Work, Energy and Power - Vocabulary Flashcards (Video Notes)

Key terms with concise definitions covering work, energy, power, potential/kinetic energy, collisions, and related concepts from the lecture notes.

Work

Energy transferred to/from a body by a force causing displacement in the force direction; W = F s cos θ (scalar product of force and displacement).

Displacement

Change in position of a body from initial to final point; a vector quantity.

Constant force

Force with fixed magnitude and direction; for constant F, work is W = F s (when θ = 0) or W = F s cos θ in general.

Variable force

Force whose magnitude or direction changes with position; work is found by integrating: W = ∫ F(x) dx (area under F–x curve).

Positive work

Work done when force and displacement are in the same direction (W > 0).

Negative work

Work done when force opposes displacement (W < 0).

Zero work

Work is zero when there is no displacement or when force is perpendicular to displacement.

Dimensional formula of work

M L^2 T^-2; derived from W = F s with appropriate dimensions.

Joule (J)

SI unit of work; 1 J = 1 N·m.

Erg

CGS unit of work; 1 erg = 1 dyne·cm; 1 J = 10^7 erg.

Kilogram–meter (kg·m)

Gravitational unit of work; 1 kg·m ≈ 9.8 J (work done against gravity moving 1 m).

Gram–centimeter (g·cm)

CGS gravitational unit; 1 g·cm = 980 erg.

Potential energy

Energy stored due to position or state (e.g., height, deformation, or electrical configuration).

Gravitational potential energy

U = m g h (reference at Earth’s surface: U = 0 at h = 0, increases with height).

Elastic potential energy

Energy stored in a spring: U = (1/2) k x^2; F = -k x (Hooke’s law).

Conservative force

Work done depends only on initial and final positions; can define a potential energy function (e.g., gravity, electrostatic, central forces).

Non-conservative (dissipative) force

Work depends on the path; energy dissipated as heat, sound, etc. (e.g., friction, viscous drag).

Central force

A force directed toward/away from a fixed point, depending only on distance to that point (e.g., gravity, electrostatic).

Conservation of mechanical energy

In a conservative force field, the sum K + U remains constant (no non-conservative losses).

Translatory equilibrium

Condition where the net force on a body is zero.

Stable equilibrium

Slight displacement leads to a restoring force toward the equilibrium position.

Unstable equilibrium

Slight displacement leads to force pushing away from equilibrium.

Neutral equilibrium

Slight displacement causes no restoring force; potential energy is locally constant.

Kinetic energy

Energy of motion: K = (1/2) m v^2.

Work–energy theorem

Work done on a body equals the change in its kinetic energy: W = ΔK.

Momentum

Linear momentum p = m v; relates to kinetic energy via K = p^2/(2m).

Power

Rate of doing work; instantaneous power P = dW/dt = F·v; average power = ΔW/Δt.

Forms of energy

Mechanical energy (K + U); plus heat, internal, electrical, chemical, and nuclear energy.

Mass–energy equivalence

E = m c^2; mass and energy are interchangeable; relativistic mass m = γ m0 increases with velocity.

Transformation of energy

Energy changes form (e.g., electrical → heat, chemical → mechanical) without violating total energy.

Elastic collision

Collision in which both linear momentum and kinetic energy are conserved.

Inelastic collision

Momentum is conserved, kinetic energy is not; some energy dissipated as heat/sound/light.

Perfectly inelastic collision

Colliding bodies stick together after impact; momentum conserved, kinetic energy not; e = 0.

Coefficient of restitution (e)

e = (relative speed of separation)/(relative speed of approach) along the line of impact; e = 1 elastic, 0 < e < 1 inelastic, e = 0 perfectly inelastic.

Line of impact

Line perpendicular to the tangent at the point of collision; direction along which impact forces act.

One-dimensional elastic collision

Collision along a straight line with both momentum and kinetic energy conserved; final velocities given by masses and initial velocities.

Two-dimensional elastic collision

Collision in a plane; momentum and kinetic energy are conserved in both directions; directions after collision depend on masses and angles.

Oblique collision

Collision where bodies do not move along the line of impact after impact; involves angles and components.

Potential energy curve

Graph of potential energy U versus position x, used to analyze equilibrium and stability.

Hooke’s law

restoring force F = -k x; energy stored in a spring is U = (1/2) k x^2.

Oscillations and total energy in spring–mass system

Total mechanical energy (K + U_spring) remains constant in the ideal, undamped case; energy continuously exchanges between K and U.

Heat energy

Internal energy due to random molecular motion; part of internal energy.

Internal energy

Sum of kinetic and potential energies of molecules; depends on temperature.

Electrical energy

Energy associated with movement of charge carriers in a conductor.

Chemical energy

Energy stored in chemical bonds; released during chemical reactions.

Nuclear energy

Energy released in nuclear reactions (fission or fusion).

Relativistic mass

Mass increases with velocity: m = γ m0; reflects mass–energy relationship at high speeds.