Physics Glossary
acceleration
The rate of change of velocity.
accuracy
Accuracy describes how relatively close a measurement is to the actual or expected value.
accurate
A measurement which is relatively close to the expected value.
action-force
In Newton's 3rd law of motion, he said that all forces are paired. This then is one of those two paired forces. The other
force is the reaction force. The action force is the initiated force, or the applied force. The other force, the reaction force,
reacts to the initiated or applied force--the action force. Sometimes, they are indistinguishable, while at other times it is
clear which force initiated the action.
For example, your foot pushes back on the ground to start walking. The ground does not initiate the action. So in this
case the action force is your foot pushing back on the ground, and the reaction force is the ground pushing forward on
your foot--this is the responding force.
angular momentum
The inertia of motion of rotating objects.
angular velocity
Angular velocity is measured in radians/sec. and has to do with the rate of spinning of an object. It follows the
equation: v = ωr where r is the radius of rotation, and v is the linear velocity (tangential velocity).
Aristotle
A Greek scientist and philosopher who lived in the 4th century BC and shaped scientific thinking for centuries. He said
that an object will not remain in motion unless a force is applied to keep it moving. Terrestrial objects tend toward their
natural state - the center of the earth, Celestial objects find their rest in outer space. Galileo declared him to be wrong,
stating that an object will keep its motion unless impinged upon by some force. Newton later defended Galileo, showing
that objects have inertia... the tendency to keep the kind of motion that they have.
axis
The line about which an object rotates or revolves.
center of gravity
The point located at an object's average position of weight. This would be at the geometric center of the object if it is
made of the same material.
center of mass
The same as the center of gravity, essentially. This will not be true if the force of gravity varies. This is the average
position of all the pieces of mass which make up the object.
centrifugal force
Centrifugal force is a fictitious outward-directed force (center-fleeing). It is not real, though it seems real. The effect is
caused by inertia and centripetal force (a real force).
centripetal force
Centripetal force is a real inward-directed force (center-pointing). It is what causes circular motion.
component
In physics, when you break a vector into its parts, those parts are called its components. The x-component is the part of
the vector acting in the x-direction, and the y-component is the part of the vector acting in the y-direction. The two
components add vectorally to give the original vector. Trigonometry and the Pythagorean theorem is used to determine
the magnitude and direction of the two components.
conservation of angular momentum
The conservation of angular momentum states that if no unbalanced external torque acts on a rotating
system, the angular momentum remains a constant.
conservation of energy
Energy can be neither created nor destroyed, by ordinary means. This last phrase needs to be added to account for
nuclear reactions, in which a small amount of mass is turned into a large amount of energy. This then becomes the
conservation of mass-energy.
Energy can change forms. An object at a height has gravitational potential energy (PE). When it falls it is converted into
more and more kinetic energy (KE) until just before it strikes the ground it is entirely KE (energy of motion). But then,
when it strikes the ground, a percentage of the KE is converted into non-mechanical energy, such as typically heat. Some
will be converted into sound energy, vibrations, etc..
When it bounces back up (assuming it is not a completely pure inelastic collision) the remaining KE begins to be
converted back into gravitational PE.
conservation of momentum
Linear momentum is a conserved quantity, meaning that if a closed system is not affected by external forces, its total
linear momentum cannot change.
conserved
When a quantity in science does not change, we say it is conserved. For example, momentum is conserved in all types of
collisions, explosions, etc.. But kinetic energy is conserved only in purely elastic collisions.
directly
When two values are related so that when one increases, the other also increases.
For example, when the force applied on an object increases, when the mass does not change, the acceleration also
increases. The force and acceleration are directly proportional.
F = ma or a = F/m
earth
The 3rd object (planet) from the sun which is falling around the sun (in orbit, or revolving, around the sun).
elastic collision
A perfectly elastic collision is defined as one in which there is no loss of kinetic energy in the collision. this occurs in
collisions in which things bounce and do not stick. Momentum is conserved in purely elastic and purely inelastic collisions
(things stick) and every degree in-between, but kinetic energy is only conserved during a collision which is perfectly
elastic.
energy
That which enables work to be done... it is the ability to do work It is measured in joules, just like work.
equilibrium
A state of balance in which the net force acting ON a free body is zero. We can also say that the acceleration is zero.
first-law-of-motion
Newton's first law of motion:
Objects at rest remain at rest and objects in motion remain in motion [in a straight line at constant
speed] unless an unbalanced force (or outside net force) acts on them.
force
A push or pull. It is measured in the metric system in newtons (N) or dynes and in the English system in pounds (lb).
free fall
A free falling object is an object that is falling under the sole influence of gravity. Any object that
is being acted upon solely by the force of gravity is said to be in a state of free fall. There are
two important motion characteristics that are true of free-falling objects:
o
o
Free-falling objects do not encounter air resistance.
