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Energy

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Energy & Work Work involves a change in a system. changing an object s position heating or cooling a building, generating a image on the TV screen, moving a speaker ... – PowerPoint PPT presentation

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Title: Energy


1
Energy Work
2
Work involves a change in a system.
  • changing an objects position
  • heating or cooling a building,
  • generating a image on the TV screen,
  • moving a speaker cone to make sound

Since different tasks require different amounts
of work, some things require more energy than
others.
3
Work is
  • F force in Newtons
  • d displacement in meters
  • The angle ? between F d
  • Joule is the unit of WORK

4
Work is
  • Work- A quantity that measures the effects of a
    force acting over a distance.
  • Work is a result of motion in the direction of
    the force.
  • There is no work without motion (d0).
  • Distance-means distance in the direction of the
    force. If a force is vertical and motion is
    horizontal, No work is done.

5
Work is
  • MAXIMUM when ? 0º
  • MAXIMUM when Force // Displacement
  • MINIMUM when ? 90º
  • MINIMUM when Force - Displacement

6
Question 2
  • If you push vigorously against a brick wall, how
    much work do you do on the wall?
  1. A lot
  2. None
  3. Without numbers, how can we know?
  4. No idea

7
Answer 2 (b) None
There is no work done on the wall as there is no
displacement of the wall.
8
What are the units of WORK?
  • Work is measured in Newton-meters (Nm) or
    foot-pounds (ftlb)
  • A Newton-meter is called a JOULE (sounds like
    jewel)
  • Named after James Prescott Joule (1818-1889)
  • British physicist who established the mechanical
    equivalence of heat and discovered the first law
    of thermodynamics.

9
Find the work done by gravity when a 2.0 kg rock
falls 1.5 m.
  • w F (d) cos(?)
  • What is the formula for Force (F)
  • F m (g) or F m (9.8 m/s2)
  • w (m g) (d) cos(?)
  • w (2.0kg 9.8m/s2)(1.5m) cos(0)
  • w 29 J

10
Negative Work...
  • ? 0
  • Cos(0) 1
  • ? 180
  • Cos(180) -1

11
How much work is done when a man pushes a car
with an 800 N constant force over a distance of
20 m?
Question 3
  1. 0 J
  2. 40 J
  3. 800 J
  4. 16000 J
  5. Im lost

12
How much work is done when a man pushes a car
with an 800 N constant force over a distance of
20 m?
Answer 3 (d) 16000 J
w F (d) cos(?) w (800 N)(20 m) cos(0) w
16 000 J
13
How much work is done by a woman pulling a loaded
dolly 100 ft with a force of 150 lb at an angle
of 45?
Question 4
  1. 0 ft-lb
  2. 7879.8 ft-lb
  3. 10606.6 ft-lb
  4. 15000 ft-lb
  5. Im lost

14
How much work is done by a woman pulling a loaded
dolly 100 ft with a force of 150 lb at an angle
of 45?
Answer 4 (c) 10606.6 ft-lb
w F (d) cos(?) w (150 lb)(100 ft)
cos(45) w 10606.6 ft-lb
15
Power Work
  • Work can be done at different rates.
  • Since work involves the transfer of energy, the
    faster work is done, the quicker energy needs to
    be transferred.
  • Power is the measure of how fast work can be
    done.
  • In other words, power is the rate at which
    energy is transferred.

16
Power is
  • W work in Joules
  • t time in seconds
  • WATT is the unit of power

17
Question 5
  • A woman exerts 100 N of force pushing a grocery
    cart 5 meters in 2.5 seconds. How much power did
    she exert?
  1. 0 watt
  2. 40 watt
  3. 200 watt
  4. 1250 watt
  5. Im lost

18
Answer 5 (c) 200 watt
  • A woman exerts 100 N of force pushing a grocery
    cart 5 meters in 2.5 seconds. How much power did
    she exert?

