Definitions Glossary — Laws of Motion & Friction — Notes

Term	Definition
Inertia	The tendency of a body to resist any change in its state of rest or uniform motion. Quantified by mass.
Newton's First Law	A body remains at rest or in uniform motion in a straight line unless acted upon by a net external force.
Newton's Second Law	The net force on a body equals the rate of change of its momentum: F = dp/dt = ma (for constant mass).
Newton's Third Law	For every action, there is an equal and opposite reaction acting on a different body: F_AB = −F_BA.
Linear Momentum	Product of mass and velocity: p = mv. Vector quantity. SI unit: kg m/s. Dimension: [ $M^{1}$$L^{1}$$T^{-1}$ ].
Impulse	The product of force and the time interval for which it acts: J = F· $\Delta t$ = $\Delta p$ . SI unit: N·s.
Free Body Diagram (FBD)	A diagram showing a single body with all external forces acting on it, used to apply Newton's laws.
Apparent Weight	The normal force experienced by a body in an accelerating reference frame. W' = m(g ± a).
Weightlessness	Condition where apparent weight is zero; occurs in free fall or orbit where a = g.
Atwood Machine	A system of two masses connected by a string over a frictionless pulley; used to study Newton's laws.
Static Friction (f_s)	Self-adjusting friction that prevents relative motion between surfaces; 0 ≤ f_s ≤ μ_s N.
Kinetic Friction (f_k)	Constant friction acting on a sliding body: f_k = μ_k N. Always less than maximum static friction.
Rolling Friction (f_r)	Very small friction acting on a rolling body: f_r = μ_r N. Much less than kinetic friction.
Coefficient of Static Friction (μ_s)	Dimensionless constant equal to (limiting static friction) / (normal force).
Coefficient of Kinetic Friction (μ_k)	Dimensionless constant equal to (kinetic friction force) / (normal force). Always μ_k < μ_s.
Angle of Repose	The maximum angle of an inclined plane at which a body can remain stationary. tan θ = μ_s.
Limiting Friction	The maximum value of static friction just before motion begins: f_s(max) = μ_s N.
Centripetal Force	The net inward force required to maintain circular motion: F_c = $mv^{2}$ /r. Not a separate force — provided by real forces.
Banking Angle	The angle at which a road is inclined to allow vehicles to negotiate a curve without friction: tan θ = $v^{2}$ /(rg).
Action-Reaction Pair	Two forces that are equal in magnitude, opposite in direction, and act on two DIFFERENT bodies (Newton's Third Law).