# Physics Glossary

### A

applied force - The push or pull of a human or machine.

average acceleration -- The slope of the velocity-time graph, or a_avg = (delta v/delta t).  This is a way to find average acceleration over any time interval.

average velocity -- For constant velocity, it is the slope of the position-time graph. Mathematical definition is v_avg=(delta x)/(delta t). This is a way to find average velocity over any time interval.

amplitude--The distance from some central reference point, without regard to direction.  The side-to-side angle of a pendulum's swing, or the height of wave crests (and depth of the troughs).

angular speed - the average angular speed is the angle moved divided by the elapsed time "delta theta"/"delta t".  The instantaneous angular speed is the magnitude of the angular velocity "absolute value of omega". SI units are rad/s.

angular velocity - The rate of change of angle with respect to time. May be average or instantaneous. The direction is typically given as positive if the object moves counter-clockwise around the axis of rotation or negative if it moves clockwise about the axis of rotation. SI units are rad/s.

### B

balanced forces - Forces that sum to zero net force. An object with balanced forces moves with constant velocity (which may be at rest). Forces may be balanced in one direction, but unbalanced in another direction.

### C

clock reading -- How you would determine an instant in time, by glancing at the timekeeper and recording what it shows at that moment.

constant -- Any physical quantity that maintains the same value over a time interval. In a lab, something that you try to keep from changing during data collection.

curve -- A function that is graphed, may actually be curved or it may be linear.

component vector -- a quantity we calculate to represent what portion of a vector is purely horizontal (X-component) or purely vertical (Y-component).

conservation/conserved -- If a physical quantity in a closed system has the same value both before and after a process, that quantity is said to be conserved. Examples may include: mass, energy, linear momentum, angular momentum, charge.

### D

delta -- Greek letter (looks like a triangle) that represents change in a physical quantity. For example, "delta X" is (X - X_0; OR X_2 - X_1; OR X_final - X_initial)

direction is denoted by whether the displacement is positive or negative. If the change is in two dimensions (or more), we specify an angle relative to our Cartesian coordinate system.

directly proportional -- Two variables that have a linear relationship.discrepancy - see percent difference

dissipated energy -- energy that spreads out in the environment and is not easily recovered

displacement -- Change in position (delta x or xf - xi). Displacement is a vector that has magnitude and direction.

### E

elastic potential energy -- energy stored due to the stretch or compression of an elastic object (an object that will regain it's original shape after being stretched or compressed)

energy -- The agent of change in the universe. Modeled as a somewhat mysterious thing that behaves like an invisible, undetectable substance. Energy can be stored in many ways. The names of the storage mechanisms may be familiar: kinetic energy (translational or linear), gravitational potential energy, chemical potential energy, elastic potential energy, rotational kinetic energy. Energy is always conserved. For any closed system (nothing can enter or leave) the total amount of energy remains constant. When energy is transferred in or out of a system, "work" is the name for the process of transferring energy. The amount of work is equal to the change in energy of the system.

### F

force - One end of the interaction between two objects.

free-body diagram - A diagram representing the external forces exerted on an object, where the center of mass of the object is shown as a dot, and vector arrows are drawn and labeled to represent the forces.

freefall - When a dropped object accelerates only due to the force of gravity on a planet (air resistance is too small to consider), it is said to be in freefall. Objects that are thrown or tossed are also in a form of freefall because they also accelerate primarily due to the force of gravity, but we are more likely to call those  projectiles.

freefall acceleration - The constant rate of acceleration of an object that falls near the surface of a planet, neglecting air resistance. On Earth, it is 9.8 m/s/s downward (32 ft/s/s downward in Imperial units).

friction - a contact force that opposes the tendency to relative motion. See kinetic/sliding friction, static friction, rolling friction, drag. Note: In introductory physics, friction usually means either kinetic or static friction.

frequency - The number of cycles per second of a process that repeats. Symbolized by an f  and measured in Hertz (Hz) or 1/s.  Equivalent to 1/T.

### G

gravitational potential energy -  the energy stored in the gravitational field between two objects with mass that are separated (local gravitational potential energy is stored when an object is at or near the surface of a planet it is written mgy; non-local gravitational potential energy is stored when  the two objects are "in space", it is written -Gmm/r.

### I

Ideal Gas --  a simplified model of gas where gas particles have no internal structure; gas particles only interact when in contact with something; -gas particles collide elastically (collisions conserve momentum and kinetic energy)

inertia -- The tendency of matter to "hold back" when you try to make it move and the tendency of matter to "keep going" in the direction it is moving.

instantaneous velocity -- The velocity at any single clock reading. The slope of a line tangent to a point on a position-time graph.

inversely proportional -- Two variables that have an inverse relationship (y=k/x).

### J

joules -- The SI unit of energy, written as capital J (no italics) or lower-case joules. Named after scientist James Prescott Joule. Joules are the same unit as newton-meters, N-m.

### K

Kinetic energy -  energy in a moving object

Kinetic theory --matter is made up of particles (atoms, molecules, subatomic particles);  the particles are always in random motion; the temperature is a measure of average kinetic energy of the particles; at any one moment in a sample of gas, an individual particle may have just about any speed, but average kinetic energy remains same

### L

length -- What you measure with a ruler, meter stick, or tape measure.

linear speed -- A synonym for "speed." The speed of something tangent to its path. If the path is a straight line, the tangent is along the path.

linearization -- The process of performing a mathematical operation on a list of data in order to make a non-linear graph into a linear function.  See the Graphical Analysis page for more information.

linear velocity -- the linear speed and direction of something.

