Potential Energy, Kinetic Energy, and Energy Transfer
Students investigate stored and moving energy, explain energy transformations, and apply conservation of energy to familiar systems.
Illustrations are auto-generated and may be placeholders. They can be refreshed to match the narration.
Energy and the Ability to Cause Change
Energy is the ability to cause change or do work. Objects can possess energy because of their motion, position, temperature, or other conditions. Energy is observed through its effects rather than seen directly. For example, a moving soccer ball has enough energy to knock over a cone. Sunlight can warm pavement, and an electric current can make a lamp produce light. Scientists identify the forms of energy present in a system and track how energy moves or changes form. A system is the group of objects being studied. When examining a wind-up toy, the system might include the spring, gears, wheels, and surrounding floor. Defining the system makes it easier to explain where energy starts and where it goes.
Potential Energy
Potential energy is stored energy related to an object's position, shape, or arrangement. Gravitational potential energy increases when an object is lifted higher above Earth's surface. A book on a high shelf has more gravitational potential energy than the same book on the floor. Elastic potential energy is stored when an elastic object is stretched or compressed. For example, pulling back a rubber band changes its shape and stores energy. A compressed spring in a toy also stores elastic potential energy. The amount of potential energy depends on factors such as height, mass, or how far a spring is stretched. Potential energy can later be transformed into motion or another form of energy.
Kinetic Energy
Kinetic energy is the energy an object has because it is moving. An object's kinetic energy depends on its mass and speed. A fast-moving skateboard has more kinetic energy than the same skateboard moving slowly. At the same speed, a loaded cart has more kinetic energy than an empty cart because the loaded cart has more mass. Kinetic energy can be transferred when moving objects interact. For example, a rolling marble can strike a stationary marble and cause it to move. Some of the first marble's kinetic energy is transferred to the second marble. Collisions may also transform part of the energy into sound and thermal energy, especially when surfaces bend, vibrate, or rub together.
Energy Transformations and Conservation
Energy can change form and move between objects, but it is not created or destroyed. This principle is called conservation of energy. On a playground swing, a rider has the most gravitational potential energy at the highest point. As the rider moves downward, potential energy decreases while kinetic energy increases. Near the bottom, the rider moves fastest and has the most kinetic energy. As the swing rises again, kinetic energy changes back into potential energy. Friction at the chain and air resistance transform some mechanical energy into thermal energy and sound. Because energy spreads into the surroundings, the swing eventually slows unless the rider pumps or receives another push that transfers additional energy into the system.