Transcript document

ENERGY
Energy

Energy- the ability to do work

SI unit: Joules
Mechanical Energy

Mechanical Energy- Energy do to the
position or movement of an object

For example, a rollercoaster has mechanical
energy in the form of both potential and
kinetic energy. Explain.
http://www.classzone.com/books/ml_science_share/vis_sim/mfm05_pg126_coaster/mfm05_pg
126_coaster.html
Potential Energy

Stored Energy / energy of position

amount depends on position or
condition of the object
Gravitational Potential Energy

Gravitational
Potential energy is
greater when the
object’s height is
greater

G. PE is greater
when the object’s
weight is greater

Which has more PE, a plant sitting on
a 5th floor window or one sitting on a
1st floor window? Why?
PE formula

PE= mass x 9.8 x height



Units
Mass= grams
Height=meters
Example PE Problem






Mass= 65 kg
Height= 35 m
Gravity accel= 9.8
PE= ?
PE= 65 x 9.8 x 35
= 22295 Joules
PE word problem






Calc. PE
1200 kg car at the top of a hill that is
42 m high
M= 1200
9.8
h= 42
1200 x 9.8 x 42 = 493920 Joules
Kinetic Energy

“Energy in Motion”

Will change more
due to velocity (bc
its squared) than
mass
Kinetic energy



KE is greater when the speed is greater.
KE is greater when the mass of the
object is greater.
Which is more kinetic energy a
motorcycle going 35mph or an 18
wheeler going 35 mph? Why?
Kinetic Energy Formula

KE= ½ mass x velocity2
Kinetic Energy Ex Problem

Mass= 44 kg
Speed= 31 m/s
KE= ?

KE= ½ mass x velocity2

22 x 961= 21142 Joules


KE Word Problem

Calc KE of 1500 kg car that is moving
at a speed of 700m/s

KE= ½ mass x velocity2

750 x 490000 = 367500000Joules
From PE to KE
Conservation of Energy

“Energy cannot be created or
destroyed”

changes forms

The total amount of energy NEVER
changes
More Forms of Energy

Chemical Energy- has to do with ions,
atoms, molecules, and bonds



Change to another form of energy when
a chemical reaction occurs
Food, Wood, Gasoline, Heating oil
Electrical Energy- associated with
voltage and current
More Forms of Energy


Thermal Energy- associated with the
movement of molecules
More Motion = More Heat

Sound Energy- associated with
longitudinal mechanical waves


Light Energy- associates with
electromagnetic waves
Light, X-rays, Lasers, Gamma Rays
Nuclear Energy



Energy associated
with Fission and
Fusion
Sun and Stars
Occurs only in
controlled
situations on earth
Energy Transformations

According the Law of Conservation of
Energy…


Energy can change forms
Energy in battery then a light turns on

Potential or chemical to electrical
More Energy Transformations

Plant sitting in the sun, then making
food then growing


Light energy to chemical energy to kinetic
energy
Making music playing the piano

Potential to kinetic to sound
Energy Conversions
Which of the seven main forms of
energy is present in each
situation?
Work



Work= the force exerted over a
distance
When work is done, energy is
transferred to the object
If no movement, Zero work!

When force is applied to an object and
it moves, work is done, and kinetic
energy is created

The greater the force, the greater the
kinetic energy of the object

If work is done and an object is lifted,
the object gains PE

The higher it is lifted, the more PE
Work Triangle

Calculating Work:

Work= Force x Distance

SI Unit: Joules (J) = 1 Newton*Meter
W
F
D
Example Work Problem



F= 30 N
d= 1.5m
W= ?
W= F *d
30 * 1.5 = 45
W= 45 J
W
F
D
Example Work Problem

A carpenter lifts a 45 N beam 1.2 m
high. How much work is done on the
beam?

F= 45 N
D= 1.2 m
W= ?


