Thinking Like a Physicist: Introduction and Vocabulary
An exploration of how physicists make sense of the natural world, with AP Physics 1 and 2 vocabulary.
The goal of physics: To describe and explain patterns order in the natural world.
Object: Anything in the universe whose internal structure, if it exists, is being ignored.
Example: A baseball has internal structure, but we can ignore its structure when determining whether a hit is a home run.
The word “particle” is sometimes is used in place of the word “object,” especially when the object’s volume is negligibly small.
Particle: An object of negligible volume.
Fundamental particle: An object with no discernable internal structure.
An electron is a fundamental particle. Protons and neutrons are not, as they are made of smaller particles called quarks and gluons. You ignored the internal structure of protons and neutrons in chemistry, and treated them as objects.
Extended object: An object whose size, shape, and density are important in defining relevant properties, such as moment of inertia.
System: A collection of objects whose internal structure is being studied.
Example: To fix an engine, we must take it apart, meaning it has to be treated as a system.
Property: An attribute of an object or a system that is measurable.
Example: The capacity of a soda bottle might be 2.0 Liters. Capacity is the property…
Quantity: An amount associated with a property.
…and 2.0 Liters is the quantity. Important properties / quantities in physics include distance, time, mass, acceleration, force, energy and momentum.
Measurement: Comparison of an unknown quantity to a better-known quantity.
Example: The lengths marked on a ruler are better known that those of the object being measured, but the ruler is not perfect, leading to measurement uncertainty.
Interaction: Event in which objects or systems influence each other.
Quantities possessed by objects may change as a result of an interaction. For example, a car accident changes the speeds of the cars. (cars = objects, accident = interaction, speed = property).
Interactions typically involve forces (pushes and pulls).
Conservation law: Rule that defines how objects interact in which a total quantity remains constant.
Examples: Conservation of energy, conservation of mass, conservation of charge and conservation of momentum.
Systems can be closed or open with respect to a given quantity.
Closed system: The quantity remains constant over time. There are no interactions that result in a quantity increase or decrease.
Open system: The quantity changes due to interactions with other objects or systems. If the quantity is conserved, the change in quantity possessed by the system equals the amount transferred to or from the system.
Example: A perfect thermos bottle at rest on a table would be a closed system for energy… no energy in or out. In reality, no thermos bottle is perfect, so the system (the thermos and hot liquid inside) loses energy to the air around the bottle.
Isolated: There are no interactions with outside objects or systems. No external forces are present.