Transcript Energy - Chapter 5-2 / 5-3

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 sun
4. 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