Download Chapter 9 Notes

Chapter 9 Notes
I. Work, Power, and Machines
A. What is work?
1. measures the effect of a force
2. must act over a distance
3. lots of force can be exerted, but no work done  how?
4. formula:
5. work is measured in joules  SI Unit
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B. Power
1. measures the rate at which work is done
2. power is determined by how fast or slow work is completed
3. formula:
4. power is measured in watts  SI Unit
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C. Machines and mechanical advantage
1. machines help us do work by redistributing the work that we
put into them
a. machines can change the direction of a force
b. machines can greatly increase an output force by
changing the distance over which the force is applied
2. different forces can do the same amount of work
a. lifting a box straight up requires a large force over a
short distance
b. using a ramp to lift a box requires a smaller force over
a longer distance
3. mechanical advantage tells how much a machine multiplies
force or increases distance
a. MA is a ratio between output and input forces
b. formula:
c. no SI Unit  it is a ratio
d. HINT: all answers should be greater than one
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II. Simple Machines
A. Broken down into two main categories
1. the lever family
2. the inclined plane family
B. The lever family
1. levers are divided into three classes
a. 1st class
b. 2nd class
c. 3rd class
2. pulleys are modified levers
a. single fixed pulley  MA of 1
b. single moveable pulley  MA of 2
c. multiple pulleys  even higher mechanical advantage
d. hint: look at the number of output force ropes bearing
the load
e. multiple pulley systems are often called a block and
tackle
3. a wheel and axle is a lever or pulley connected to a shaft
a. wheel turns  axle also turns
b. small input force  large output force
c. example: steering wheel of a car
C. The inclined plane family
1. inclined planes multiply and redirect force
a. changes magnitude and direction of force
b. example: wheelchair ramp
2. a wedge is a modified inclined plane
a. functions like two inclined planes back to back
b. one downward force becomes two forces directed out
to the sides
c. example: axe blade
3. a screw is an inclined plane wrapped around a cylinder
a. threads of a screw  inclined plane
b. steeper threads  more input force required
D. Compound machines
1. machine made up of more than one simple machine
2. examples
a. pair of scissors  two 1st class levers joined at a
fulcrum; each lever has a wedge for a blade
b. car jack  lever in combination with a large screw
III. What is Energy?
A. Important Concepts
1. whenever work is done, energy is transferred or transformed
to another system
2. energy can be present in an object or system even when
nothing is happening
3. because energy is a measure of the ability to do work,
energy and work are both expressed in joules
B. Potential energy
1. observations
a. stretching a rubber band requires work
b. releasing a stretched rubber band  it flies away
c. energy used to stretch the rubber band is stored in the
rubber band  able to do work at a later time
d. where is the energy before you release it?
2. potential energy is stored energy
a. common names include:
1) stored energy
2) energy of position
3) energy of rest
b. types of potential energy
1) elastic potential energy  rubber band
2) gravitational potential energy  apple on a tree
3) chemical potential energy  batteries/chemical
bonds
3. gravitational potential energy depends on both mass and
height
a. because it results from the force of gravity, GPE
depends on the mass of the objects in a system and on the
distance between them
b. formula:
c. note that GPE is measured in joules  SI Unit
d. also note that “mg” in the equation is the weight of the
object  GPE = weight x height
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C. Kinetic energy
1. observations
a. apple falls from the branch of a tree  ability to do
work
b. energy of a moving object  kinetic
2. kinetic energy depends on mass and speed
a. falling apple > falling cherry
Why?
b. apple falls  kinetic energy increases Why?
c. formula:
3. kinetic energy depends on speed more than mass
a. because speed is squared, a greater value for speed
will cause a large increase in KE
b. example: violence of a car crash
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D. Other forms of energy
1. mechanical energy
a. sum of potential and kinetic energy
b. ME = PE + KE
2. atoms and molecules have kinetic energy
a. all atoms are constantly in motion
b. can be affected by temperature
3. chemical reactions involve potential energy
4. living things get energy from the sun
5. the sun gets energy from nuclear reactions  nuclear fission
6. electricity is a form of energy
7. light can carry energy across empty space
IV. Conservation of Energy
A. Energy transformations
1. potential energy can become kinetic energy
2. kinetic energy can become potential energy
3. energy transformations explain several everyday occurrences
a. a bouncing ball
b. the flight of a ball
4. mechanical energy can change to other forms of energy
a. elastic PE  kinetic energy
b. heat energy  result of friction (KE)
B. The law of conservation of energy
1. energy cannot be created or destroyed
2. energy does not simply appear or disappear
3. systems may be open or closed
C. Efficiency of machines
1. not all of the work done by a machine is useful work
a. friction  heat
b. sound
2. efficiency is the ratio of useful work to work in
a. no SI unit
b. formula:
c. no machine is 100% efficient
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