Physics
Energy
Big Picture
Energy is the measure of an object’s ability to do work (exert a force over a distance) on another system or object. Energy can never be created or destroyed; it can only be transformed into a different type of energy. When somebody says energy has been lost, the person really means that the energy was turned into some form that is unusable. The conservation of energy is one of the fundamental laws of our universe.
Key Terms
Potential Energy: The potential energy of an object/system is the energy that is stored and ready to be used in the object/ system. SI units: J
System: Any object or group of objects that we are studying. A system where energy is conserved is called a closed system.
Kinetic Energy: A measurement of the energy associated with an object in motion. Kinetic energy can also refer to energy associated with the vibration or motion of atoms or molecules. SI units: J
Work: If a force is exerted over a distance to move an object, the force is said to be doing work. SI unit: J
Conservative Forces: A force in which the work done to move a particle between two points is not affected by the path taken.
Non-Conservative Forces: A force in which the work done to move a particle between two points is affected by the path taken.
Power: The rate at which work is done by a system or object. SI units: J/s or W
Forms of Energy
There are many different types of potential energy that are differentiated by the way that the energy is stored.
- Gravitational potential energy is based on an object’s position relative to some reference point, and chemical potential energy is stored in the chemical bonds between atoms.
- Gravitational potential energy is the potential for an object/system to move due to the force of gravity.
- Near the surface of the planet, gravitational potential energy \(U_g = mgh\). For Earth, \(g = 9.8 m/s^2\).
All moving objects have kinetic energy. \(KE = \frac{1}{2}mv^2\)
- Because the velocity is squared in the formula for kinetic energy, faster objects have much more kinetic energy than slower ones.
Energy Diagrams
Energy diagrams allow us to visualize how energy is being transferred.
Below is a diagram for a swinging pendulum that shows how energy changes forms from gravitational potential energy to kinetic energy.
Work
In the work-energy theorem, the work done on an object is always equal to the change in kinetic energy of the object.
- No work is done if there is no motion or if the applied force is perpendicular to the direction of motion.
- If the force is a conservative force, the work done to move an object depends only on the displacement, not the total distance traveled. The work done on an object is equal to the displacement multiplied by the component of the force along the direction of motion.
For example, gravity is a conservative force. When an object’s gravitational potential energy changes, only the object’s displacement determines the work done. The diagram on the right shows how the change in energy is the same whether the red ball takes the green path or the blue path. Gravity does work mgh along either paths.
- Friction is an example of a non-conservative force because the work done actually depends on the total distance the object travels.
- Power is the rate work is done.
Image Credit: CompuChip, Public Domain
