Energy
Energy is the ability to do work - make things move.
does not have mass or volume (matter)
described by the things that it does - its
interaction with matter
can move by itself or through matter
measured in joules (J)
E=MC2
James Prescott Joule
1818-1889
Newton’s Second Law: F=ma; force = mass times acceleration
A force (f) is any push or pull or any influence
that can change the motion of an object such as
gravity. Forces are measured in the metric system by a
unit called a newton (N). Weight is the force due to
gravity. 1 newton equals a force that will give a 1
kilogram mass an acceleration (a = m/s2) of 1 meter per
of saying: kg×m/s2
second per second (kg)(m)/(s2).Instead
We say: N named after Sir Isaac Newton
What is the difference between mass and weight?
Work (W) is defined as force that acts over a
distance, or the movement of an object from one
location or position to another. Work equals force
times distance. The equation for work is: W = f × d
Distance (d) in the metric system is measured in
meters (m) and force is measured in newtons (N).
Instead of using the unit n×m, work is measured using
the metric unit joule (J).
Ex. If a 500N box is lifted 2 meters, how much work has
been done? 500N × 2m = 1000J
Metric unit: J = newton× meter
English unit: calories
1 BTU = 255 calories = 1055J
Energy - 2
Each time work is done energy is transferred or
given by one object to another that allows it to do
work. So work is also understood to be the transfer of
energy. When a certain amount of work is done, the
same amount of energy is involved. Energy is therefore
measured in joules just like work.
All energy travels in waves - either through
matter or by itself in the form of electromagnetic
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radiation (pure energy). An energy wave that travels
through matter is called a mechanical wave. The
material a wave travels through is called a medium.
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Mechanical waves form when a source of energy
causes a medium to vibrate. A vibration is a repeated
back-and-forth or up-and-down motion. Although the
particles of matter move as the energy passes through
it, the particles do not travel with the wave. There are
three main types of mechanical
waves that are classified by the
direction of movement of the
particles: transverse, longitudinal,
and surface.
Energy - 3
A transverse wave vibrates
the medium particles at right
angles to its movement through the
material. S waves, also called
secondary waves or shear waves,
are a type of seismic wave and are
an example of a transverse wave. A vibrating string is
another example of a transverse wave. All points on
the string itself are fixed to a
point and move only up and
down, but the wave pulses move
at a right angle along the length
of the string. The wave speed
is the speed with which a pulse
moves along the string.
Electromagnetic or radiant energy (RE) also
travels in transverse waves, only this form does NOT
need a medium. It is a wave of pure energy defined by
its wavelength and frequency (the amount of energy in
a space).
Energy - 4
The electromagnetic spectrum includes radio
waves, TV waves, infrared (heat), visible light, ultra
violet, X-rays, gamma rays. Radiant energy travels at
about 186,000 miles per second in a vacuum (absence
of matter - space).
A longitudinal wave, sometimes called a
compression wave or a push-pull wave, vibrates the
medium particles in the same direction as the
movement of the wave. As the energy moves through,
some particles get close together called compression.
At the same time some particles are spread out called
rarefaction. The compressions and
rarefactions travel along the whole way
with the wave, but the individual
particles do not move with the wave.
Energy - 5
After the wave passes, the particles return to
their original position. P waves or primary waves are a
type of seismic wave and are an example of a
longitudinal wave.
Sound energy (SE) is
the vibration of particles
along a path. Energy is
transmitted from particle
to particle forming a chain
or compression wave or
longitudinal wave.
What are the differences between light and sound?
Surface waves are a combination of transverse
and longitudinal waves. This type of wave travels along
a surface that separates two mediums. Ocean waves
are the most familiar. When a wave passes through
water, the water moves up-and-down and also back-andforth. The individual particles actually make a circular
motion as the wave passes by.
Another type of surface wave is the Rayleigh wave. These are seismic waves that
are the most easily felt during an earthquake. They cause the most damage.
Energy – 6
Waves are
measured by their
wavelength,
amplitude, frequency,
and speed.
Transverse waves
have a high point
called a crest and low
point called a trough.
Amplitude measures
how big the wave is or
the height of the wave from the crest to the trough.
In sound, amplitude describes the loudness or amount
of energy.
Wavelength ( ) (the Greek letter lambda is the
symbol for wavelength) is the distance between
individual waves or the distance between two crests or
troughs of the wave. It is measured in meters (m).
Energy – 7
Frequency (f) is the number of waves that pass a
point in a certain time period. Frequency is measured in
Hertz (Hz). One Hz is one wave per second (f = 1/t).
As the frequency of the wave is increased, its waves
are closer together so the wavelength is smaller. As
the frequency of the wave decreases, the waves are
farther apart so the wavelength increases.
The observation of a specific frequency is
commonly referred to as the pitch of a sound. A high
pitch sound matches to a
high frequency sound wave
and a low pitch sound
matches to a low frequency
sound wave.
An average human ear is capable of detecting sound
waves with a wide range of frequencies, ranging
between approximately 20 Hz to 20,000 Hz. Any sound
with a frequency lower than the normal hearing range
(less than 20 Hz) is known as an infrasound, and any
sound with a frequency higher than the normal hearing
range (more than 20,000 Hz) is known as an ultrasound.
Energy - 8
The speed (v) of a wave depends on the density of
the material that it is passing through. It is measured
in meters per second (m/s), or feet per second (ft/s),
or miles per hour (mph),or miles per second (mi/s). The
closer the particles, the faster the wave.
The speed of sound:
air at 68F = 1130 ft/s, 770 mph
water = 5000 ft/s, 3409 mph
steel = 20,000 ft/s, 13,636 mph
The speed of light:
space = 186,000 mi/s
air = slightly less than 186,000 mi/s
water = 140,000 mi/s
glass = 122,000 mi/s
Since frequency is measured by waves per second
or 1/t and since velocity (speed in a certain direction) is
measured by distance divided by time or wavelength
divided by period (r=d/t), we can calculate the velocity
of a wave as:
If v = λf then
wavelength times frequency
v wave = λf
λ = v/f
f = v/λ
and
For example: If a wave has a wavelength of 11 meters and a
frequency of 10 Hz, what is its speed? 11m×10Hz = 110 m/s
Note the unit is meters per second.
In a sound wave if 8 compressions pass a point in 2 seconds and the
distance between the compressions is 12 meters. What is the speed of
the wave?
Frequency: 1/t ; 8/2 = 4Hz
Speed: wavelength times frequency; 12m*4Hz = 36 m/s
Energy - 9