All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (9.8 m/s2) or 32
ft/s2.
free-body
A free-body diagram is a diagram which shows all the forces acting on a body. It ONLY shows forces acting on it. It does
not show any forces of the body acting on anything else.
free-fall
A free falling object is an object that is falling under the sole influence of gravity. Any object that
is being acted upon solely by the force of gravity is said to be in a state of free fall. There are
two important motion characteristics that are true of free-falling objects:
o
o
Free-falling objects do not encounter air resistance.
All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (9.8 m/s 2) or 32
ft/s2.
friction
A resistive force which always acts to oppose motion or attempted motion, acting in the opposite direction of the intended
motion, due to contact between the surface of the object and some other surface.
Galilee Galileo
The first physicist to mathematically analyze gravitational attraction and acceleration. Also considered the inventor of the
telescope (1609), though others were using it before he put it to astronomical use. He died in 1642... the year Newton
was born. He is recognized as the first to refer to the inertia of objects, and also developed the equivalence principle--the
concept that all falling objects accelerate at the same rate.
gravity
According to Newton, the force between any two bodies due to their masses and distance of separation. According to
Einstein it is the bending of the space-time continuum.
hang-time
What total time a person is able to remain in the air when jumping up, then falling back down.
hypothesis
An educated guess. A scientific hypothesis must be testable.
impulse
Impulse is a product of the force applied and the time applied. The units are Ns (Newton-seconds). Impulse causes a
change in momentum.
Impulse --> Ft (in Ns)
If we look at the 1st law of motion, which says that objects in motion remain in motion unless an unbalanced force is
applied, here we see that unbalanced force which is applied. The impulse formula tells us exactly how much the motion
of an object is changed by the applied force:
Ft = mΔv
inelastic collision
A perfectly inelastic collision is defined as one in which there is a partial loss of kinetic energy in the collision. It is
changed into internal energy or some other form of energy (such as sound energy) during the collision. This occurs in
collisions in which things stick and do not bounce. Momentum is conserved in purely elastic and purely inelastic collisions
(things stick) and every degree in-between, but kinetic energy is only completely conserved during a collision which is
perfectly elastic.
inertia
The tendency of an object to keep the kind of motion that it has. So then things in motion remain
in motion (in a straight line at a constant velocity) unless acted upon by an outside force, and things at rest stay at
rest unless acted upon by an outside force.
This is Newton's first law of motion, which was actually first proposed by Galileo.
instantaneous speed
The speed at a moment in time... not the average speed over a period of time.
inverse-square law
The inverse-square law states that the gravitational force of attraction is inversely proportional to the square of the
distance of separation. There is an inverse-square law for electrostatics as well.
inversely
When two values are related so that when one increases, the other decreases, and vice-versa.
For example, when the mass of an object increases, when using the same applied force, the acceleration decreases. The
mass and acceleration are inversely proportional.
a = F/m
kilogram
The unit of mass in the metric system, abbreviated "kg". A kilogram is 1000 grams.
kinetic energy
Kinetic energy is energy of motion. It is defined as the work needed to accelerate a body of a given mass from rest to its
stated velocity.
KE = 1/2 mv2
lever arm
The distance from the turning axis to the point of the applied force.
linear velocity
Linear velocity is the velocity of an object moving in a circular manner, tangent to the circle of motion. It is measured in
m/s.
lunar eclipse
A lunar eclipse (eclipse of the moon) is produced when the earth is directly between the sun and the moon and the earth's
shadow covers part or all of the moon.
mass
The amount of matter of an object, measured in kilograms (kg). It is also a measure of the amount of inertia of an object.
mechanical energy
In science, mechanical energy is the sum of potential energy and kinetic energy. It is the energy associated with the
motion and position of an object. The law of conservation of mechanical energy states that in an isolated system that is
only subject to conservative forces the mechanical energy is conserved (unchanged). In all real systems, however, nonconservative forces, like frictional forces, will be present, but sometimes they are negligible values and the mechanical
energy's being constant can therefore be a useful approximation. In elastic collisions, the mechanical energy is
conserved but in inelastic collisions, some mechanical energy is converted into heat and other forms of non-mechanical
energy (such as sound). The equivalence between lost mechanical energy and an increase in temperature was
discovered by James Prescott Joule in his work in the family-owned brewery.
mechanics
In physics, the study of the motion of objects.
moment
The product of a force and the lever arm applied. Also referred to as torque. It is measured in N-m.
moment of inertia
The moment of inertia is another term for the rotational inertia of an object. It has to do with the inertisa of a rotating
object which depends not just on the mass, but the location of each piece of mass.
momentum
"Inertia in motion," or more precisely the product of an object's mass and its velocity. It is a vector quantity and points
in the same direction as its velocity. The letter "P" is used to represent momentum, and stands for "progress":
P = mv
moon
The large satellite which is falling around the earth. (The moon is in orbit around the earth.)
net-force
The total resultant force acting on a free body - the accumulated effect of all forces acting on an object, whether it is in
motion or not.
neutral equilibrium
Neutral equilibrium is an equilibrium of an object balanced such that its equilibrium is neither lowered or raised by
displacement. A cone or top laying on its side is an example.
