Energy Although "work" and "energy" are two words commonly used in conversation, they can mean different things depending upon the circumstances. For example, digging weeds in the garden and balancing a checkbook are both "work", but the labor involved in each is quite different. In physics work has a very specific meaning: when a force is applied to an object in such a way that the object moves in the direction of the force, we say work is done on the object. Thus, pulling a wagon along the ground is doing work on the wagon (it moves in the direction it is pulled) but holding the wagon still on a hillside is not (the wagon doesn't move).
Energy is sometimes defined as the ability to do work. For example, a truck moving along the road can smash into a trash can and move the can several meters along the road- the truck applies a force that does work on the trash can. All moving objects have this kind of energy, which we call kinetic energy, the energy of motion.
The SI (Metric) unit of energy is the joule (j), named after British physicist James Joule. A baseball traveling 30 meters/second (about 60 miles per hour) has around 110 joules of kinetic energy. A 1000 kg (2200 pound) car traveling at the same speed has 450 000 joules of kinetic energy.
Energy takes many other forms. For example, the energy stored in a tank of gasoline or the energy of a raised hammer. These are both forms of potential energy. Under the right circumstances, this energy can be released to do work.
An important law of Physics is the Conservation of Energy which states that energy can change from one form to another, but the total amount of energy in a system remains constant. A 1 kg rock lifted 0.5 meters above the ground has about 5 joules of potential energy. When the rock is released, the potential energy is converted into kinetic energy (motion). A negligible amount is also converted to heat, which appears in the increased motion of the air as the rock falls. As the rock nears the ground, the potential energy content decreases (potential energy depends on the height) and the kinetic energy content increases (as speed increases) but the sum of the two is still 5 joules. Just before the rock hits the ground, its potential energy is zero and its kinetic energy is 5 joules. What happens to that energy when the rock hits the groung? It is converted to heat energy, and the ground become slightly warmer.
Another unit for enerty that is useful when studying elementary particles such as electrons is the electron volt (eV). This is a very tiny unit, appropriate for particles with very tiny mass. The conversion between eV and joules is
1 eV = 1.6 x 10-19 joules One additional energy unit you are no doubt familiar with is the Calorie, which is equivalent to 4186 joules. (There is another calorie, spelled with a lower case c, that is equal to 4.186 joules.) The upper case c Calorie is also called a nutritionist's calorie, and is used to measure the energy content of food. It's a rather large unit -a 100 Calorie apple contains 418 600 joules of energy, also written 418.6 kilojoules (kj). So, when you consume calories, you increase your internal energy content so you can do work.