Download Work = force x distance, or: W = f x d work Power = time , or: W P = t

Integrated Science
Chapter 8 Notes
Section 1: Work, Power, and Machines
1. Work –a quantity that measures the effects of a force acting over a distance
• Work is done only when force causes a change in the motion of an object
• The amount of work is calculated using the equation:
•
Work = force x distance, or: W = f x d
kgm2
The unit for work is the joule (J): 1 N ⋅ m = 1J = 1
s2
2. Power – a quantity that measures the rate at which work is done
• Work can be done in any length of time, however, the faster the work is done the greater the poer
used to do the work
Power =
work
W
P=
time , or:
t
⇒ The equation for power:
3. Machines and Mechanical Advantage
• Machines help us do work by redistributing the work that we put into them in one of two ways
⇒ Machines can change the direction of an input force
⇒ Machines can increase an output force by changing the direction over which the force is
applied
• The amount of work put into the machine equals the amount of work done by the machine
• It is possible to calculate how much is force or distance is multiplied by a machine
• Mechanical advantage – a quantity that measures how much a machine multiplies force or
distance
• The equation for mechanical advantage:
output force
input distance
=
, or:
input force
output distance
F
D
MA = o = i
Fi
Do
mechanical advantage =
Section 2: Simple Machines
1. Simple Machines – six basic types of machines of which all other machines are composed
• The six simple machines are: the lever, the pulley, the wheel and axle, the inclined plane, the
wedge, and the screw
• Simple machines can be divided into families: the lever family and the inclined plane family
2. The lever family
• All levers have a rigid arm that turns around a point called the fulcrum
• The original input force can be multiplied or redirected onto an output force
• Levers are divided into three classes:
⇒ The 1st class lever has the fulcrum between the input arm and output arm. A hammer is an
example of a 1st class lever.
⇒ The 2nd class lever has the fulcrum on one end of the lever and the input force on the other
end, while the output force acts between them. The wheelbarrow is an example of a 2nd
class lever.
•
•
⇒ The 3rd class lever has the fulcrum at one end of the lever, the output force is applied at the
other end, and the input force is applied between them. A person pressing a dumbbell is an
example of a 3rd class lever.
Pulleys are modified levers. The point in the middle of a pulley is the fulcrum, and the rest of the
pulley behaves like the rigid arms of the lever
The wheel and axle is a lever or pulley connected to a shaft
3. The inclined plane family
• Inclined planes multiply and redirect force
• An inclined plane turns a small input force into a large output force by spreading the work out
over a large distance
• A wedge is a modified inclined plane
⇒ A wedge turns a downward force into two forces directed out to the sides
• A screw is an inclined plane wrapped around a cylinder
4. Compound machine – a machine made of more than one simple machine
• An example of a compound machine is a pair of scissors. Scissors use two levers joined at a
common fulcrum, and each lever arm is a wedge that cuts the paper.
Section 3: What is Energy
1. Energy can be defined as the ability to do work
• Energy can be present in a system when nothing is happening, but it can only be observed when
it is transferred from one system to another.
• The amount of energy being transferred can be measured, and because energy is the ability to
work, energy and work are expressed in the same unit—joules.
2. Potential Energy – the stored energy resulting from the relative positions of objects in a system
• Energy stored in an elastic material (a rubber band, a bungee cord) is elastic potential energy
• Energy stored in any material due its position above a surface is called gravitational potential
energy
⇒ Gravitational potential energy depends on both the mass on the object and its height above
the surface.
⇒ The equation for gravitational potential energy is:
Potential Energy = mass x acceleration due to gravity x height , or
PE = mgh
Where:
PE is Potential Energy (in Joules)
m is mass (in kilograms)
g = 9.8 m/s2 (a constant value)
h is height (in meters)
3. Kinetic Energy – the energy of a moving object due to its motion
• Kinetic energy depends on the object’s mass and speed
⇒ The equation for kinetic energy:
Kinetic Energy =
Where:
KE is Kinetic Energy (in Joules)
m is mass (in kilograms)
v is speed (in meters per second)
Kinetic energy depends more on the speed of the object than its mass
KE =
•
1
x mass x speed squared , or
2
1
mv2
2
4. Mechanical Energy – the sum of the kinetic and potential energy of large-scale objects in a system.
Mechanical Energy = Potential Energy + Kinetic Energy , or
ME = PE + KE
or
1
ME = mgh + mv2
2
5. Other forms of energy
• Atoms and molecules have kinetic energy
• Chemical bonds involve potential energy
• Living things get energy from the sun
• The sun gets energy from nuclear reactions
• Electricity is a form of energy
• Light can carry energy across empty space
Section 4: Conservation of Energy
1. Energy can change from one form into another
• example: on a roller coaster, the ride starts by the cars being pulled up a steep incline. The
farther up the incline the cars go, the more potential energy they acquire. At the top of
the incline the potential energy of the cars is at its greatest value. As soon as the cars
start rolling down the other side of the incline the potential energy is converted to kinetic
energy. The faster the cars go, the greater the kinetic energy. At the bottom of the
incline the cars start up the next incline and the kinetic energy is converted back to
potential energy.
• Like the roller coaster, the bouncing of a ball is the conversion of potential energy to kinetic
energy and back.
• In these examples not all of the energy is converted to kinetic or potential energy. Some of the
energy is lost in air resistance, friction, and sound.
2. The Law of Conservation of Energy:
Energy is neither created nor destroyed, but can change form
3. Efficiency of machines
• Because of friction and other factors, only some of the work done by a machine is applied to the
task.
• In other words, there is a difference between the total work done by a machine and the useful
work done by the machine
• Efficiency – a quantity, usually expressed as a percentage, that measures the ratio of useful work
output to work input
efficiancy =
useful work output
work input
⇒ The equation for efficiency:
4. Perpetual motion machines are impossible
• Perpetual motion machines are supposed to be able to run forever without any energy input.
⇒ All machines lose energy (due to friction), so to keep them going some energy is always
necessary to keep the machine running.