waves.
Some sounds, like very high-pitched whistles, are beyond the
range of human hearing.
When you consider all these different forms of kinetic energy,
one fact becomes clear: Our lives are full of motion!