W= F * D
45 * 1.2 = 54
W
W= 54 J
F
D
Multiple Step

A dancer lifts a 45 kg ballerina
overhead a distance of 1.4 m. How
much work is done?
W= F * d
441 * 1.4 = 617.4 J



F= ? 45 * 9.8= 441 N
D= 1.4m
W= ?
W= 617.4 J
W
F
D
Last one

The same dancer holds the ballerina
overhead for 5 seconds. How much
work is being done?
None, no distance is being traveled.
Power

The rate at which work is done
SI unit: Watts (w) = 1 Joule per second

Formula= Power = Work / Time

W
P
T
Example Problem:



W= 500 J
T= 20 s
P= ?
P= W / T
500 / 20 = 25
P= 25 watts
W
P
T
Example Problem # 2





F= 450 N
d= 1.0 m
t= 3 s
W= ?
P=?
W= F * d
450 * 1 = 450
W= 450 J
P=W/t
450 / 3 = 150 watts
W
P
T
P = 150 watts
Word Problem

A mover carries a chair up the stairs in
30 seconds. His work totals 300
Joules. What was his power?
P=w/t
300 / 30 = 10
W
P
P= 10 watts
T
Last one

Mary runs up the stairs in 22 seconds.
Carrie runs up the stairs in 27
seconds. Each girl has a work total of
240 Joules. Which has more power?
Mary
W= 240
T= 22
240 / 22= 10.91
P= 10.91 Watts
Carrie
W= 240
T= 27
240 / 27 = 8.89
P= 8.89 watts
Machines

change the force that you exert in
either size or direction.

Simple Machine- one movement
Compound Machine- more than one
movement

Machines at work

2 forces involved with
machines

Input Force (In) – force
applied to the machine

Output Force (Out)- force
applied by the machine to
overcome resistance
Machines at Work

Mechanical Advantage- # of times the
machine multiplies input force

MA= input force / output force

MA= input / output
Out
MA
In
Example Problem




Output = 500 N
Input = 20 N
MA= ?
MA= Output / Input
500 / 20 = 25
MA= 25
Out
MA
In
Another example




Output = 2000 N
Input= ?
MA= 10
MA= Output/ Input
Input= Output / MA
2000 / 10 = 200 N
Input= 200 N
Out
MA
In
Word Problem

The power steering in an car has a
mechanical advantage of 75. If the
input force to turn the steering wheel
is 49 N, what is the output force of
the car’s front wheels?
Output= 3675 N
MA= 75
Input= 49 N
Out
Output= ?
Output= MA * Input
75 * 49 = 3675 N
MA
In
2 families of Simple Machines

Levers

Inclined planes
Simple Machines

Lever- arm that turns around a
fixed point

FULCRUM- fixed part
Types of Levers




1st Class
2nd Class
3rd Class
classified on location Input force,
Output force, and fulcrum
First Class Lever

Fulcrum between Input force
and Output force

Can multiple force or distance

Ex: Scissors, pliers, clothes
pin
2nd Class Lever

Output between Input force and
fulcrum

Multiply force

Examples: Wheel Barrel
3rd Class lever

Input force between
Output force and fulcrum

Increase distance

Examples: bicep, fishing
rod, hockey stick
Pulleys

Modified Lever

grooved wheel with a rope
or chain running along the
groove

Single, fixed pulley has an
MA of 1
Pulley

Moving Pulley- has an MA of 2

Block and Tackle Pulley- multiple
pulleys put together. Increases MA
Wheel and Axle

Lever family

2 wheels of different
sizes that rotate
together

Smaller wheel is called
the axle
Inclined Plane

Inclined Plane- a sloping surface used
to raise objects (such as a ramp)

you exert less input force over a
greater distance
Screw and Wedge

Screw- inclined plane wrapped in a
spiral around a cylindrical post


Ex:
Wedge- inclined plane with one or two
sloping slides

Ex: Chisel, knives, ax blade
COMPOUND MACHINES

Compound Machine- a combination of
two or more simple machines

Examples: Bicycle (2 wheel and axle),
the axle (wedge and lever)
Efficiency in Machines

With machines, not all work is useful

Some lost as heat, through friction,

Efficiency- measure of how much
useful work a machine can do

Efficiency= useful work output
work input
Efficiency Problem

Work input= 180 J
Useful work output= 140 J
Efficiency= ?

140/180= .78 J

