Energy and Conservation

Physics

Chapter 5-2 (p172-178)

Chapter 5-3 (p179-183)

Types of Energy

- Energy is measured in Joules

- Kinetic energy (KE) – energy of a moving object due to its motion

KE = ½ mv2

KE kinetic energy (J)

m mass (kg)

v velocity (m/s)

- KE depends on speed (velocity) and mass More speed (velocity) or

more mass = higher KE

- BUT velocity is more important

- Potential energy (PE) – stored energy; energy of position

2 kinds of potential energy

Types of Potential Energy

1. Gravitational (PEg)

Associated with an objects’ position / height

PEg = mgh (do not use negative sign with gravity here)

m = mass (kg)

g = magnitude of gravity (m/s2)

h = height above the ground (m)

• The higher the object above the ground, the more gravitational potential energy the object has.

Types of Potential Energy

2. Elastic Potential Energy (PEe)

energy in a stretched or compressed spring

PEelastic = ½ kx2

k = spring constant (N/m)

x = distance compressed or stretched (m)

- Spring constant – (force constant) – the measure of a spring’s resistance to being stretched or compressed measured in N/m

Work and Energy

ǽ⅞. Other forms of energy

1. Kinetic molecular energy energy of moving atoms and molecules

2. Chemical energy

energy released in chemical reactions

energy stored in the food you eat

Work and Energy

3. Nuclear energy nuclear reactions (fission,

fusion), the sun4. Solar energy sunlight,

electromagnetic waves

5. Electrical energy

electricity, movement of charged ions

and electrons

Conservation of Energy

A. The Law of Conservation of Energy – Energy cannot be created or destroyed, but can only change form.

1. Potential energy changes to Kinetic energy as a ball rolls down a hill (stored energy changing slowly to energy of motion.)

2. This transformation occurs slowly, NOT all at once.

3. At the top of the hill all the energy is potential, at the bottom of the hill all the energy is kinetic (until the ball stops rolling)

4. Some energy is lost to friction and heat, but it is not destroyed

- Mechanical Energy – the sum of all kinetic energy and all forms of potential energy

ME = KE + ΣPE ΣPE = ΣPEg + ΣPEe

- In the presence of friction ME is not conserved, because some of the energy is lost to friction, and changes into other forms of energy, such as light, and heat

***The total energy of a system is always conserved!!

Energy Transformations

Conservation of Energy

Homework:

Chapter 5-2

Page 173 #3

Page 177 #2, 4

Page 178 #1, 3

Chapter 5-3

Page 183 #2, 3, 4