Chapter 9 Section 4 pg 358 Energy Pg

Chapter 9 Section 4 – pg 358 Energy

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

Pg 359 • Kinetic Energy = ½ x Mass x Speed 2 • 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

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

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

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

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

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

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

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

2 A. What is potential energy?

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

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

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

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

3 C. 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?