Transcript New Energy Powerpoint (Power Point)

Work, Energy and Power
-In the previous units/chapters, we utilized
Newton's laws to analyze the motion of
objects.
-In this unit, motion will be approached from
the perspective of work and energy. The
effect that work has upon the energy of an
object (or system of objects) will be
investigated; the resulting velocity and/or
height of the object can then be predicted
from energy information.
Work
• A force acting upon an object to cause a
displacement. - three key words - force,
displacement, and cause. In order for a force to
qualify as having done work on an object, there
must be a displacement and the force must
cause the displacement.
• Mathematically,
where F = force, d = displacement, and
the angle (theta) is defined as the angle
between the force and the displacement
vector.
Three scenarios. Give examples
Scenario A:
Scenario B:
Scenario C:
Read the following statements and determine whether or not
they represent examples of work.
1. A student applies a force to a wall and
becomes exhausted.
No.This
is not an example of work. The wall is not
displaced. A force must cause a displacement in order
for work to be done.
2. A book falls off a table and free falls to the
ground.
Yes.
This is an example of work. There is a force (gravity)
which acts on the book which causes it to be displaced
in a downward direction (i.e., "fall").
Examples
3.
A waiter carries a tray full of meals above his head
by one arm straight across the room at constant
speed.
No.
This is not an example of work. There is a force (the waiter
pushes up on the tray) and there is a displacement (the tray is
moved horizontally across the room). Yet the force does not cause
the displacement. To cause a displacement, there must be a
component of force in the direction of the displacement.
Examples
4. A rocket accelerates through space.
Yes.
This is an example of work. There is a force (the expelled
gases push on the rocket) which cause the rocket to be displaced
through space.
What about this?
• A chain pulling upwards and rightwards upon
Fido in order to drag Fido to the right. It is only
the horizontal component of the tensional force
in the chain which causes Fido to be displaced
to the right.
Work is the transfer of energy.
• In physics we say that work is done on an object
when we transfer energy to that object.
• If you put energy into an object, then you do work
on that object.
• If a first object is the agent that gives energy to a
second object, then the first object does work on
the second object. The energy goes from the first
object into the second object. At first we will say
that if an object is standing still, and you get it
moving, then you have put energy into that object.
Example
• A golfer uses a club and gets a stationary golf ball moving when he or
she hits the ball. The club does work on the golf ball as it strikes the
ball. Energy leaves the club and enters the ball. This is a transfer of
energy. Thus, we say that the club did work on the ball.
• And, before the ball was struck, the golfer did work on the club. The
club was initially standing still, and the golfer got it moving when he or
she swung the club.
Conclusion
• So, the golfer does work on the club,
transferring energy into the club, making it
move. The club does work on the ball,
transferring energy into the ball, getting it
moving.
Energy transfer
Work is a kind of energy transfer.
When a force moves something the
energy transfer is called work.
• Work isn't a form of energy - it's one of the ways
that energy can be transferred.
• The amount of work done is the same as the
amount of energy transferred.
• The amount of work is measured in joules (J).
• The distance is measured in the direction of the
force.
work = force x distance moved
Example
Q.By dragging a sledge up a slope, how
work are you doing against friction?
much
Work = size of frictional force x distance moved in
direction of frictional force
= 40N x 100m
= 4,000 J
Where did this 4 000 J come from and where did it go?
Energy Types
Since energy comes in so many forms
and, as we will see, is also constantly
changing from one form into another,
selecting a perfect set of 10 basic types is
not easy.
Types of Energy
In science, we say that energy is the
ability to do work.
We need energy to do all the things we
do.
• Reading a book needs energy.
• Running needs energy.
• Riding a bicycle needs energy.
• Watching TV needs energy.
Even sleeping needs energy.
Everything that happens in the world
involves movement and for something
to move, energy is required.
If something takes energy in, then it also
gives energy out.
A light bulb can get energy from the mains electricity
supply.
The bulb gives energy back as light and as heat.
Examples
• A person riding a bicycle gets their energy
from the food they eat.
• Energy is given out as sound and as
heat.
Work Concepts
Energy Concepts
Potential Energy
• An object can store energy as the result of its
position.
• For example, the heavy ram of a pile driver is
storing energy when it is held at an elevated
position. This stored energy of position is
referred to as potential energy.
Example
• A drawn bow is able to store energy as the
result of its position. When assuming its
usual position (i.e., when not drawn), there is
no energy stored in the bow. Yet when its
position is altered from its usual equilibrium
position, the bow is able to store energy by
virtue of its position. This stored energy of
position is referred to as potential energy.
