Loading...

PPT – Energy PowerPoint presentation | free to download - id: 653f34-ZjE3O

The Adobe Flash plugin is needed to view this content

Energy Work

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.

Work is

- F force in Newtons
- d displacement in meters
- The angle ? between F d
- Joule is the unit of WORK

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.

Work is

- MAXIMUM when ? 0º
- MAXIMUM when Force // Displacement
- MINIMUM when ? 90º
- MINIMUM when Force - Displacement

Question 2

- If you push vigorously against a brick wall, how

much work do you do on the wall?

- A lot
- None
- Without numbers, how can we know?
- No idea

Answer 2 (b) None

There is no work done on the wall as there is no

displacement of the wall.

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.

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

Negative Work...

- ? 0
- Cos(0) 1
- ? 180
- Cos(180) -1

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

- 0 J
- 40 J
- 800 J
- 16000 J
- Im lost

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

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

- 0 ft-lb
- 7879.8 ft-lb
- 10606.6 ft-lb
- 15000 ft-lb
- Im lost

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

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.

Power is

- W work in Joules
- t time in seconds
- WATT is the unit of power

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?

- 0 watt
- 40 watt
- 200 watt
- 1250 watt
- Im lost

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?

Horsepower

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

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.

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.

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

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.

Energy

- The ability to do work.
- An object has energy if it is able to produce

change in itself or its surroundings.

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).

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)

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

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.

Mechanical Energy

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

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.

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.

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.

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?

SI units?

- m kg
- g m/s2
- h m
- PE (kg m/s2) m Nm J

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?

- 0 joules
- 2.50 joules
- -2.50 joules
- 24.525 joules
- -24.525 joules

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?

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?

- 0 joules
- 2.50 joules
- -2.50 joules
- 24.525 joules
- -24.525 joules

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?

Kinetic Energy

- Energy that appears in the form of motion.
- Depends on the mass and speed of the object in

motion.

Kinetic Energy

- KE (1/2)mv2
- m mass v velocity
- Unit for energy is Joule (J) it is defined as a

Newton Meter.

SI units?

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)

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.

Question 8

- What is the KE of a 1140 kg (2513 lb) car driving

at 8.95 m/s (20 mph)?

- 0 joules
- 5101.5 joules
- 10203 joules
- 4.57x104 joules
- Im lost

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)?

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.

- 0 joules
- 5101.5 joules
- 10203 joules
- 4.57x104 joules
- Im lost

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.

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.

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.

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?

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

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

The End ?