19
Horsepower
  • Horsepower (hp) is a commonly used unit of power.
  • 1hp 746 watts(W)

20
For example
  • Let's carry a box of books up a set of stairs.
  • From experience, we know that running the books
    up the stairs takes more energy than walking the
    same distance (you would be more tired if you
    ran).
  • But the amount of work done is the same since the
    books weighed the same and moved the same
    distance each trip.
  • However, the work is done much faster if we run,
    so energy must be converted faster.
  • Therefore, more power is required.

21
For example 2
  • Think of a racecar versus an economy car.
  • They both can travel the same distance, but the
    race car does it much faster since it is capable
    of expending much more energy in much less time.
  • This is because the more powerful car can convert
    energy quicker.

22
For example 3
  • Think of an 18-wheeler versus an economy car.
  • They both can travel the same distance, but the
    economy car does it much faster since it is
    capable of expending much more energy in much
    less time.
  • BUT
  • The truck can carry more weight (exert a greater
    force) and is more powerful

23
Electrical Power
  • Electrical Power is defined the same way.
  • Work must be done to move electrons through the
    electrical devices (i.e.,resistance).
  • More resistance means more work must be done to
    allow the device to operate.
  • More electrical power means more energy is being
    converted.
  • This electrical energy is supplied by the source
    of the electrical current, like a battery or
    generator.

24
Energy
  • The ability to do work.
  • An object has energy if it is able to produce
    change in itself or its surroundings.

25
Energy lets us do work
  • Energy is the ability to do Work
  • Energy is important to all living things in
    order to maintain life functions.
  • Humans use energy to modify their environment and
    perform work.
  • Energy is measured by the amount of work it is
    able to do.
  • The units of energy are joules (J).

26
Energy exists in different forms
  • Mechanical energy (moving objects and their
    positions)
  • Radiant energy (light and solar energy)
  • Chemical energy (including the food you eat and
    fuels we burn)
  • Thermal or heat energy (molecules moving faster
    means more heat)
  • Electrical energy (electrons moving through a
    wire)
  • Nuclear energy (energy locked in the nucleus of
    an atom)

27
Energy can be transferred
  • Fossil fuels like coal and oil can be burned to
    heat water that boils into steam that turns a
    turbine to generate electricity that you use to
    operate a stereo.
  • Chemical energy ? Thermal energy
  • Thermal energy ? Kinetic energy
  • Kinetic energy ? Electrical energy

28
Energy cannot be created or destroyed.
  • In the example of riding a bicycle down a steep
    hill, you begin with a lot of potential energy at
    the top of the hill and gain kinetic energy as
    you coast down the hill.
  • If you are not making the kinetic energy
    (movement down the hill), where does it come
    from? The answer is simple your potential
    energy at the top is transformed into kinetic
    energy as you speed along.

29
Mechanical Energy
  • Kinetic Potential
  • Kinetic is the energy of moving objects.
  • Potential Energy is stored energy.
  • Gravitational PE is energy due to position.

30
Mechanical Energy - II
  • As you speed down a steep hill on a bicycle, you
    are moving and therefore have kinetic energy.
  • But where did this energy come from? You
    probably already know that it came from your
    position at the top of the hill.
  • At the top of the hill, you had the ability to
    do work (move the bicycle) purely because of
    where you were. You had the potential to perform
    the work of moving the bicycle.
  • Whenever you work with mechanical energy, you
    probably are dealing with both forms together in
    the same system.

31
Potential Energy
  • Energy that is a result of an objects position
    or condition.
  • All potential energy is Stored Energy.
  • Pull back on a bow string and bend the bow. The
    object then possesses potential energy.

32
Potential Energy
  • A rock on a table top has more potential energy
    than when it is on the ground due to its
    position.
  • This is a form of gravitational potential energy.
  • Fuel is an example of chemical potential energy,
    due to its ability to burn.

33
Gravitational Potential Energy
  • Depends on mass and height.
  • GPE m(g)h
  • m mass
  • g acceleration due to gravity
  • h height
  • -What are the Units of GPE?

