AP Physics C: Mechanics - Newton's Laws and Gravitation

Vocabulary review flashcards covering Newton's First, Second, and Third Laws, free-body diagrams, universal gravitation, gravitational fields, shell theorem, and apparent weight based on Grade 11 AP Physics C Mechanics course materials.

Newton's Third Law

The law stating that whenever an object exerts a force on another object, the second object exerts a force on the first object of equal magnitude and opposite direction: $F_{\text{A on B}} = -F_{\text{B on A}}$.

Action–Reaction Pair

A pair of simultaneous forces of the exact same type that are equal in magnitude, opposite in direction, and act on two different interacting objects, meaning they can never cancel each other.

Third-Law Force Pairs vs. Balanced Forces Diagram

A diagram illustrating that balanced forces act on the same object ($F_N$ and $F_g$ on a resting book) and can cancel ($\Sigma F = 0$), whereas third-law pairs act on two different objects (table pushes book up while book pushes table down; Earth pulls book down while book pulls Earth up) and never cancel.

Balanced Forces

Forces acting on the same object that sum to zero ($\Sigma F = 0$), resulting in zero acceleration ($a = 0$) such that the object remains at rest or moves at a constant velocity.

Newton's First Law

The principle stating that if the net force exerted on a system is zero ($\Sigma F = 0$), the velocity of the system's centre of mass remains constant ($a = 0$), meaning an object at rest stays at rest and an object in motion continues moving in a straight line at constant speed.

Inertia

The tendency of an object to maintain its velocity and resist any change in its state of motion; it depends solely on the object's mass and does not depend on its speed.

Translational Equilibrium

The state in which the net force acting on an object in every coordinate direction is zero ($\Sigma F_x = 0$ and $\Sigma F_y = 0$), resulting in zero acceleration.

Newton's Second Law

The principle stating that the acceleration of an object is directly proportional to the net force acting on it, inversely proportional to its mass, and directed along the net force: $\Sigma F = F_{\text{net}} = ma$.

Newton (Unit)

The standard SI unit of force, denoted $N$, defined as the net force that imparts an acceleration of $1\,m/s^2$ to a mass of $1\,kg$ ($1\,N = 1\,kg\cdot m/s^2$).

Free-Body Diagram (FBD)

A diagram representing an object as a dot and showing only the external forces exerted on that object as arrows starting at the dot, never including '$ma$' or a 'force of motion'.

Tension in a Massless String

The uniform pulling force transmitted throughout a string of negligible mass, where $\Sigma F = (0)a = 0$ for any segment so that the pull at one end equals the pull at the other end.

Contact Force in a Two-Block System

The contact force exerted between two pushed blocks of masses $m$ and $2m$ on a frictionless horizontal floor under an applied force $F$, having a magnitude of $F_{\text{contact}} = \frac{F}{3}$ regardless of the value of $m$.

Negative Acceleration

An acceleration vector directed in the chosen negative coordinate direction; it indicates slowing down only when velocity and acceleration have opposite signs, and speeding up when both share the same sign.

Newton's Law of Universal Gravitation

The law stating that the gravitational force between two masses $m_1$ and $m_2$ is directly proportional to their masses and inversely proportional to the square of the distance $r$ between their centres: $F_g = G\frac{m_1 m_2}{r^2}$.

Universal Gravitational Constant ($G$)

The fundamental constant of proportionality in universal gravitation, equal to $G = 6.67 \times 10^{-11}\,N\cdot m^2/kg^2$.

Inverse-Square Law Graph (Gravitation)

A graphical representation showing that gravitational force $F_g$ drops inversely with the square of the separation between centres $r$, diminishing from $F$ at $r$ to $F/4$ at $2r$, $F/9$ at $3r$, and $F/16$ at $4r$.

Gravitational Field

A region of space around a mass where any other mass experiences a gravitational force, represented by vector field lines directed radially inward toward the mass.

Gravitational Field Vector Map

A visual vector diagram surrounding a mass $M$, where field arrows point inward toward $M$ and diminish in length as radial distance increases according to $g \propto \frac{1}{r^2}$.

Gravitational Field Strength ($g$)

The gravitational force per unit mass exerted at a specific point in space, defined as $g = \frac{F_g}{m} = \frac{GM}{r^2}$ and measured in $N/kg$ or $m/s^2$.

Weight ($W$)

The downward gravitational force exerted on an object by a much larger astronomical mass (such as Earth or Mars), calculated as $W = mg$, which varies with the local gravitational field strength while mass remains invariant.

Apparent Weight

The magnitude of the supporting contact force (such as the normal force $F_N$ or scale reading) acting on an object, which differs from actual gravitational force ($mg$) whenever the object accelerates vertically.

Free Fall Weightlessness

The apparent absence of weight ($F_N = 0$) observed when an object and its surroundings are in free fall under gravity alone, such as astronauts orbiting on the ISS where $g \approx 8.7\,N/kg$.

Newton's Shell Theorem

A theorem establishing that a uniform spherical shell of mass attracts an external object ($r \ge R$) as if all its mass were concentrated at the centre ($g = \frac{GM}{r^2}$), while inside the hollow shell ($r < R$), the gravitational forces from all segments cancel completely ($g = 0$).

Gravitational Field Inside a Uniform Solid Sphere

The internal gravitational field strength of a solid sphere of radius $R$ at a distance $r \le R$ from its centre, expressed as $g = \frac{GMr}{R^3}$, which increases linearly with $r$ from zero at the center to a maximum at the surface.