Transcript Energy unit general notes jan 2013

Energy can change from one
form to another without a net
loss or gain.
Energy Notes
•
Definition of energy:
•
Energy can be stored, transferred and used.
•
Definition of work:
•
Work is done when a force causes something to move, and, as a result, energy moves from
place to place or changes from one form to another.
•
SI Unit for Work and Energy:
•
Difference between kinetic energy and potential energy
Kinetic energy
The energy of motion
The ability to cause change. Every change involves energy.
W = Fd (Work equals force times distance)
Joules (J)
Potential energy
The energy of position or stored energy
Energy Notes
•
What is the equation for KE?
•
How does KE increase or decrease?
KE = 1/2mv2 Kinetic energy = ½ mass x velocity2
Increase or decrease the velocity of the object (change speed).
Double your speed, Quadruple the KE
Prove it: Calculate the KE of a 2500 kg car traveling at 20 m/s and at 40 m/s
•
•
KE at 20 m/s
(500,000 J)
KE at 40 m/s
(2,000,000 J)
More mass, same speed, more KE.
Double the mass, double the KE
Prove it: Calculate the KE of a 100 kg cart and a 200 kg cart, each traveling at 15 m/s
•
•
100 kg cart at 15 m/s
200 kg cart at 15 m/s
•
(11,250 J)
(22,500 J)
9.3 Mechanical Energy
Mechanical energy is the energy due to the position of
something or the movement of something.
Total mechanical energy is the total amount of energy
related to the motion of an object! TME = KE + PE
PE can be gravitational PE or elastic PE (or both)
9.7 Conservation of Energy
Same energy
transformation applies
10 J of PE does 8 J
The 2 J of heat can be called non-useful work (work that is not useful work on the
part of the object’s total mechanical energy). This “loss” of
arrow and 2 J of
TME really represents a transfer from TME to DE)
non-useful work on
the molecules that
Dissipated energy (DE) is amount of
compose the bow
and string and
energy transferred away from the total
arrow. The arrow
mechanical energy. More DE means
has 8 J of KE.
less TME, which means less speed!
9.4 Potential Energy
Gravitational Potential Energy
•Energy is stored in an object as the result of increasing its
height.
•Work is required to elevate objects against Earth’s gravity.
•Example: Water in an elevated reservoir and the raised ram
of a pile driver have gravitational potential energy.
9.5 Kinetic Energy
If an object is moving, then it is capable of doing work. It
has energy of motion, or kinetic energy (KE).
• The kinetic energy of an object depends on the mass
of the object as well as its speed.
Kinetic Energy
Energy
of
motion
KE
increases
with speed
9.4 Potential Energy
Elastic Potential Energy—potential to do work
•Energy stored in a stretched or compressed spring or
material.
•When a bow is drawn back, energy is stored and the bow can
do work on the arrow.
•These types of potential energy are elastic potential energy.
Definitions
•
•
•
•
Energy (the ability to cause change)
can be transferred, stored or used
Work (net force x distance moved)
method of energy use, transfer and
storage
• Power (work per unit time)
• the rate at which work is done or energy
is transferred
Chapter 9.1
Work
9.1 Work
Energy is the ability to cause change.
Work is done when a net force acts on an
object and the object moves in the direction of
the net force (force causes displacement).
Work involves a transfer of energy between
something and its surroundings.
9.1 Work
Two categories of work:
1) Work done against another force
a) Transfer of energy to change position or store
energy—work against another force
Ex: Pulling back the string on a bow stores
energy.
b) Transfer of energy (one form to another)
Ex: Rubbing your hands together transfers one
form (kinetic) of energy to another (thermal).
2)
Work done to change the speed of an object
Ex: Bringing an automobile up to speed increases
its (kinetic) energy.
9.1 Work
= force × distance
Did the weightlifter do work on the barbell
and weights?
•Yes, when he first lifted them above his
head.
Is the weightlifter currently doing work on
the barbell and weights?
No, the barbell and weights are not moving.
•Explain two ways that the work done by
the weightlifter be increased.
1) Increase the weight on the ends
of the barbell
2) Increase the distance over
which the weightlifter pushes
the barbell and weights.
9.1 Work
Work has the same units as energy
Joules
Newton x meter
J
Nxm
•One joule (J) of work is done when a
force of 1 N is exerted over a distance
of 1 m (lifting an apple over your
head).
9.1 Work
If we lift two loads, we do twice as much work as lifting
one load the same distance, because the force needed is
twice as great.
If we lift one load twice as far, we do twice as much work
because the distance is twice as great.
9.1 Work
While the weight lifter is holding a
barbell over his head, he may get
really tired, but he does no work on
the barbell.
Work may be done on the muscles
by stretching and squeezing them,
but this work is not done on the
barbell.
When the weight lifter raises the
barbell, he is doing work on it.
9.1 Work
think!
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom floor.
How much work do you do on the package?
9.1 Work
think!
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom floor.
How much work do you do on the package?
Answer:
W = Fd = 60 N × 4 m = 240 J
9.1 Work
When is work done on an object?
When is work not done on an object?