Glossary
Boundaries
Conceptual or physical lines that separate a system from its surroundings, helping to identify which forces are external or internal.
Example:
When studying a ball falling, the air around it and the ground below are outside the boundaries of the ball's system.
Closed System
A system that does not exchange matter or energy with its surroundings.
Example:
A perfectly insulated thermos containing hot coffee is an ideal example of a closed system because no heat or coffee leaves or enters.
Dynamic Equilibrium
A state where a system is in motion, but its velocity is constant, meaning the net force acting on it is zero.
Example:
A car cruising at a steady 60 mph on a straight highway is in dynamic equilibrium because the engine's thrust balances air resistance and friction.
Equilibrium
A state where a system experiences no net change over time, meaning its net force and net torque are zero.
Example:
A book resting motionless on a table is in equilibrium because the gravitational force pulling it down is balanced by the normal force pushing it up.
External Forces
Forces that originate *outside* a defined system and act upon objects within it, causing the system's overall motion or acceleration.
Example:
When a person pushes a box across the floor, the pushing force, friction, gravity, and normal force are all external forces acting on the box system.
Internal Forces
Forces that act between objects *within* a defined system, which do not affect the overall acceleration of the system as a whole.
Example:
The tension in the string connecting two blocks in an Atwood machine is an internal force if both blocks are considered part of the same system.
Net External Force
The vector sum of all forces acting on a system from outside its defined boundaries, which causes the system to accelerate.
Example:
For a car accelerating on a road, the engine's thrust minus air resistance and friction constitutes the net external force causing its acceleration.
Neutrons
Electrically neutral subatomic particles found in the nucleus of an atom, composed of one up quark and two down quarks.
Example:
Isotopes of an element have the same number of protons but different numbers of neutrons.
Newton's Second Law (ΣF = ma)
States that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
Example:
When a soccer player kicks a ball, the ball's acceleration is directly proportional to the force of the kick and inversely proportional to the ball's mass, as described by Newton's Second Law.
Open System
A system that exchanges matter and/or energy with its surroundings.
Example:
A rocket launching is an open system as it expels exhaust gases (matter) and releases heat (energy) into the environment.
Protons
Positively charged subatomic particles found in the nucleus of an atom, composed of two up quarks and one down quark.
Example:
The atomic number of an element is determined by the number of protons in its nucleus.
Quarks
Fundamental particles that combine to form composite particles like protons and neutrons, held together by the strong force.
Example:
Scientists use particle accelerators to smash atoms apart and study the behavior of quarks and other fundamental particles.
Subatomic Particles
Particles that are smaller than an atom, such as protons, neutrons, and electrons.
Example:
The study of nuclear physics often delves into the interactions and properties of subatomic particles within the nucleus.
System
A collection of objects chosen for analysis, treated as a single unit for the purpose of studying its motion or interactions.
Example:
When analyzing a car crash, the car and its occupants might be defined as a single system to study the overall impact.
Total Mass
The sum of the masses of all individual objects within a defined system, used when applying Newton's Second Law to the entire system.
Example:
If a train consists of a locomotive and ten cars, the total mass of the train system is the sum of the mass of the locomotive and all ten cars.