Energy Chapter 11 Physics I

Energy Energy is the property that describes an object’s ability to change itself or the environment around it. Energy can be found in many forms. Kinetic Energy (KE) – energy of motion. Potential Energy (PE) – energy gained by a change in position or structure

Kinetic Energy (KE) Moving objects possess Kinetic Energy. KE = ½ mv 2 Energy is a scalar quantity and has the unit of Joule (J) (1 J = 1 Nm)

Work Work = Force x Displacement W = Fd Unit for Work = Newton Meter (Nm) –1 Nm = 1 Joule (J) (Same as Energy) Work is a scalar quantity (no direction) In doing Work the Displacement has to be in the same direction as the Force!

Work-Energy Theorem Work and Energy are closely related. Work and Kinetic Energy can be connected with the kinematics equations and Newton’s 2 nd Law W =  KE (Work-Energy Theorem) W = KE f – KE i = ½ mv f 2 – ½ mv i 2

Work and Energy Work and Energy are closely related. Energy is required for Work to be done Energy and Work share the same unit –Unit for Energy – Joule (J) = 1Nm –Since a Joule (J) is small we often use kilojoule kJ. 1 kJ = 1000J

Kinetic Energy (KE) Moving objects possess Kinetic Energy KE is the energy of Motion KE depends on: –Mass of the object, more m = more KE –Velocity of the object, more v = more KE KE = ½ mv 2

Kinetic Energy (KE) Double the Mass (2m)  KE Doubles (2KE) Double the Velocity  KE Quadruples (4KE ) Energy is a scalar quantity and has the unit of Joule (J) (1 J = 1 Nm)

Potential Energy (PE) Potential Energy (PE) – is the stored energy gained by a change in position or makeup of the object –Gravitational PE - Gained by position –Chemical PE – Gained by structure

Gravitational Potential Energy Gravitational PE – is the work done against gravity. PE gained by position. –PE G = Work against Gravity –PE G = W = Fd = Weight (F) x Height (d) –PE G = mgh –PE G is always found with respect to an Equilibrium Position or Reference Point where PE = 0

Elastic Potential Energy Elastic PE (PE E ) is the energy gained by an object when it is stretched or compressed. Examples: Rubber band, Spring, Bow and Arrow Stretching an object increases PE E and this energy can change to other forms like KE.

Mechanical Energy Mechanical Energy is the sum of the PE and the KE acting on an object. Mechanical Energy = PE +KE Mechanical Energy deals with the motion and position of an object.

Closed Isolated System In order to observe the conservation of momentum or energy we need to have a Closed Isolated System (CIS). In a Closed Isolated System: 1.You must have a collection objects 2.Nothing enters or leaves the system 3.There are no NET EXTERNAL FORCES acting on your system

Conservation of Energy Conservation means the total amount remains constant! Law of Conservation of Energy – in a Closed Isolated System, Energy can change form but the total amount of Energy (E T ) remains constant Energy can neither be created or destroyed. It can only change form!

Conservation of Energy Equations E T = PE + KE (Mechanical Energy) E Ti = E Tf (Conservation of Energy) PE i + KE i = PE f + KE f (No Friction) mgh i + ½ mv i 2 = mgh f + ½ mv f 2 Most of the time; PE TOP = KE BOTTOM

Conservation of Energy Examples 1. Juggling (Throwing objects into the air) 2. Waterfall (Hydroelectric power) 3. Pole Vault (More Speed More Height) 4. Pendulum (Equilibrium Position PE = 0 at the Bottom)

Conservation of Energy in Collisions To look at the Conservation of Energy in Collisions, we need to redefine Elastic and Inelastic Collisions. Elastic Collision – occurs if the Mechanical Energy is completely conserved in the collision. All the PE and KE from one object goes to the other.

Conservation of Energy in Collisions Inelastic Collision – If during the collision some of the energy transforms to other forms (Not Mechanical Energy). Other forms of energy can be, Sound, Light, Heat, Dents in the Object.

Conservation of Energy in Collisions Elastic Collisions are rare and not very realistic. Example: Collisions between molecules or atoms can be considered Perfectly Elastic! Inelastic Collisions are more complicated but are also more common! We need to use both the Conservation of Momentum and Energy in collision problems!