Newton's Second Law, Gravitational Force, and Friction

Vocabulary practice flashcards defining core terms, laws, units, forces, and friction concepts from chapters on Newton's second law of motion and applications of forces.

Newton's Second Law of Motion

The law stating that if the net external force acting on an object is not zero, the object will accelerate in the direction of the net force; the magnitude of the acceleration is directly proportional to the magnitude of the net force and inversely proportional to the mass of the object, expressed as $\vec{F}_{\text{net}} = m\vec{a}$.

Newton ($\text{N}$)

The SI unit of force, defined as the net force required to accelerate an object with a mass of $1\,\text{kg}$ at a rate of $1\,\text{m/s}^2$ ($1\,\text{N} = 1\,\text{kg}\cdot\text{m/s}^2$).

Slope of a Net Force versus Acceleration Graph

The physical property represented by the slope of a linear graph of net force ($\vec{F}_{\text{net}}$) plotted against acceleration ($\vec{a}$), which is equal to the mass ($m$) of the object ($\text{slope} = \frac{\Delta \vec{F}_{\text{net}}}{\Delta \vec{a}} = m$).

Pulley

A small, frictionless, and light wheel used to change the direction of the tension force in a rope or string without altering its magnitude.

Free Fall

The motion of a falling object where the only force acting on the object is gravity.

Cross-Sectional Area

The surface area seen when looking directly up at a falling object; objects with larger cross-sectional areas encounter greater air resistance than objects with smaller cross-sectional areas.

Air Resistance

A frictional force exerted by air that acts in a direction opposite to the velocity of an object moving through the atmosphere.

Terminal Speed

The maximum constant speed reached by a falling object when the upward force of air resistance becomes equal in magnitude to the downward force of gravity, resulting in zero net force and zero acceleration.

Force Field

A region of space surrounding an object that can exert an action-at-a-distance force on other objects placed within that region that are able to interact with that force.

Gravitational Field Strength ($\vec{g}$)

The force per unit mass acting on an object placed in a gravitational field, calculated as $\vec{g} = \frac{\vec{F}_g}{m}$; at Earth's surface, its average value is $9.8\,\text{N/kg [down]}$.

Mass

The quantity of matter in an object, measured in kilograms ($\text{kg}$) using a balance, which remains constant regardless of location or changes in gravitational field strength.

Weight

A measure of the force of gravity ($\vec{F}_g$) acting on an object, which is a vector quantity measured in newtons ($\text{N}$) using a spring scale or force sensor and varies with geographic location and gravitational field strength.

Static Friction ($\vec{F}_S$)

The force of friction exerted on a stationary object by a surface that prevents the sliding of the two surfaces relative to one another and keeps the object at rest.

Maximum Force of Static Friction ($\vec{F}_{S\text{max}}$)

The peak magnitude of static friction that must be overcome by an applied horizontal force to initiate the motion of a stationary object across a surface.

Kinetic Friction ($\vec{F}_K$)

The force exerted on a moving object by a surface, which always acts in a direction opposite to the direction of motion of the object.

Friction versus Applied Force Graph

A plot showing that static friction increases linearly with applied force with a slope of $1$ until reaching $\vec{F}_{S\text{max}}$, after which the object begins moving and the frictional force abruptly drops to a constant level of kinetic friction ($\vec{F}_K$).

Sliding Friction

A specific type of kinetic friction that occurs when a solid object scrapes or slides across a surface.

Rolling Friction

A specific type of kinetic friction that occurs when a round object rolls across a surface.

Fluid Friction (Drag)

The resistive force that opposes the motion of an object travelling through a fluid, such as a boat passing through water or an aircraft travelling through air.

Coefficient of Friction ($\mu$)

The dimensionless ratio of the magnitude of the force of friction ($F_f$) to the magnitude of the normal force ($F_N$) acting between two surfaces, defined as $\mu = \frac{F_f}{F_N}$.

Coefficient of Static Friction ($\mu_S$)

The ratio of the maximum force of static friction to the normal force, defined mathematically as $\mu_S = \frac{F_{S\text{max}}}{F_N}$.

Coefficient of Kinetic Friction ($\mu_K$)

The ratio of the force of kinetic friction to the normal force, defined mathematically as $\mu_K = \frac{F_K}{F_N}$.

Synovial Fluid

A specialized fluid found within the flexible joints of the human body that reduces friction between moving bones and cartilage layers, functioning similarly to motor oil or ski wax.

Forward Static Friction Force

The reaction force exerted by the ground on an athlete's foot when pushing backward against the ground from rest, which propels the person forward without slipping.

Combined Maximum Force of Static Friction

The total threshold force required to initiate motion in two or more attached objects, calculated by multiplying the coefficient of static friction by the total normal force of all combined masses ($F_{S\text{max}} = \mu_S m_T g$).