Chapter 13 Work and Energy
13.1 Work, Power, Machines
Work the transfer of energy to an object by the application of a force that causes the object to move in the
direction of the force. Calculated by multiplying the force by the distance over which the force is applied.
Work = Force x distance
W=Fxd
Symbol for work is an Italic W
Joule = Newton x meter
Joule = kg*m/s2 x m/1
Joule = kg x m2/s2
Power a quantity that measures the rate at which work is done or energy is transformed. Calculated by
dividing Work by time.
Power = Work / time P = W / t
Watt = Joule / seconds
W=J/s
1 kilowatt = 3 600 000 Joules
Mechanical Advantage a quantity that expresses how much a machine multiplies force or distance. Defined
as the ratio between the output force and the input force, or the ratio between the input distance and
output distance.
Mechanical Advantage equation
MA = output force = input distance
Input force = output distance
Different forces can do the same amount of work. Increasing the distance will reduce the force that is
needed (ramp), but the overall work will be the same.
13.3 What is Energy?
Energy; the capacity to do work (cause movement). Whenever work is done, energy is transferred from one
system to another and/or transformed into another type.
Energy is measured in Joules Work = Force x distance
W=Nxm
1. Mechanical Energy the amount of work an object can do because of the object’s kinetic and potential
energies. Calculated by the sum total of the Potential and Kinetic Energy in a system.
Mechanical Energy = Potential Energy + Kinetic Energy
a. Gravitational Potential Energy is the energy an object has because of the position, shape, or
condition of the object. Sometimes called the “energy of position” because it results from the
relative position of the objects in the system.
Gravitational Potential Energy = height x mass x free-fall acceleration (g = 9.8 m/s2)
9.8 m/s2
gPE
Joules =
m
x kg
gPE
=
m
x weight (w = m x g)
gPE
Joules =
kg
x
m
x
x
m/s2
Elastic Potential Energy lies in the elastic properties of rubber, springs, and bungee cords.
b. Kinetic Energy is the energy of an object due to an objects motion. The energy an object has
because of its movement. Change in velocity has greater affect on amount of energy a system has.
(joules)
Kinetic Energy = ½ x mass
KE Joules = ½ x
kg
x velocity squared
x (m/s) 2 (velocity squared)
KE Joules = ½ x
kg x m2/s2
Joule = ½ x
kg x m2/s2
Other Forms of Energy exist.
Non-Mechanical Energy that lies at the level of atoms. Chemical Energy, due to the arrangement of the
bonds that hold the atoms together. This is a type of Potential Energy. The energy is released when the
bonds of the substance are rearranged, thus the potential energy is reduced.
Example; wood (stored potential energy in bonds of wood) to ash (energy transferred and transformed)
Electromagnetic spectrum Energy = can travel through empty space, Light Energy (Photon – particle of this
energy) Living things on Earth us this energy from the sun in a process called Photosynthesis to transform
EM energy into chemical energy (glucose).
Nuclear Energy – the sun combines (light atomic nuclei into heavier nucleus) This fusion of atoms releases
tremendous amounts of energy. Nuclear Power plants perform the opposite process; fission is when a single
large nucleus breaks into two smaller ones. In both process, a small amount of matter releases large
quantities of energy.
Electrical Energy can be stored in fields. Electric energy results from the location of charge particles in an
electric field. Electrons move from an areas of high potential (powerplants) to areas of low potential (your
house).
Thermal Energy – Energy due to the motion of an object’s particles. Also known as Heat Energy
Sound Energy – a measure of a sound’s Amplitude (loudness)
13.4 Conservation of Energy
Energy Transformation – during an action, energy changes from one form to another, but the total amount
of energy stays the same.
Mechanical Energy (PE and KE) can easily change into other forms of energy.
Kinetic Energy can change in heat energy, sound energy, or light energy
The Law of Conservation of Energy – Energy cannot be created or destroyed.
The total amount of energy in the Universe never changes; although the energy may change from
one form to another, the energy of the system is always conserved.
Energy does not appear or disappear. Energy is never lost, just changes form.
To make studying energy transfers easier, scientists limit their view to a small area or a small number of
objects. The boundaries define a system.
Open system – energy and matter can be exchanged with the surrounding (you in gym)
Closed system – energy but not matter exchanged. (car cooling system)
Isolated system – not energy nor matter is exchanged with the surrounding.
Thermodynamics describes energy conservation. Energy is can be transferred into Work or as heat. For any
system, the net change in energy equals the energy transferred as work and as heat.
When no energy is transferred as work or heat, then we say energy is conserved. This form of the law of
energy conservation is called the first law of thermodynamics.
Efficiency of Machines
Only a portion of work done by a machine is useful work, some of the energy going into the system is
converted into heat, instead of work(movement)
Efficiency of a machine is a measure of how useful work a machine can do. Defined as the ratio of useful
work output to work input.
Efficiency = useful work output / work input Always a decimal, expressed as a percentage
Perpetual motion machines are impossible. No machine is 100% efficient
All machines need energy input.