# Glossary

Acceleration: The rate of change of an object's velocity with respect to time. Also, change in velocity divided by the time interval over which it occurred.

Air Density: Density is defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume. Ratio of an air sample's mass to its volume.

Air Resistance (also Air Drag and Drag): A resistive (frictional) force, which is the result of a moving object colliding with air molecules.

Boundary Layer: The stable layer of air on the leading surface of an object moving through the air.

Center of Mass: 1. The point in a system of bodies or an extended body at which the mass of the system may be considered to be concentrated and at which external forces may be considered to be applied. 2. Point representing the mean position of the matter in a body.

Coefficient of Restitution (COR): The ratio of the velocity of an object rebounding from the surface of a hard, immovable object to its incident velocity. The coefficient of restitution is used to measure the elasticity of the collision between two objects (for example: a baseball and a bat).

Conservation of Energy: Principle stating that energy can be converted from one form into another but it cannot be destroyed or created; the total amount of energy remains constant.

Drag Coefficient: A measure of the aerodynamic sleekness of an object. Drag coefficient is signified by (Cd). The lower the number, the greater the aerodynamic efficiency.

Dynamics: The branch of physics concerned with the study of motion.

Elastic Collision: A collision in which objects rebound from each other with negligible loss of kinetic energy.

Elastic Potential Energy: Energy that is due to compression or deformation of an elastic material such as a spring.

Energy: The capacity for doing work or causing changes in matter.

Force: A physical quantity that results in accelerating or deforming an object.

Friction: A force that resists the relative motion of objects that are in contact with each other.

Gravitational Acceleration: At the earth's surface all objects with mass accelerate downward at the rate of g = 9.8 m/s2 (32 ft. per sec per sec).

Heat Energy: A form of energy representing the kinetic energy of atoms and molecules in an object.

Humidity: The measure of the amount of water vapor contained in a sample of air.

Inelastic Collision: A collision in which the colliding objects stick together after impact.

Inertia: The property of matter that causes it to resist any change in its motion. An object's inertia is directly proportional to its mass.

Kinematics: The part of dynamics that describes motion without regard to its causes.

Kinetic Energy: Energy possessed by an object because of its motion.

Magnus Effect, The: The Magnus effect is the name given to the physical phenomenon whereby an object's rotation affects its path through a state of matter, in particular, air. The perpendicular force associated with the change in path is commonly referred to as the Magnus force.

Mass: A measure of the quantity of matter in an object; its inertia or resistance to change in motion.

Momentum: The product of the mass and velocity of a moving body.

Net Force: The vector sum of all forces acting upon an object.

Newton's Three Laws of Motion

• Newton's First Law of Motion: A body at rest or in uniform motion in a straight line will remain at rest or in the same uniform motion unless acted upon by an external force; also called the law of inertia.

• ·Newton's Second Law of Motion: The acceleration of a body is directly proportional to the net force exerted on the body, is inversely proportional to the mass of the body, and has the same direction as the net force; also called the law of acceleration.

• Newton's Third Law of Motion: If one body exerts a force on a second body, then the second body exerts a force equal in magnitude and opposite in direction on the first body; also called the law of interaction.

Parabolic: The shape of a projectile's trajectory, neglecting air resistance.

Pressure: Force per unit area.

Projectile Motion: Any object moving freely through the air without the aid of any sort of propulsion system. A typical example would be a thrown object (javelin, shot put, baseball).

Resultant Vector: A single vector that represents the sum total of several component vector quantities.

Rotational Velocity: Time rate of change of an object's angular position. It is commonly defined in terms of the number of revolutions the object makes in a given time period (i.e., RPM or revolutions per minute).

Smooth Air Flow: Airflow that is primarily in one direction; sometimes called laminar flow.

Speed: Rate of change of an object's position.

Temperature: The average kinetic energy of atoms and molecules in an object.

Trajectory: The path of a projectile or other moving body through space.

Turbulent Air Flow: Airflow that is smooth, or laminar, in nature. Evidenced by swirling, multidirectional airflow patterns.

Vector: A quantity in which both the magnitude and the direction must be stated. Force, velocity and field strength are examples of vector quantities. Distance and speed are scalar quantities but displacement and velocity are vector quantities.

Velocity: Speed in a specified direction.

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