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Potential and Kinetic Energy What is Energy • Energy is the ability to do work. • Measured in Joules (J). • Mechanical energy is the energy due to the position of something or the movement of something. • Energy is never created or destroyed, it is only transformed into a different form. Forms of Energy • • • • • • • Chemical - gasoline Electrical - lightning Heat – burner on a stove Light – flash light Mechanical - car Nuclear - sun Sound – music on the radio Mechanical Energy • What are the two forms of mechanical energy? Potential and Kinetic Energy • Energy that is stored is called potential energy (PE) because in the stored state it has the potential for doing work. • Three examples of potential energy are elastic potential energy, chemical energy, and gravitational potential energy. Examples of Potential Energy: A stretched rubber band.. Water at the top of a waterfall.. Yo–Yo held in your hand.. A drawn Bow and Arrow… Potential Energy • Work is required to elevate objects against Earth’s gravity. • The potential energy due to elevated positions is gravitational potential energy (GPE). • The higher an object, the more potential energy. • The more mass an object has, the more potential energy it has. Potential Energy • The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity to lift it. GPE = mgh • The height is the distance above some chosen reference level, such as the ground or the floor. Potential Energy • The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. a. b. The boulder is lifted with 100 N of force. The boulder is pushed up the 4-m incline with 50 N of force. c. The boulder is lifted with 100 N of force up each 0.5-m stair. Kinetic Energy • When stored energy begins to move, the object now transfers from potential energy into kinetic energy. Kinetic Energy • If an object is moving, it has energy. (Be careful, the converse of this statement is not always true!) • This energy is called kinetic energy - the energy of motion. Kinetic Energy • An object’s kinetic energy depends on: the object’s mass. – Kinetic energy is directly proportional to mass. the object’s speed (velocity). – Kinetic energy is directly proportional to the square of the object’s speed. Potential Energy Check • If I want to drop an apple from the top of one of these three things, where will be the most potential energy? A B C Since the tower is the highest off the ground, the apple would have the most PE (PE = mgh) Kinetic Energy Check When these objects move at the same speed, which will have more kinetic energy? The truck will have a higher KE at the same speed because it has a larger mass KE = ½ mv2 Work – Energy Theorem • When you throw a ball, you do work on it to give it speed as it leaves your hand. The moving ball can then hit something and push it, doing work on what it hits. • Common units of kinetic energy: Joules Work – Energy Theorem • The work-energy theorem describes the relationship between work and energy. • Work = ∆KE; ΔPE = -Work • Work equals the change in kinetic energy. • The work in this equation is the net work—that is, the work based on the net force.