Transcript Energy and Its Conservation

Energy and Its
Conservation
Introduction
• Energy is always present, but
never visible!
• Instead, we see the evidence of
energy: movement, sound, heat,
light
Work-Energy Correlation
• In order for work to be done, a
force must be applied to an
object and cause it to move in
the same direction as the force.
• Work transfers energy from one
object to another.
What is energy?
• Energy is the ability of an object
to produce a change in itself or
the surroundings. It is the
ability to do work.
Types of Energy
• Mechanical Energy (ME)-enables
an object to do work.
A. Kinetic Energy (KE)
B. Potential Energy (PE)
Kinetic Energy
• Kinetic energy is defined as the
energy of a moving object.
• KE = 1/2mv2
• m is mass (kg) and v is velocity
(m/s)
• The unit for energy is the joule.
• Kinetic energy is a scalar
quantity.
Practice Problem
• What is the kinetic energy of a 45 kg
object moving at 13 m/s?
Example 2
• The kinetic energy of a boat is
calculated at 52,000 J. If the
boat has a mass of 39,000 kg,
with what velocity is it moving?
Potential Energy
• Potential energy is defined as the
energy in matter due to its position
or the arrangement of its parts.
• Often referred to as stored energy.
• There are many types of stored
energy:
1) Elastic potential energy
2) Chemical potential energy
3) Gravitational potential energy
Work and Energy
• Work and energy are related.
• When work is waiting to be
done, we call the energy
potential.
• When work is being done, we
call the energy kinetic.
Elastic Potential Energy
• Occurs when an object resists being
stretched or compressed.
• Examples: a stretched rubber band, a
spring
• The elastic potential energy in a rubber
band can be used to do work-such as a toy
plane-A rubber band untwists and causes a
propeller to turn.
• If you do more work on the rubber band
and pull it back a greater distance (or twist
it more), it will have greater potential
energy, and as a result, can do more work.
Chemical Potential
Energy
• Chemical potential energy is the
energy stored in molecules.
• Examples: gasoline, food
• When gasoline is burned through
combustion, the bonds of molecules
are broken and reformed, and energy
is released.
• The released energy is used to do
work.
Gravitational Potential
Energy
• If an object moves away from the earth,
energy is stored in the system as a result
of the gravitational force between the
object and the Earth.
• Work is done in moving the object to its
new location-the energy required to do the
work against gravity is now stored in the
object as a form of potential energy called
gravitational potential energy.
• PE = mgh (m is the object’s mass (kg), g is
the acceleration due to gravity (9.8 m/s2),
and h is the height (in meters)above the
reference level .
Practice Problem
• A 3.8 kg object is lifted to a height of
3 meters. What is the potential
energy of the object?
Example 4
• A 30 kg child climbs 15 m up a
tree. When he stops to look
around, what is the child’s
potential energy?
Let’s Review:
State whether each
of the following has kinetic energy, potential
energy, or both.
1. If an object is at rest, it certainly does NOT possess this
form of energy.
2. Depends upon object mass and object height.
3. The energy an object possesses due to its motion.
4. The amount is expressed using the unit joule (abbreviated
J).
5. The energy stored in an object due to its position (or
height).
6. The amount depends upon the arbitrarily assigned zero
level.
7. Depends upon object mass and object speed.
8. If an object is at rest on the ground (zero height), it
certainly does not possess this form of energy.