Energy Notes Energy is one of the most important concepts in science. An object has energy if it can produce a change in itself or in its surroundings. OR The ability to do work

Energy Notes The SI unit of energy is Joules. It takes work to make an object move and a moving object can do work. If positive net work is done on an object, its speed increases. If negative net work is exerted on an object, its speed decreases.

Energy Notes Kinetic energy (KE) Energy due to the motion of an object; if an object has speed, it has kinetic energy. KE = 1/2 m v 2 Where m is mass in kilograms and v is the object's Velocity m/s If an object's mass is doubled, its kinetic energy is doubled, but if its speed is doubled, its kinetic energy is quadrupled!

Energy Notes Work-Energy Theorem Work causes a change in an object's kinetic energy; if you want to change the kinetic energy of an object, you must do work on it W = ∆KE = KE f - KE i W = 1/2 m v f 2 - 1/2 m v i 2

Energy Notes Potential energy (PE) Energy due to an object's position. Energy can be stored for later use in the form of potential energy. The mathematical formula for the potential energy depends upon the type of force involved.

Energy Notes Gravitational potential energy Energy of an object due to its position in a gravitational field PE grav = m g h Where m is mass in kilograms, g is the acceleration due to gravity, and h is the object's height. (mg is equivalent to the object's weight.) Base level Position where the potential energy of an object is defined to be zero. The higher an object is above base level, the greater its gravitational potential energy. Notice that gravitational potential energy depends upon an object's vertical height above base level. The work done by the gravitational force only depends upon the vertical distance between two points, not the path taken to get from one point to another.

Energy Notes Elastic Potential Energy A simple spring has potential energy when compressed (or stretched). When it is released, it is capable of doing work. Hooke's Law describes the "restoring force" of a spring. F s = - kx where k represents the spring constant and x represents the distance stretched elastic PE = 1/2 kx 2

Energy Notes Law of Conservation of Energy Energy can be transformed from one form to another in an isolated system, but it cannot be created or destroyed. The total energy of the system is constant. The total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one body to another, but the total energy remains constant. Principle of Conservation of Mechanical Energy The total mechanical energy of an object remains constant as an object moves provided that no work is done by forces other than gravity. E tot = KE + PE

Energy Notes Energy Before = Energy After 1/2 kx 2 + 1/2 mv i 2 + mgh i = 1/2 kx 2 + 1/2 mv f 2 + mgh f

Energy Notes Work Work is done in physics when a constant force (both in magnitude and direction) is applied on an object and there is a displacement parallel to the direction of the force. Work is a scalar quantity - it has only magnitude. W = F d

Energy Notes A box being pulled by a constant force along a horizontal surface and moved a displacement d. The force is applied at an angle Θ to the surface. Only the component of the force (F cos Θ) parallel to the displacement does work in the direction of the displacement. The amount of work done is given by W = F d cos Θ

Energy Notes Joule The SI unit of work. 1 J = 1 N m, or one Joule equals one Newton meter

Energy Notes Power The rate of doing work. P = W / t P is the power dissipated, W is the work done, and t is time in seconds Since Work is the product of force times displacement, power can also be expressed in terms of velocity P = F v Watts the SI unit of power 1 W = 1 J/ 1 sec, or one watt equals one joule per second Another unit of power is the horsepower. 1 hp = 746 W