CHAPTER 4: ENERGY

SECTION 1: The Nature of Energy

WARM-UP:  What is energy?  List some different types of energy.

LEARNING GOALS  Distinguish between kinetic and potential energy.  Calculate kinetic and gravitational potential energy.  Describe different forms of potential energy.

WHAT IS ENERGY?  Changes are constantly occurring in the world.  Examples: light bulbs heating the air, sunlight on window, you breathing

WHAT IS ENERGY?  Every change requires energy.  Energy: ability to cause change

FORMS OF ENERGY  Electromagnetic energy: energy that travels in waves  Example: UV, Infrared, Radio  Mechanical energy: energy in physical movement or position  Example: moving car, flying baseball

FORMS OF ENERGY  Electrical energy: energy carried by moving electrons  Example: any kind of electricity  Thermal energy: energy related to temperature  Example: cup of hot tea

UNITS OF ENERGY  Joule (J) – SI unit of energy  Other units:  calorie (cal)  Calorie (Cal)  Kilowatt Hour (kWh)

KINETIC ENERGY  Kinetic Energy: the energy that a moving object has because of its motion.  Depends on the mass and speed of the object.

KINETIC ENERGY  KE = ½ mv 2  KE = kinetic energy in joules (J)  m = mass in kg  v = velocity in m/s

KINETIC ENERGY  KE = ½ mv 2  m = 2(KE/v 2 )  v = √2(KE/m)

EXAMPLE  A jogger with the mass of 60 kg is moving at a speed of 3 m/s. What is the jogger’s kinetic energy?

EXAMPLE  A sprinter has a mass of 80.0 kg and a kinetic energy of 4,000 J. What is the sprinter’s speed?

EXAMPLE  A baseball is moving at a speed of 40 m/s and has 120 J of kinetic energy. What is the mass of the baseball?

POTENTIAL ENERGY  Potential Energy: stored energy due to an object’s position.  Three types:  elastic  chemical  gravitational

POTENTIAL ENERGY  Elastic Potential Energy: energy stored by something that can stretch or compress  Ex: rubber bands, springs

POTENTIAL ENERGY  Chemical Potential Energy: energy stored in chemical bonds  Ex: gasoline, food, batteries

POTENTIAL ENERGY  Gravitational Potential Energy: energy stored in objects due to their position above the Earth’s surface  Ex: a boulder on a cliff

POTENTIAL ENERGY  GPE = mgh  GPE = gravitational potential energy (J)  m = mass in kg  g = 9.8 m/s 2  h = height in m

POTENTIAL ENERGY  GPE = mgh  m = GPE/(gh)  h = GPE/(mg)  g = GPE/(mh)

EXAMPLE  What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

EXAMPLE  How high above the ground is a baseball with a mass of 0.15 kg and a GPE of 73.5 J?

EXAMPLE  A rock climber is 200 m above the ground and has a GPE of 117,600 J. What is the rock climber’s mass?

CHANGING ENERGY  If all of the object on the shelves have the same mass:  Which has the most potential energy?  Which will be moving faster if they were to all fall?

CHANGING ENERGY  As an object begins to fall, it has both GPE and KE.  As the object gets closer to the ground, it has less GPE and more KE.

EXAMPLE  An 80 kg diver jumps from a 10 m platform.  What is the GPE of the diver at the top of the platform?  What is the GPE of the diver after falling 5 m?

CHECK-IN:  Explain how the kinetic energy of a truck could be increased without increasing the truck’s speed.