34
SI units?
  • m kg
  • g m/s2
  • h m
  • PE (kg m/s2) m Nm J

35
Question 6
  • A man lifts a 2 kilogram book from the floor to
    the top of a 1.25 meter tall table. What is the
    change in the books gravitational potential
    energy?
  1. 0 joules
  2. 2.50 joules
  3. -2.50 joules
  4. 24.525 joules
  5. -24.525 joules

36
Answer 6 (d) 24.525 J
  • A man lifts a 2 kilogram book from the floor to
    the top of a 1.25 meter tall table. What is the
    change in the books gravitational potential
    energy?

37
Question 7
  • A mouse now pushes a book (2 kg) off the table
    (1.25m). What is the change in the books
    gravitational potential energy?
  1. 0 joules
  2. 2.50 joules
  3. -2.50 joules
  4. 24.525 joules
  5. -24.525 joules

38
Answer 7 (e) -24.525 J
  • A man lifts a 2 kilogram book from the floor to
    the top of a 1.25 meter tall table. What is the
    change in the books gravitational potential
    energy?

39
Kinetic Energy
  • Energy that appears in the form of motion.
  • Depends on the mass and speed of the object in
    motion.

40
Kinetic Energy
  • KE (1/2)mv2
  • m mass v velocity
  • Unit for energy is Joule (J) it is defined as a
    Newton Meter.

41
SI units?
42
Kinetic Energy
  • Energy due to motion.
  • A brick falling at the same speed as a ping pong
    ball will do more damage.
  • KE is dependent on mass.
  • KE also depends on speed (v)

43
Which would affect the kinetic energy of an
object more, doubling its mass or its velocity?
Kinetic Energy
  • doubling the mass would result in a doubling of
    the KE.
  • doubling the velocity would quadruple the KE.

44
Question 8
  • What is the KE of a 1140 kg (2513 lb) car driving
    at 8.95 m/s (20 mph)?
  1. 0 joules
  2. 5101.5 joules
  3. 10203 joules
  4. 4.57x104 joules
  5. Im lost

45
Answer 8 (d) 45658.4 J
  • What is the KE of a 1140 kg (2513 lb) car driving
    at 8.95 m/s (20 mph)?

46
Question 9
  • What is the KE of 11 pound rabbit running at
    302.6 mph? Note 1 ton 907.185 kg and 1 mph
    0.447 m/s.
  1. 0 joules
  2. 5101.5 joules
  3. 10203 joules
  4. 4.57x104 joules
  5. Im lost

47
Answer 9
  • What is the KE of 11 pound rabbit running at
    302.6 mph? Note 1 pound 0.454 kg and 1 mph
    0.447 m/s.

48
Recall, Law of Conservation of Energy
  • Energy can not be created nor destroyed.
  • Energy can change from one form to another.
  • The total energy in the universe is constant.

49
Conservation of Energy
  • In a roller coaster all of the energy for the
    entire ride comes from the conveyor belt that
    takes the cars up the first hill.

50
Examples
  • A 400 kg roller coaster car sits at the top of
    the first hill of the Magnum XL200. If the hill
    is 151 ft (46 m) tall, what is the potential
    energy of the cart?
  • What is the speed of the cart at the bottom (what
    do you need to ignore?)
  • How much KE and PE does the car have half way
    down the hill?

51
Answers
  • Energy at Top Energy at Bottom
  • Ignoring friction assume 100 energy conversion
  • Energy at Top
  • GPE 400 kg x 9.8 m/s2 x 46 m 180 320 Joules
  • KE 0
  • Energy at Bottom
  • GPE 0
  • KE 1/2 x 400 kg x v2
  • 180 320 200 v2
  • Velocity at Bottom 30 m/s 67 mph

52
Answers
  • Energy at Top Energy at Bottom
  • Energy at Halfway point?
  • 1/2 PE 90 160 J
  • 1/2 KE 90 160 J
  • Speed at Halfway point ?
  • 1/2 of 30 m/s 15 m/s 33.5 mph
  • NO !!!!!!
  • 1/2 mv2 90 160
  • Velocity 21.2 m/s 47.5 mph

53
The End ?
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