## Glossary (work in progress, new terms will be added throughout the semester)

**acceleration**- Acceleration is the rate that velocity is changing with respect to time. Acceleration is a vector quantity, i.e. it has a magnitude and direction. Typical units: m/s² (SI) or ft/s² (USCS).

**aeronautics**- Aeronautics is the science/engineering of systems capable of flight within a planetary atmosphere, including fixed wing aircraft, rotorcraft, lighter-than-air vehicles and missiles.

**aerospace engineering**- Aerospace engineering is the field of engineering that focuses on the design, construction, testing, analysis, and operation of aircraft and spacecraft, i.e. vehicular systems that operate within and beyond a planetary atmosphere. Aerospace engineering is an inherently interdisciplinary field that involves, at its core, the study of complex engineering systems and the phenomena that affect the behavior of complex engineering systems.

**astronautics**- Astronautics is the science/engineering of systems capable of flight beyond a planetary atmosphere; including launch vehicles, spacecraft, and other space related technology.

**dimensions**- Dimensions are generalized, measurable characteristics of a physical quantity.

**drag**- Drag is one of the 4 principal forces acting on an aircraft or any aerospace system that operates within a planetary atmosphere. Drag is the aerodynamic force (a vector quantity) exerted on an aircraft in the direction rearward and parallel to the relative wind. The effect of the drag force is to decelerate the aircraft. Typical units: N (SI) or lbf (USCS)

**drag coefficient**- The drag coefficient is a dimensionless number that is proportional to the drag force over a surface or body.

**dynamic pressure**- Dynamic pressure is the kinetic energy per unit volume of a fluid element, defined as one-half the product of the density times the square of the freestream fluid speed. Typical units: Pa (SI) or psi (USCS).

**engineering**- Engineering is the profession that uses mathematical, computational, and/or physical models to create, design, test, analyze, and/or predict the behavior of technology and technological systems. It is a profession that is fundamentally about creating practical knowledge and problem solving.

**force**- A force is the push or pull experienced by an object during its interaction with another object. When the forces acting on an object are not in balance, i.e. not in equilibrium, the object will accelerate. Force is a vector quantity, i.e. it has a magnitude and a direction. Typical units: N (SI) or lbf (USCS). In a much broader sense, a force may refer to any phenomenon that is capable of changing the state of the system.

**free body diagram**- A free body diagram is a schematic representation of the forces and moments acting on an object (including the direction of the forces/moments and ideally the relative magnitude of the forces). Such diagrams are used to conceptualize the impact that support structures, weight, drag, lift, thrust, etc. have on an object.

**equilibrium**- A system is in equilibrium when all the phenomena that can cause the state of a system to change are in balance. For example, mechanical equilibrium implies that there are no unbalanced forces acting on a system.

**length**- Length refers to the distance between 2 points in space. Typical units: m (SI) or ft (USCS)

**lift**- Lift is one of the 4 principal forces acting on an aircraft or any aerospace system that operates within a planetary atmosphere. Lift is the aerodynamic force (a vector quantity) exerted on an aircraft in the direction upward and perpendicular to the relative wind. Lift is used to overcome the weight of the aircraft during flight. Typical units: N (SI) or lbf (USCS)

**mass**- Matter refers to the amount of matter in an object. Typical units: kg (SI) or lbm (USCS)

**mechanics (engineering)**- Engineering mechanics (also known as engineering science or applied physics) is a field of study that focuses on the application of mathematical, scientific, and engineering principles to understand how forces fundamentally affect the behavior and/or motion of a system. Engineering mechanics is the broad foundation on which the study of most engineering disciplines is built.

**momentum**- An object's momentum, a vector quantity, is the product of its mass times its velocity. Typical units: kg*m/s (SI) or lbm*ft/s (USCS).

**moment (also known as torque)**- Moment, a vector quantity, is defined as the product of the force acting on an object times the distance between where the force is applied and a reference point in space. Typical units: N*m (SI) or ft*lbf (USCS).

**Newton's First Law of Motion**- An object will remain at rest or will remain in motion with the same speed & direction, i.e. velocity, unless acted upon by an unbalanced force.

**Newton's Second Law of Motion**- The sum of the forces acting on object (a constant mass system in an inertial reference frame) is equal to the rate that the object's momentum is changing with respect to time. The object's momentum is the product of the object's mass times its velocity. In simplest terms, Newton's Second Law of Motion says that the sum of the forces acting on an object is equal to the object's mass times its acceleration.

**Newton's Third Law of Motion**- For every action, there is an equal and opposite reaction. In other words, if an objects exerts a force on a second object, then the second object also exerts a force of equal magnitude and opposite direction on the first object.

**temperature**- Temperature is a measure of energy and is proportional to the average internal energy of matter. Typical units: °C, K (SI) or °F, °R (USCS)

**terminal velocity**- Terminal velocity is the speed of a free-falling object when the drag force is equal to the weight of the object, i.e. there is no net force on the object. By Newton’s Second Law, when the object reaches its terminal velocity, it is no longer accelerating. Typical units: m/s (SI) or ft/s (USCS).

**thrust**- Thrust is one of the 4 principal forces acting on an aerospace system. Thrust is the force (a vector quantity) acting on an object that is forward and generally taken to be parallel to the relative wind. The effect of the thrust force is to accelerate an aerospace system, thrust is used to overcome drag (and possibly weight depending on the flight direction). Typical units: N (SI) or lbf (USCS).

**time**- Time is a measurable period during which an event or process occurs. Typical units: s (SI) or s (USCS)

**torque**- see the definition for moment

**vector**- A vector is a quantity that has both a magnitude and a direction. Force, velocity, and acceleration are examples of vectors.

**vector (unit)**- A unit vector is the concept used to characterize the direction of vector. For example, in Cartesian coordinates a unique unit vector is used to represent each of the 3 principal axes.

**velocity**- Velocity is the rate that position is changing with respect to time. Velocity is a vector quantity, i.e. it has a magnitude and direction. Typical units: m/s (SI) or ft/s (USCS).

**scalar**- A scalar is a quantity that has only a magnitude. Temperature, area, density (mass per unit volume), mass, and volume are all scalar quantities.

**state**- The state of a system is specified by the set of properties needed to fully describe the condition of the system in equilibrium. For example, the thermodynamic state of a "simple compressible" system is completely specified by knowledge of only 2 intensive (or specific) properties. Knowledge of these properties allows one to determine all the other properties of the system.

**units**- Units are the magnitudes (based on arbitrary, but widely accepted scales) assigned to physical dimensions.

**weight**- Weight is one of the 4 principal forces acting on an aerospace system. Weight is the force (a vector quantity) acting on an object in the presence of a gravitational force field. The magnitude of the weight is equal to the product of the mass times the acceleration due to gravity, i.e. W=mg. Weight always acts in the same direction as the vector associated with the acceleration due to gravity, i.e. "downward." Typical units: N (SI) or lbf (USCS).