Energy and Work
Where did the energy of the avalanche come A from? Where did it go? Energy is known by the changes it causes. You can hear the roar of an avalanche and see the movement of the snow. Sound and motion are examples of energy in action. In order to define energy, you need to return to the definition of a related topic, work. Recall that work is done when a force moves an object through a distance. Energy is the ability to do work. In other words, energy is transferred by a force moving an object through a distance.
Work and energy are closely related. When work is done on an object, energy is transferred to that object. 
Work is a transfer of energy. Both work and energy are typically measured in joules (J). Recall that 1 joule equals 1 newton-meter, the work done when an object is moved 1 meter by a 1-newton force. Although energy can take many different forms, it can always be measured in joules.
Think about the work and energy involved in doing something as simple as carrying your backpack up a flight of stairs. You do work on the backpack by lifting it against the force of gravity. This work requires energy. The energy to do the work comes from your muscles. Your muscles receive energy from the food you eat. The energy contained in your food comes from plants that have used the energy of sunlight, or animals that have eaten such plants. Figure 2 shows some of the many forms of energy.
Kinetic Energy
Many forms of energy can be classified into two general types: kinetic energy and potential energy. The energy of motion is called kinetic energy. The word kinetic comes from the Greek word kinetos, meaning “moving.”
The kinetic energy of any moving object depends upon its mass and speed. To calculate the kinetic energy of an object in joules, multiply by the object's mass (m) in kilograms and the square of its speed (v) in meters per second.
Notice that doubling the mass in the formula would double the kinetic energy. However, doubling the speed would quadruple the kinetic energy, since kinetic energy is proportional to the square of an object's speed.
Figure 2 Energy has many different forms. A The sun gives off energy in the form of heat and light. B Plants convert sunlight into food that we can process and eat. C People convert food energy into muscle movement.
Applying Concepts How did the skiers in the photo obtain the energy to climb the mountain slope?
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