Newton
The unit of force in the metric system. Isaac Newton, born December 25th, 1642, is considered
to be the father of physics.
normal-force
In math, normal refers to the perpendicular. Hence the normal force is the force pushing against an object in contact with
some surface which is perpendicular to that surface upon which it rests. We typically use "n" to represent this force.
parallelogram
A 4-sided figure for which the opposite sides are parallel to one another. The parallelogram
method is one method used to add two vectors.
pascal
The unit of measure of pressure in the metric system. Pressure is a measure of force per unit of area. It is named after
Blaise Pascal.
1 Pa = 1N/m2
Pelton wheel
The Pelton wheel is a more efficient waterwheel. It was invented by Lester Pelton during the California gold rush in 1849 1851. It has curved paddles, which causes the water to "bounce" (elastic collision) or make a "u-turn" and stay on the
paddle longer, causing a greater impulse. It was much more effective than the existing waterwheels, and many of the
miners did not strike it rich, but Pelton sure did with this nice invention.
perturbation
When planets deviate from their normal, expected orbits.
The first such deviation of consequence was regarding the orbit of mercury. As mercury is the closest planet to the sun,
it is the most likely one to see such perturbations, due to the inverse-square law (see this term in the glossary).
Newtonian mechanics do not explain why Mercury's orbit is off if you predict its location over centuries. Albert Einstein
used his general relativity principle to account mathematically for this extremely minor deviation, and in essence gave a
very minor adjustment to newton's universal law of gravity. His mathematics is very complex, and hence scientists do not
need to use his adjustments in calculating orbits for vehicles sent anywhere in our solar system, but our GPS systems do
need to use it, or they would be off by about 7 miles per day.
To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30
nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted
by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond
accuracy.
physics
The most basic, fundamental, of the sciences. It is the study of the physical world and principles that apply.
It is the science of matter and energy and of interactions between the two, grouped in traditional fields such
as mechanics, thermodynamics, waves, optics, relativity theory, quantum mechanics and electromagnetism.
potential energy
When a state exists in which there is the potential to do work. For example, a spring when stretched or compressed can
do work, as can an object raised above the ground level and placed on a shelf. The first image below has elastic potential
energy and the latter has gravitational potential energy.
power
Power is the rate at which work is done; it is work done over a time interval.
A powerful
automobile engine can get the car up to 60 mph faster than a less powerful one.
P = W/t
The unit of power is the watt, in honor of James Watt--the inventor of the steam engine. .
precise
A measurement which is repeatable with close to the same values.
Accuracy
Precision
precision
In science, precision refers to how well a measurement can be repeated with close to the same results.
Accuracy
Precision
pressure
Pressure is a measure of force per unit of area. The unit of measure of pressure in the metric system is the "pascal"
(Pa), named after Blaise Pascal. The deeper you are in a fluid the greater the pressure due to the weight of the fluid
above you.
1 Pa = 1N/m2
projectile
A fired, thrown, or otherwise propelled object, such as a bullet, having no capacity for self-propulsion. Any object for which
after it is propelled into the air the only force acting upon it is gravity.
Pythagorean theorem
c2 = a2 + b2 The square of the hypotenuse is equal to the sum of the squares of the other two sides of a right-triangle.
resultant
The vector addition of two or more vectors.
revolve
To go around an external axis. To spin a ball on a string around your head is to revolve, while
a spinning top does not. It rotates instead.
rotate
To go around an internal axis. A spinning top rotates, while to spin a ball on a string around your
head does not. It revolves instead.
rotational inertia
Rotational inertia is the moment of inertia of an object. It has to do with the amount of mass and also where that mass is
located. Arranging the mass further from the center of gravity (CG) on the average makes this term greater. A moment is
a torque, which is a force times its lever arm. So the moment of inertia depends not just on the total amount of mass, as
inertia does, but also on the location of that mass... the distance it is from the CG. The man with his arms extended has
increased his rotational inertia (I) below.
rotational velocity
Rotational velocity is measured in radians/sec. It describes the rate of the circular motion. It can also be listed in
revolutions per minute or per second, but in physics and math it is listed in radians per second. The rotational velocity is
greater in the second image below.
satellite
An object which is continually falling around the earth or some other astronomical body, such as the sun. The image
below is from Newton’s famous Principia, which illustrates why satellites, such as the moon, remain in orbit.
scalar
A quantity which has only magnitude (amount), not direction, such as a gallon (volume), a kilogram (mass) or a
second (time).
science
The process of discovering facts, relationships and how things work and interact.
scientific method
A process for recognizing a problem, suggesting a solution (hypothesis), testing this hypothesis through
experiments and then interpreting your results.
second-law-of-motion
Newton's second law of motion:
The acceleration produced by the net force on a body is directly proportional to the resulting force producing it
and is inversely proportional to the mass of the body. The acceleration will be in the same direction as the
applied force.
