AP Physics C: Mechanics key terms

The change in position of an object, calculated as final position minus initial position. It is a vector, so direction matters, and it does not depend on the path traveled.

The rate at which an object's position changes with time. Average velocity uses displacement over a time interval; instantaneous velocity is the limit as the interval becomes very small.

The rate at which velocity changes with time. Because velocity is a vector, acceleration can change speed, direction, or both; it is not required that an object be speeding up.

The two-dimensional motion of an object moving under only gravity after launch, with negligible air resistance. Horizontal velocity stays constant while vertical motion has constant downward acceleration.

Motion along a circular path at constant speed. The velocity direction changes continuously, so the object has a centripetal acceleration pointing toward the center even though its speed is constant.

The vector sum of all individual forces acting on an object. It determines the object's acceleration according to Newton's second law and is zero when the object's velocity is constant.

A simplified drawing that represents one object and shows each external force acting on it as an arrow. It helps identify directions and magnitudes before applying Newton's laws.

The relationship stating that an object's acceleration equals the net force on it divided by its mass. It is a vector equation, so force and acceleration point in the same direction.

A contact force exerted by a surface perpendicular to that surface, pushing on an object to prevent it from passing through. Its magnitude adjusts to the situation and is not always equal to weight.

A contact force parallel to a surface that opposes attempted relative sliding while the object remains at rest relative to the surface. Its magnitude can vary up to a maximum value.

A contact force parallel to a surface that opposes sliding when one surface moves relative to another. Its magnitude is often modeled as a constant coefficient times the normal force.

A pulling force transmitted through a string, rope, cable, or similar connector. In ideal massless connectors, the tension magnitude is often the same throughout and acts along the connector.

The gravitational force exerted on an object by a nearby massive body, commonly Earth. Near Earth's surface its magnitude is mass times the gravitational field strength, and its direction is downward.

The energy transferred to or from an object by a force acting through a displacement. It is calculated from the force component along the displacement times the displacement.

The energy an object possesses due to its motion, equal to one-half its mass times the square of its speed. It is a scalar and is always nonnegative in classical mechanics.

The principle that the net work done on an object equals its change in kinetic energy. It connects force and displacement over a process to the resulting speed change.

Energy associated with the configuration of a system and the conservative forces within it, such as gravity or a spring. Only changes in potential energy are physically meaningful; a reference zero must be chosen.

A force for which the work done between two points depends only on those endpoints, not on the path taken. Such a force allows a potential energy function to be defined.

The sum of an object's or system's kinetic and potential energies. It remains constant when only conservative forces do work, but changes when nonconservative forces such as friction act.

The rate at which work is done or energy is transferred. Average power is work divided by time; instantaneous power can also be expressed as force times velocity.

A vector quantity equal to an object's mass times its velocity. The total momentum of an isolated system remains constant, making it useful for analyzing collisions and explosions.

The change in momentum produced by a force acting over a time interval. For a constant force it equals force times time; graphically it is the area under a force-time graph.

The principle that the total linear momentum of a system stays constant when the net external force on the system is zero. Internal forces can redistribute momentum among objects without changing the total.

A collision in which both total linear momentum and total kinetic energy are conserved. Objects may deform temporarily but return the energy to motion, so no net kinetic energy is lost.

A collision in which total linear momentum is conserved but total kinetic energy is not. Some kinetic energy becomes deformation, heat, sound, or other internal energy; perfectly inelastic collisions stick together.

The mass-weighted average position of the parts of a system. The system's translational motion under external forces can often be analyzed as if all mass were at this point.

A measure of a force's tendency to cause rotation about an axis, equal to the force times its perpendicular lever arm. Its sign indicates the rotation direction according to a chosen convention.

A measure of how an object's mass is distributed about a chosen axis, determining its resistance to angular acceleration. It depends on both the mass and the distance of mass from the axis.

The rate at which angular velocity changes with time. It is related to net torque by the rotational form of Newton's second law, with moment of inertia playing the role of mass.

A vector measure of rotational motion, equal to moment of inertia times angular velocity for a rigid body about a fixed axis. For a particle it equals position crossed with linear momentum.

The kinetic energy an object has because of rotation about an axis, equal to one-half its moment of inertia times the square of its angular velocity. Rolling objects can also have translational kinetic energy.

The principle that a system's total angular momentum stays constant when the net external torque on it is zero. Internal rearrangements can change the distribution of mass and thus angular speed.

A periodic oscillation in which the restoring force is directly proportional to displacement from equilibrium and points opposite to it. The resulting motion is sinusoidal with a fixed period independent of amplitude.

The maximum magnitude of displacement from equilibrium during an oscillation. It describes the size of the oscillation and is related to the energy stored in the oscillating system.

The time required for one complete cycle of a periodic motion or oscillation. It is the reciprocal of frequency and, for simple harmonic motion, is independent of amplitude in the ideal case.

The number of complete cycles of a periodic motion occurring per unit time, usually measured in hertz. It is the reciprocal of period and relates to angular frequency by a factor of two pi.

For a periodic motion, two pi times the ordinary frequency, describing phase change per unit time in radians. In simple harmonic motion it is set by the system's mass and restoring-force constant.

The gradual decrease in the amplitude of an oscillation due to energy loss from the oscillating system, often through friction or drag. It can be light, critical, or heavy depending on the strength.

A condition in which an oscillating system responds with large amplitude when driven near its natural frequency. The response depends on damping, which limits the peak amplitude.

An idealized oscillating system consisting of a mass attached to an ideal spring. For a Hooke's-law spring, the motion is simple harmonic with angular frequency equal to the square root of stiffness divided by mass.