Download Name NOTES – CHAPTER 12 WORK AND ENERGY HONORS

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NOTES – CHAPTER 12
WORK AND ENERGY
HONORS PHYSICAL SCIENCE
I.
Work, Power and Machines
a. What is Work?
i. Work is the transfer of energy to a body by the application of a force that causes
the body to move in the direction of the force
ii. Work is done only when a force causes an object to move in the direction of the
force. This is different from the everyday meaning of work.
iii. Work Equation
1. Work = force x distance
iv. Work is measured in joules
1. Because work is calculated as force times, it is measured in unites of
newtons times meter, N x m
2. These units are also called joule (j) in terms of SI base units, a joule is
equivalent to 1 kg m2/s2
b. Power
i. Power is a quantity that measures the rate at which work is done or energy is
transformed
ii. Power Equation
iii. Power is measured in watts
1. A watt (W) is equal to a joule per second (I J/s)
c. Machines and Mechanical Advantage
i. Machines multiply and redirect forces
1. Machines help people by redistributing the work put into them
2. They can change either the size or the direction of the input force
ii. Different forces can do the same amount of work
1. A machine allows the same amount of work to be done by either
decreasing the distance while increasing the force or by decreasing the
force while increasing the distance
iii. Mechanical advantage tells how much a machine multiplies force or increase
distance
iv. Mechanical Advantage Equation
II.
III.
Simple Machines
a. The Lever Family
i. The most basic machines are called simple machines
ii. The six types of simple machines are divided into two families
1. The lever family
a. Simple lever
b. Pulley
c. Wheel and axis
2. The inclined plane family
a. Simple incline plane
b. Wedge
c. Screw
iii. Levers have a rigid arm and a fulcrum
iv. Levers are divided into three classes
1. All first-class levers have a fulcrum located between the points of
application of the input and output forces
2. In a second-class lever, the fulcrum is at one end of the arm and the
input force is applied to the other end
3. Third-class levers multiply distance rather than force. As a result, they
have a mechanical advantage of less than 1.
4. Pulleys are modified levers
a. The point in the middle of a pulley is like the fulcrum of a lever
b. A single, fixed pulley has a mechanical advantage of 1.
c. Multiple pulleys are sometimes put together in a single unit
called a block and tackle
5. A wheel and axle and axle is a lever or pulley connected to a shaft
a. The steering wheel of a car, screwdrivers, and cranks are
common wheel-and-axel machines
b. The Incline Plane Family
i. Incline planes multiply and redirect force
1. An incline plane turns a small input force into a large output force by
spreading the work out over a large distance
ii. A wedge is a modified inclined plane
iii. A screw is an inclined plane wrapped around a cylinder
c. Compound Machines
i. A machine made of more than one simple machine is called a compound
machine
ii. Examples of compound machines are:
1. Scissors, which use two first class levers joined at a common fulcrum
2. A car jack, which uses a lever in combination with a large screw
Energy and Work
a. Energy is the ability to do work
i. When you do work on an object, you transfer energy to that object
ii. Whenever work is done, energy is transformed or transferred to another system
b. Energy is measured in joules
i. Because energy is a measure of the ability to do work, energy and work are
expressed in the same units
c. Potential Energy
i. The energy than an object has because of the position, shape, or conditionof the
object is called potential energy
ii. Potential energy is stored energy
1. Elastic potential energy is the energy stored in any type of stretched or
compressed elastic material, such as a spring or a rubber band
2. Gravitational potential energy is the energy stored in the graviatational
field which exists between any two or more objects
iii. Gravitational potential energy depends on both mass and height
iv. Gravitational Potential Energy Equation
v. The height can be relative
1. The height used in the above equation is usually measured from the
ground
2. However, it can be a relative height between two points, such as
between two branches in a tree
d. Kinetic Energy
i. The energy of a moving object due to the object’s motion is called kinetic energy
ii. Kinetic energy depends on mass and speed
iii. Kinetic Energy Equation
iv. Kinetic energy depends on speed more than mass
e. Other Forms of Energy
i. The amount of work an object can do because of the object’s kinetic and
potential energies is called mechanical energy
ii. Mechanical energy is the sum of the potential energy and the kinetic energy in a
system
iii. In addition to mechanical energy, most systems contain non-mechanical energy
iv. Non-mechanical energy does not usually affect systems on a large scale
v. Atoms and molecules have kinetic energy
1. The kinetic energy of particles is related to heat and temperature
IV.
vi. Chemical reactions involve potential energy
1. The amount of chemical energy associated with a substance depends in
part on the relative positions of the atoms it contains
vii. Living things get energy from the sun
1. Plants use photosynthesis to turn the energy in sunlight into chemical
energy
viii. The sun gets energy from nuclear reactions
1. The sun is fueled by nuclear fusion reactions in its core
ix. Electricity is a form of energy
1. Electrical energy is derived from the flow of charged particles, as in a
bolt of lightning or in a wire
x. Light can carry energy across empty space
1. Light energy travels from the sun to Erath across empty space in the
form of electromagnetic waves
Conservation of Energy
a. Energy Transformations
i. Energy readily changes from one form to another
ii. Potential energy can become kinetic energy
1. As a car goes down a hill on a roller coaster potential energy changes to
kinetic energy
iii. Kinetic energy can become potential energy
1. The kinetic energy a car has at the bottom of a hill can do work to carry
the car up another hill
iv. Energy transformations explain the flight of a ball
v. Mechanical energy can change to other forms of energy
1. Mechanical energy can change to non-mechanical energy as a result of
friction, air resistance, or other means
b. Law of Conservation of Energy
i. The law of conservation of energy states that energy cannot be created or
destroyed
ii. Energy doesn’t appear out of nowhere
1. Whenever the total energy in a system increases, it must be due to
energy that enters the system from an external source
iii. Energy doesn’t disappear, but it can be changed to another form
iv. Scientists study energy systems
1. Boundaries define a system
v. Systems may be open or closed
1. When the flow of energy into and out of a system is small enough that it
can be ignored, the system is called a closed system
2. Most systems are open systems, which exchange energy with the space
that surrounds them
c. Efficiency of Machines
i. Not all of the work done by a machine is useful work
1. A machine cannot do more work than the work required to operate the
machine
2. Because of friction, the work output of a machine is always somewhat
less than the work input
ii. Efficiency is the ratio of useful work out to work in.
1. Efficiency is usually expressed as a percentage
2. The efficiency of a machine is a measure of how much useful work it can
do
3. Efficiency Equation
4. Perpetual motion machines are impossible
a. Energy is always lost to friction or air resistance
5. Machines need energy input
a. Because energy always leaks out a system, every machine needs
at least a small amount of energy input to keep going