Transcript What is Energy?

What is Energy?
In the chapter on matter, you
learned that matter and
energy is conserved. Instead
of being created or destroyed,
it is just changed from one
form to another.
The energy of the sunlight
that reaches Earth is the
ultimate source of most of
the energy around us. Look at
the illustration and identify all
the types of energy involved.
What is Energy?
1. How did energy from sunlight provide the energy
the girl needed to swing the bat? (Hint: What do you
need to have energy?)
2. When the girl hits the ball, she exerts a force on it.
Does she do work on the ball in the scientific sense of
the term? Explain why.
3. After the girl hits the ball, the ball moves very fast
and has energy. When the ball hits the fielder’s glove,
it stops moving. Given that energy can never be
destroyed but merely changes form, what happened
to the energy the ball once had? (Hint: If you are the
fielder, what do you hear and feel as you catch the
ball?)
Energy and Work
• Energy is the ability to do work.
• When you do work on an object, you transfer
energy to that object.
• Whenever work is done, energy is transformed or
transferred to another system.
• Energy is measured in joules.
• Because energy is a measure of the ability to do
work, energy and work are expressed in the same
units.
Potential Energy
• The energy that an object has because of the
position, shape, or condition of the object is
called potential energy.
• Potential energy is stored energy.
• Elastic potential energy is the energy stored in any
type of stretched or compressed elastic material, such
as a spring or a rubber band.
• Gravitational potential energy is the energy stored in
the gravitational field which exists between any two
or more objects.
Potential Energy
• Elastic potential energy depends on the distance
an object can be compressed or stretched.
• Elastic Potential Energy Equation
elas. PE = ½ x spring constant  (stretch distance)2
Es = ½
2
kd
 units N/m x m2 or J
• The spring constant (k)
• Is determined by measuring the force needed to stretch
or compress a spring.
k = Force/distance  units N/m
Potential Energy
• Gravitational potential energy depends on both
mass and height.
• Gravitational Potential Energy Equation
grav. PE = mass  free-fall acceleration  height
Epg = mgh
 units kg * m/s2 x m or J
• The height can be relative.
• The height used in the above equation is usually
measured from the ground.
• However, it can be a relative height between two
points, such as between two branches in a tree.
Math Skills
Gravitational Potential Energy A 65 kg rock
climber ascends a cliff. What is the climber’s
gravitational potential energy at a point 35 m
above the base of the cliff?
1. List the given and unknown values.
Given:
mass, m = 65 kg
height, h = 35 m
free-fall acceleration, g = 9.8 m/s2
Unknown: gravitational potential energy, Epg = ? J
Math Skills
2. Write the equation for gravitational potential
energy.
Epg = mgh
3. Insert the known values into the equation, and solve.
Ep = (65 kg)(9.8 m/s2)(35 m)
Ep = 2.2  104 kg•m2/s2
Ep = 2.2  104 J
Kinetic Energy
• The energy of a moving object due to the
object’s motion is called kinetic energy.
• Kinetic energy depends on mass and speed.
• Kinetic Energy Equation
1
kinetic energy   mass  speed squared
2
1
 units kg * m2/s2 or J
KE  mv 2
2
• Kinetic energy depends on speed more than mass.
Math Skills
Kinetic Energy What is the kinetic energy of a 44
kg cheetah running at 31 m/s?
1. List the given and unknown values.
Given:
mass, m = 44 kg
speed, v = 31 m/s
Unknown: kinetic energy, KE = ? J
Math Skills, continued
2. Write the equation for kinetic energy.
1
kinetic energy   mass  speed squared
2
1
KE  mv 2
2
3. Insert the known values into the equation, and solve.
1
KE  (44 kg)(31 m/s)2
2
KE  2.1 10 4 kggm2 /s 2
KE  2.1 10 4 J
Other Forms of Energy
• The amount of work an object can do because of
the object’s kinetic and potential energies is
called mechanical energy.
• Mechanical energy is the sum of the potential
energy and the kinetic energy in a system.
• In addition to mechanical energy, most systems
contain nonmechanical energy.
• Nonmechanical energy does not usually affect
systems on a large scale.
Other Forms of Energy, continued
• Atoms and molecules have kinetic energy.
• The kinetic energy of particles is related to heat
and temperature.
• Chemical reactions involve potential energy.
• The amount of chemical energy associated with a
substance depends in part on the relative
positions of the atoms it contains.
• Living things get energy from the sun.
• Plants use photosynthesis to turn the energy in
sunlight into chemical energy.
Other Forms of Energy, continued
• The sun gets energy from nuclear reactions.
• The sun is fueled by nuclear fusion reactions in its
core.
• Electricity is a form of energy.
• Electrical energy is derived from the flow of charged
particles, as in a bolt of lightning or in a wire.
• Light can carry energy across empty space.
• Light energy travels from the sun to Earth across
empty space in the form of electromagnetic waves.