Energy

KEY CONCEPTS How are energy and work related? What factors does the kinetic energy of an object depend on? How is gravitational potential energy determined? What are the major forms of energy?

ENERGY is the ability to do work What is work? Work is a transfer of energy by a force moving an object through a distance Energy and work both measured in JOULES (J); 1 J = 1 N. m

Most forms of energy can be classified into two general types: 1. KINETIC ENERGY 2. POTENTIAL ENERGY

KINETIC ENERGY-energy an object has because of its motion KE = ½ mv 2 where KE is kinetic energy (in J), m is mass (in kg) and v is velocity (in m/s)

SEE “Math Skills-Calculating Kinetic Energy” p. 448

POTENTIAL ENERGY-energy something has because of its position or shape; two types A. GRAVITATIONAL POTENTIAL ENERGY-potential energy that depends on an object’s height PE = mgh where PE is potential energy (in J), m is mass (in kg), g is acceleration due to gravity (9.8 m/s 2 ) and h is height above a reference point (in m) What is the potential energy of a 10 kg rock 5 m above the ground?

B. ELASTIC POTENTIAL ENERGY- potential energy an object has because it is stretched or compressed SEE FIG. 5, P. 450

All energy can be considered to be KE, PE or energy in fields such as those produced by electromagnetic waves

The major forms of energy are 1. MECHANICAL-associated with motion and position of everyday objects 2. THERMAL-total of potential and kinetic energy of all microscopic particles in an object 3. CHEMICAL-energy stored in chemical bonds 4. ELECTRICAL-energy associated with electric charges (+ and -) 5. ELECTROMAGNETIC-form of energy that travels through space in the form of waves (light, X-rays, etc.) 6. NUCLEAR-energy stored in atomic nuclei

SECTION 15.1 ASSESSMENT

KEY CONCEPTS Can energy be converted from one form into another? What is the law of conservation of energy? What energy conversion takes place as an object falls toward Earth? How are energy and mass related?

An ENERGY CONVERSION is the process of changing energy from one form to another What type of energy conversions take place when a match is lit? When a battery-powered radio is turned on?

The LAW OF CONSERVATION OF ENERGY states that energy cannot be created or destroyed; the total amount of energy present before a change is equal to the total amount after (in a closed system)

Work done by friction changes kinetic energy into thermal energy. Friction reduces efficiency. Falling objects PE is converted into thermal energy because of air resistance. Sometimes small amounts of friction can be ignored.

Gravitational PE is converted to KE as an object falls Ex. pendulum (SEE FIG. 13, P. 456), pole-vaulter (SEE FIG. 14., P. 457)

Mechanical energy = KE + PE For a closed, frictionless system (KE + PE) i = (KE + PE) f SEE “Math Skills-Conservation of Mechanical Energy”, “Math Practice” p. 458

Einstein’s famous equation E = mc 2 where E is energy (in J), m is mass (in kg) and c is the speed of light (3 X 10 8 m/s), says that energy and mass are equivalent, and can be converted into each other When do mass-energy conversions happen?

SEE SECTION 15.2 ASSESSMENT