Potential energy is the stored energy
of position possessed by an object.
Two forms of potential energy
1. Gravitational potential energy
The energy stored in an object as the result of its vertical
position (i.e., height). The energy is stored as the result of
the gravitational attraction of the Earth for the object. The
gravitational potential energy of the heavy ram of a pile
driver is dependent on two variables - the mass of the ram
and the height to which it is raised. There is a direct
relation between gravitational potential energy and the
mass of an object; more massive objects have greater
gravitational potential energy. There is also a direct
relation between gravitational potential energy and the
height of an object; the higher that an object is elevated,
the greater the gravitational potential energy. These
relationships are expressed by the following equation:
PEgrav = mass * g * height
PEgrav = m * g * h
How to determine the GPE?
To determine the gravitational potential energy
of an object, a zero height position must first
be arbitrarily assigned. Typically, the ground is
considered to be a position of zero height.
Try this example
• Since the gravitational potential energy of an
object is directly proportional to its height
above the zero position, a doubling of the
height will result in a doubling of the
gravitational potential energy. A tripling of the
height will result in a tripling of the
gravitational potential energy. Knowing that
the potential energy at the top of the tall pillar
is 30 J, what is the potential energy at the
other positions shown on the hill and the
stairs.
Are you ready?
Check your understanding
• A cart is loaded with a brick and pulled at
constant speed along an inclined plane to the
height of a seat-top. If the mass of the loaded
cart is 3.0 kg and the height of the seat top is
0.45 meters, then what is the potential
energy of the loaded cart at the height of the
seat-top?
P.E = m*g*h
= (3.0kg)*(9.8m/s2)*(0.45m)
Translational kinetic energy
The energy of motion. An object which has motion whether it be vertical or horizontal motion - has
kinetic energy. The amount of translational kinetic
energy which an object has depends upon two
variables: the mass (m) of the object and the
speed (v) of the object.
A twofold increase in speed, the kinetic energy will
increase by a factor of four; for a threefold increase
in speed, the kinetic energy will increase by a factor
of nine; and for a fourfold increase in speed, the
kinetic energy will increase by a factor of sixteen.
The kinetic energy is dependent upon the square of
the speed.
Check Your Understanding
Q.1 Determine the kinetic energy of a 1000-kg
roller coaster car that is moving with a speed
of 20.0 m/s.
K.E = (½)(1000-kg)(20.0m/s)2 = 200 000 Joules
Q.2 If the roller coaster car in the above
problem were moving with twice the speed,
then what would be its new kinetic energy?
If the speed is doubled, then the K.E is
quadrupled.
Thus, K.E = 4 (200 000 Joules) = 800 000
Joules
Examples
Q.3 A 750-kg compact car moving at 100 km/hr
has approximately 290 000 Joules of kinetic
energy. What is the kinetic energy of the
same car if it is moving at 50 km/hr?
The K.E is directly related to the square of
the speed. If the speed is reduced by a
factor of 2 (as in from 100 km/h to 50
km/h) then the K.E will be reduced by a
factor of 4. Thus, the new KE is (290 000
J/4) or 72 500 J
The Law of Conservation of Energy
Energy in a system may take on various forms (e.g.
kinetic, potential, heat, light). The law of
conservation of energy states that energy may
neither be created nor destroyed. Therefore the
sum of all the energies in the system is a constant.
Pendulum:
• mass, m = 1kg
height, h = 0.2 m
gravity, g = 9.8 m/s2
• PE = mgh
PE = 1.96J
Check your knowledge
• Describe the position of BLUE BALL.
The position of the blue ball is where the
Potential Energy (PE) = 1.96J while the Kinetic
Energy (KE) = 0. As the blue ball is approching
the purple ball position the PE is decreasing
while the KE is increasing. At exactly halfway
between the blue and purple ball position the
PE = KE.
Describe the position of PURPLE BALL.
The position of the purple ball is where
the Kinetic Energy is at its maximum while
the Potential Energy (PE) = 0. At this point,
theoretically, all the PE has transformed
into KE> Therefore now the KE = 1.96J
while the PE = 0.
Describe the position of PINK BALL.
The position of the pink ball is where the
Potential Energy (PE) is once again at its
maximum and the Kinetic Energy (KE) = 0.
The sum of PE and KE is the total mechanical
energy:
Total Mechanical Energy = PE + KE
Energy Transformation for a Pendulum
Energy Conservation on an Incline
How High Will It Go?
A Roller Coaster is waiting for you!
Energy Transformation on a Roller Coaster