1. Energy Chapter 11

2. Forms of Energy  There are two forms of energy.  Kinetic energy is energy due to the motion of the object. Energy of movement.  Potential energy is energy stored in an object because of its state. Energy of position.

3.  Work is the transfer of energy by mechanical means.  Energy in food and the body is stored in chemical bonds and is called chemical energy.  There are many forms of potential energy which depends on the position, shape, or form of an object.

4.  Kinetic energy depends on an objects mass and velocity.  KE = ½ mv 2.  Work done equals the kinetic energy gained by an object.  W = KE.  Ex Pr 250 Prac Pr 251

5.  Work - Energy theorem states that the net work done on an object is equal to its change in kinetic energy.  Wnet = KE f - KE i =  KE.  If the net work is +, the KE .  If the net work is -, the KE .  The total energy is the sum of the potential and kinetic energies.

6.  Total energy equals the sum of KE and PE.  E = KE + PE  Gravitational potential energy depends on an object’s position above Earth’s surface.  PE = mgh, is valid only if g is constant.

7.  When above Earth, the PE no longer  linearly with the height.  Ex Prob 254 Prac Pr 254  Elastic potential energy - energy can be stored in the bending or stretching of an object.  Examples: bow, rubber band, pole vaultpole vault

8. 11-2 Conservation of Energy  The law of the conservation of energy states that within a closed, isolated system, energy can change form, but the total amount of energy is constant.  Energy cannot be created or destroyed.  The sum of PE and KE is called mechanical energy.

9.  A  in PE means an  in KE.  The equation describing the conservation of energy is  KE i + PE i = KE f + PE f  The KE of a dropped ball at the bottom of a plane is the same whether the ball falls vertically or slides down without friction.  Fig 11-9 259

10.  The top hill of a roller coaster must be the highest, if not the PE needed to reach the top would be larger than the mechanical energy stored in the car at the top of the first hill.roller coaster first hill  SHM demonstrates the conservation of energy. Fig 11-12 260 SHM  The sum of the PE and KE is constant.

11.  The PE and KE change to other forms of energy in the case of the pendulum and other such systems. pendulum  Some is changed to heat energy.  Some is changed to light energy.  Some is changed to sound energy.  Ex Prob 261 Prac Pr 261

12.  Albert Einstein stated that mass itself is a form of PE. This is given in his equation Eo = mc 2. Eo is the rest energy.  Stretching a spring or bending a bow causes them to gain mass. The change in mass is too small to be easily detected.  When strong nuclear forces are involved, the energy released by the changes in mass can be very large.

13. Analyzing Collisions  If the PE is completely converted back to KE, the collision is elastic.  Ex: billiard balls & marblesbilliard balls  KE is conserved in elastic collisions.elastic  Momentum is conserved in all collisions that occur in closed systems.

14.  If KE is changed to another form of energy, the collision is inelastic.  Ex:objects that stick together.  Many collisions are neither completely elastic nor inelastic.  Velocity can be calculated only if the amount of energy loss is known.

15.  Ex Prob 264 Prac Prob 265