### M

magnitude -- The size of something (a physical quantity, like velocity), without regard to its other characteristics (direction, for instance).

mass-- Inertia quantified. A quality of mass that tends to resist changes in motion. What a balance measures.

mechanism -- The way we describe how something happens.  "Force is the mechanism for energy transfer."

medium -- The material through which a wave is propagated. May be a liquid, solid, or gas, as in mechanical waves, or it may be space itself, as in electromagnetic waves.

model -- A representation of an object, system, or concept that has some of the characteristics of the actual object, system, or concept. Constructed b/c it can make predictions about the actual thing, communicate meaning, or be used to test ideas. Often we make models when the real thing is unavailable, difficult to work with, or we are looking for a shorthand way to communicate about something.

momentum -- Another physics quantity that is conserved in a closed system. A vector quantity that is the product of mass and velocity. SI units are kg*m/s.

### N

negligible -- Anything that can be neglected/ignored in the physics of a situation. It's there, but not enough to matter for our analysis.

Newton's First Law - If the net external force on an object or system is zero, the object moves with constant velocity (which may be zero).

Newton's Second Law - In words, an acceleration results when a net force is exerted on an object/system. The acceleration is proportional to the net force and inversely proportional to the mass of the object/system. (Equation a = F_net/m)

Newton's Third Law - If object A exerts a force on object B, then Object B exerts a force on Object A. The amount of the force each object exerts is identical, but the directions are opposite each other. (In equation form, F_A on B = - F_B on A). The reason this is true is because forces are the result of interactions between two objects. The interaction is the same for both objects, but directed oppositely (you push me, I push you back in the opposite direction).

### O

operational definition -- Many physical quantities don't exist as things, but only when a human performs a certain operation to measure them. For example, area is defined by the process of finding out how many unit squares (1 meter on a side, for example) fit into a two-dimensional shape.

### P

paradigm -- A pattern. An example that can be used for comparison.

percent difference/percent error/experimental discrepancy - the difference between an experimental result and the theoretical prediction, calculated as a percentage:

(experimental result - theoretical prediction)/theoretical prediction  X 100

period - the time for one complete cycle of a process that repeats. Symbolized by a T. Measured in seconds (s) and equivalent to 1/f.

position -- The distance and direction (typically given as negative or positive) from a reference point.

proportionality -- Any two variables that change in a constant ratio  (k=y/x)  have a relationship that may be described as being proportional. Any two variables that produce a linear graph that passes through (0,0) are proportional (y=kx) .

### Q

quantity -- Something that can be measured that conveys information about a physical system.  Examples: length, mass, time.

### R

ray or ray approximation - A model of light in which light travels only in straight lines and is represented as thin lines with arrowheads showing the direction of travel. Useful in tracing images and deducing image characteristics

reference point -- A point in space designated as zero for measurements of position.

representation -- Words, equations, graphs, or pictures intended to communicate essential information about a model.

### S

scalar -- Anything you might measure that can be designated by a single number. Physical quantities without direction. Examples: time, temperature, mass.

speed --  Average speed is often interpreted as total distance/total time. This is a scalar, and an nswer to the question "How fast?" Instantaneous speed is the magnitude of the velocity "absolute value of v".

symmetry -- Something that can be folded across one or more axes (or lines) and what is folded on itself matches up. The human body is roughly "symmetric," meaning the right side is almost exactly the same as the left side.

system - Any object or objects that you are interested in.  If there are multiple objects, then these objects interact in some way.

### T

tension -- The name we use for changes caused by the stretching or extension of an object. Think about a piece of string  with forces applied at opposite ends, directed away from the center of the string. The length of the string will change very slightly, and the ends of the string will pull back in the opposite direction on the other objects applying force to string. Energy will have been transferred to the string. The tension in the string does not have a single direction, but each force of tension of the ends of the string on the other objects connected to it will have a direction. Objects that may be under tension and will exert tension forces include (but are not limited to) ropes, chains, rods, beams, and springs.

test plot -- This is the graph produced when you "try out" a mathematical operation to linearize data that plots as a curve.  If the test plot is successful, then it will be linear.

time -- What a clock measures.

time interval -- Delta t, or change in time (t2 - t1).

trajectory - The path of an object moving through space, usually dealt with in this class by a representation (a graph, picture, or motion map).

### U

unit -- The standardized system for measuring a physical quantity.  "Standard" means people agree upon what is the basic amount.  For instance, the kilogram is the unit of mass, and in Paris, there is a chunk of metal that everybody agrees has a mass of one kilogram.  Examples: for length, meters or feet, for time seconds or minutes.

uncertainty -- For this class, an estimate of the amount of variation in a measurement. Since all honest measurements vary due to circumstances beyond the scientists' control, there is always some uncertainty. See http://www.nist.gov for more info.

### V

variable -- A symbol that stands for a physical quantity.  Examples: m for mass, x for horizontal length, y for  a vertical length, t  for a time.

vector -- A physical quantity with magnitude (size) and direction.

### W

wave -- A disturbance in a medium that transmits energy and momentum through coupled oscillations.

work(ing) -- Energy transferred into or out of a system.

By Marc Reif - Any errors or omissions are my fault.  Please let me know so I can correct them.