Transcript document

I. Work is the use of force to move an object.
A.Force is necessary to do work.
* You do work only when you exert
a force on an object to move it.
* Turning a page in a book or writing an answer
is doing work.
1. Force, Motion, and Work
a. Work done is related to size of
force & distance over which
force is applied
b. Work is done by the applied force in
the direction of motion
2. Calculating Work
-W=F d
-Work= Force x distance
-Work (joule)
-Force (Newton)
-distance (m)
-1 Joule (J) = Newton X Meter
B. Objects that are moving can do work.
1. Moving objects can do work also.
* A bowling ball moving pins
* A windblown leaf landing in moving water
that moves it downstream
2. On a water wheel, gravity does the work
on the water; work is also by water to turn
gears that grind grain
II. Energy is transferred when work is done.
A. Work transfers energy.
1. Changing position & speed
transfer energy.
2. Energy- ability of
object or person to do work
or cause a change.
***Work - transfer of energy
and measured in joules!!!
B. Work changes potential and kinetic energy.
***Throwing a ball transfers kinetic
energy (motion);
The faster it moves, the more kinetic
energy it has.
***Lifting a ball from the ground,
gives it potential energy (stored); it now
has the potential to fall to the ground!
You changed the position.
You can also have PE by changing
the shape!  elastic PE!
(Ex: a spring or a rubber band)
1. Calculating Gravitational Potential Energy
***caused by gravity
***used for launching spacecraft and roller
coasters (must have enough PE at the
top of the hill to reach the top of the next hill)
Potential Energy= mass x gravity x height
PE=mgh
***g=9.8m/s2
2. Calculating Kinetic Energy
Kinetic energy = 1/2 mass x velocity2
KE=
2
1/2mv
***increasing the velocity has a greater effect
than increasing the mass
3. Calculating Mechanical Energy
***combined KE and PE
***can do WORK!
Mechanical Energy = PE + KE
C. The total amount of energy is constant.
LAW OF CONSERVATION OF ENERGY
No matter how energy is transferred or
transformed..ALL energy is still present
somewhere in one form or another!
1. Conserving Mechanical Energy
Ex. RollerCoaster
2. Losing Mechanical Energy
Some energy is transferred into
heat.
See example of pendulum p. 430
3. Forms of Energy
a. Thermal energy- motion of
molecules
b. Chemical energy- chemical bonds
c. Nuclear energy- nucleus of an
atom
d. Electromagnetic energy- electrical
and magnetic interactions
See examples of each on p. 432
13-3 Notes
POWER
III. Power is the rate at which work is done.
A.Power can be calculated from work and time.
***The RATE at which you do work.
Example: A runner breaking into a sprint to
cross the finish line in a shorter time.
***Depends on force, distance and time
***The more force, the more power, shorter time
Power
• There are 2 formulas to calculate power.
– Power from Work
– Power from Energy
Power from Work
1. Calculating Power from Work
•
Power = Work
P=W
time
t
• Power (Watt)
• 1 Watt = 1 joule per second
Power from Energy
• B. Power can be calculated from energy
and time.
• ***amount of energy transferred over time
1. Calculating Power from Energy
• Power = Energy
time
• ***measured in Watts
P= E
t
2. Everyday Power
Appliances in your home, hair
dryer, etc.
See page 439 for examples and
explanations!
2. Horsepower
***a unit of measurement for power
*** the amount of work a horse can do
in a minute
1 horsepower = 745 Watts
***Larger than a Watt!
It is primarily used
today with engines and motors.
1. What is the power of a kitchen
blender if it can perform 3750J of
work in 15s?
2. How much power is done using
a 500J microwave in 5min?
3. How much power is done using
a 60J light bulb for one hour?
Calculating Power
4. A set of pulleys is used to lift a
piano weighing 1000N. The piano
is lifted 3 meters in 60 s. How
much power is used?
5. How much power is used if
force of 35N is used to push a box
a distance of 10m in 5s?
Quiz Chp. 13
• Clear your desks!! NOTHING SHOULD
BE ON THEM EXCEPT A PENCIL/PEN
AND A PIECE OF PAPER!
• 10 QUESTIONS
• 45 Seconds a Question
1
• What is power?
– A. Rate at which work is done
– B. Force at which work is done
– C. Work
– D. Strength
2. What is the formula for
power?
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A. P=ma
B. P=Et
C. P= Wt
D. None of the above
3. What is the unit for power?
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A. joule
B. Work
C. Watt
D. None of the above
4. What type of unit dealing with
power do we use in the engine
of our cars?
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A. Watts
B. Joules
C. Horsepower
D. Newtons
5. How many formulas are their
to find power?
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A. 1
B. 4
C. 0
D. 2
6. Some appliances we have
that require power at home are?
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A. Clothes
B. Furniture
C. Lightbulbs
D. Doors
7. There are two types of
energy. What are they?
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A. Kinetic and Magical
B. Potential and Kinetic
C. Forceful and Work
D. Power and Force
8. When a person is running at
a quarter of their top speed,
their potential energy and
kinetic energy are
_______________?
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A. 50% and 50 %
B. 75% and 25%
C. 25% and 75%
D. 40% and 60%
9. What is the formula for
mechanical energy?
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A. PE = mgh
B. KE = 1/2mv^2
C. ME = PF + mgh
D. ME = PE + ME
10. What law says that energy
can’t be created nor destroyed?
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A. Law of conservation of momentum
B. Law of conservation
C. Boyles Law
D. Law of conservation of energy