Chapter 9 Section 4 – pg 358 Energy

Energy: the ability to do work or cause change (measured in joules) Pg 358 Work: an object is caused to move a certain distance; the transfer of energy (measured in joules) Energy: the ability to do work or cause change (measured in joules) A pitcher does work on a ball when pitching – energy is transferred to the ball from the pitch

Pg 359 Kinetic Energy Kinetic Energy: the energy an object has due to its motion kinetos Greek for “moving” The kinetic energy of an object depends on both its mass and its speed Kinetic energy increases when mass increases This is why a bowling ball dropped on your toes hurts more than a tennis ball Kinetic energy also increases when speed increases A fast traveling bowling ball will knock down more pins than a slow one

Kinetic Energy = ½ x Mass x Speed2 Pg 359 Kinetic Energy = ½ x Mass x Speed2 Note that changing the speed of the object will have a greater effect on its kinetic energy than changing its mass by the same factor

Pg 360 Potential Energy Some objects have stored energy as a result of their positions or shapes Potential Energy: stored energy that results from the position or shape of an object This type of energy has the potential to do work

Gravitational Potential Energy Pg 360 Gravitational Potential Energy Gravitational Potential Energy: potential energy related to an object’s height An object’s gravitational potential energy depends on its weight and on its height relative to a reference point Gravitational Potential Energy = Weight x Height

Elastic Potential Energy Pg 360 Elastic Potential Energy Elastic Potential Energy: the potential energy of objects that can be stretched or compressed

Energy Transformation and Conservation Pg 361 Energy Transformation and Conservation Mechanical Energy: an object’s combined kinetic energy and potential energy Mechanical Energy = Kinetic energy + Potential energy A ball thrown into the air has both kinetic (from its motion) and potential (from its height) energy

Transformations Between Potential and Kinetic Energy One of the most common energy transformations is the transformation between potential energy and kinetic energy Any object that rises or falls experiences a change in its kinetic and gravitational potential energy In a pendulum, a continuous transformation between kinetic and potential energy takes place Pg 362

Conservation of Energy Pg 363 Conservation of Energy The law of conservation of energy states that when one form of energy is transformed to another, no energy is destroyed in the process According to the law of conservation of energy, energy can not be created or destroyed If you add up all the new forms of energy after a transformation, all of the original energy will be accounted for http://www.pbs.org/opb/circus/classroom/circus-physics/conservation-energy/

Chapter 9 Section 4 Homework – pg 363

1A. What is kinetic energy? In what unit is it measured?

1B. What factors affect an object’s kinetic energy?

1C. A 1,500-kg car is moving at a speed of 10 m/s 1C. A 1,500-kg car is moving at a speed of 10 m/s. What is the car’s kinetic energy?

2A. What is potential energy?

2B. What factors affect an object’s gravitational potential energy?

2C. What is the potential energy of a 500-N pole-vaulter when she is 4 m above the ground?

3A. State the law of conservation of energy in your own words

3B. Describe the energy transformations that occur when you bounce a ball

3C. Suppose a ball had a potential energy of 5 J when you dropped it 3C. Suppose a ball had a potential energy of 5 J when you dropped it. What would be its kinetic energy just as it hit the ground?