F = ma
significant figure
A digit in a number which can be relied upon. Also referred to as a "significant digit." We learned the Atlantic
Ocean and Pacific Ocean method of determining the number of sig figs.
simulated gravity
Simulated gravity is a form of artificial gravity created by spinning a space station, using the centripetal
acceleration to emulate gravitational acceleration.
slope
The rise over the run or rate of change of a graph. The slope of a distance vs. time graph is velocity. The
slope of a velocity vs. time graph is the acceleration.
solar eclipse
A solar eclipse (eclipse of the sun) is produced when the moon comes between the sun and the earth, blocking
out all or part of the sun to the view of people in certain locations on the earth.
speed
The rate of change of displacement or rate at which one travels a certain distance. It is a scalar
quantity. Velocity also incorporates the direction of the speed of motion.
stable equilibrium
Stable equilibrium is an equilibrium of an object balanced such that it cannot easily lose its equilibrium,
because it is difficult to lower its center of gravity. A book laying on its back is such an example.
tangential velocity
Tangential velocity is measured in m/s and is directed along the outer path of motion. It is a velocity vector
tangent to the circle describing the circular motion. It is measured in m/s.
technology
The practical application of science. Science discovers and examines principles. Technology applies these
principles.
tension
A force on a line or rope produced by forces pulling on that line, measured in newtons.
terminal-velocity
With a falling object, when the air resistance (upward) equals the weight of that falling object (downward), we
say it has reached its terminal velocity. Once this has happened, it is no longer accelerating as it falls and is in
equilibrium.
theory
A synthesis of information involving tested hypotheses. A theory is a hypothesis which has been tested and
verified numerous times. Typically many changes to the hypothesis result.
third-law-of-motion
Newton's third law of motion:
"For every action force there is an equal (in magnitude) and opposite (in direction) reaction force."
Explanation:
In Newton's 3rd law of motion, he said that all forces are paired. The two paired forces are the action
force and the reaction force. The action force is the initiated force, or the applied force. The other force,
the reaction force, reacts to the initiated or applied force--the action force. Sometimes, they are
indistinguishable, while at other times it is clear which force initiated the action.
For example, your foot pushes back on the ground to start walking. The ground does not initiate the
action. So in this case the action force is your foot pushing back on the ground, and the reaction force is the
ground pushing forward on your foot--this is the responding force.
It is important to recognize that the action-reaction forces are always equal (in magnitude) and opposite (in
direction), whether or not an object is being accelerated or even moving! The third law of motion is not about
equilibrium.
topple
To tip over. If a plumb line hanging from the center of gravity does not dangle over the base of the object, it
will tip over.
torque
The product of a force and the lever arm applied. It is also referred to as the moment. It is measured in N-m.
universal gravitational principle
Or the universal law of gravitation. Sir Isaac Newton stated a law of gravity which applied to all objects in the universe. It
states that every object attracts every other object in the universe. His equation has a universal constant, G = 6.11 x 1011 Nm2/kg2.
Fg = G(m1m2)/r2
where r = the distance of separation of m1 and m2. m1 and m2 are the two
masses attracted to one another, and G is the universal gravitation constant
(see the equation).
unstable equilibrium
Unstable equilibrium is an equilibrium of an object balanced such that it can easily lose its equilibrium, because it is easy
to lower its center of gravity. A book balanced on its end is an example.
vector
A quantity which has both magnitude (amount) and direction.
For example, speed refers to the magnitude of the rate of change of displacement and is a scalar, while
velocity also incorporates the direction the object is moving, and is hence a vector.
velocity
Speed with direction. Hence it refers to the rate of change of displacement, but also incorpoprates the direction
of the motion.
volume
The amount of space an object takes up. Do not confuse with either mass or weight.
weight
A measurement of the force of gravity on an object. Mass is not a force. Weight is.
weightlessness
A person in orbit around the earth is continually falling around the earth and hence would experience a
sensation of weightlessness He appears to weigh nothing, though his mass has not changed.
work
Work is defined to be force times distance. From a physics perspective, if we don't move any object, we did
not work. It is also expressed as the product including the cos () when the angle of pulling is not 0 degrees.
W = Fd
W = Fd cos()