(Maths) of a point: the rate of change with respect to time of the velocity of the point.

(Mech.) The rate of change of velocity, expressed in metres (or feet) per second squared.  Certain restricted and special applications of the word occur in Astronomy; e.g., secular acceleration.

(Mech.).  Acceleration with which a body would fall freely under the action of gravity in a vacuum. This varies according to the distance from the earth's centre, but the internationally adopted value is 9.80665 m/s², or about 35.30394 (km/h)/s (≈32.174 ft/s² or ≈21.937 mph/s).

F = ma

Second law: The sum of the forces on an object is equal to the total mass of that object multiplied by the acceleration of the object. In more technical terms, the acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. Thus, F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object. Force and acceleration are both vectors (as denoted by the bold type). This means that they have both a magnitude (size) and a direction relative to some reference frame.

First law: When viewed in an inertial reference frame, an object either is at rest or moves at a constant velocity, unless acted upon by an external force.

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Consequently,

• An object that is at rest will stay at rest unless an external force acts upon it.
• An object that is in motion will not change its velocity unless an external force acts upon it.

The second law can also be stated in terms of an object's acceleration. Since the law is valid only for constant-mass systems, the mass can be taken outside the differentiation operator by the constant factor rule in differentiation. Thus,

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where F is the net force applied, m is the mass of the body, and a is the body's acceleration. Thus, the net force applied to a body produces a proportional acceleration. In other words, if a body is accelerating, then there is a force on it.

(maths.)  A dynamical quantity conserved within a closed system. A body of velocity v has a linear momentum of Mv.  It has an angular momentum about a point O defined as the moment of the linear momentum about O. About G this reduces to Iω where I is the moment of inertia about G and ω the angular velocity of the body.

(phys.) the property of a body, proportional to the mass but independent of gravity, which opposes the change in the state of motion of a body. 

Centrifugal and Coriolis forces are manifestations of inertia.

(mech.) That which, when acting on a body which is free to move, produces an acceleration in the motion of the body, measured by rate of change of momentum of body. The unit of force is that which produces unit acceleration in unit mass.

In physics, a force is any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or a flexible object to deform, or both. Force can also be described by intuitive concepts such as a push or a pull. A force has both magnitude and direction, making it a vectorquantity. It is measured in the SI unit of newtons and represented by the symbol F.

The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. As a formula, this is expressed as:

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where the arrows imply a vector quantity possessing both magnitude and direction.

Related concepts to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque which produceschanges in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the so-called mechanical stress. Pressure is a simple type of stress. Stress usually causes deformation of solid materials, or flow in fluids.

Torque, moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object. Mathematically, torque is defined as the cross product of the lever-arm distance and force, which tends to produce rotation.

Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

The symbol for torque is typically τ, the Greek letter tau. When it is called moment, it is commonly denoted M.

The magnitude of torque depends on three quantities: the force applied, the length of the lever arm^[2] connecting the axis to the point of force application, and the angle between the force vector and the lever arm. In symbols:

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where

τ is the torque vector and τ is the magnitude of the torque,

r is the displacement vector (a vector from the point from which torque is measured to the point where force is applied),

F is the force vector,

× denotes the cross product,

θ is the angle between the force vector and the lever arm vector.

The length of the lever arm is particularly important; choosing this length appropriately lies behind the operation of levers, pulleys, gears, and most other simple machines involving a mechanical advantage.

The SI unit for torque is the newton metre (N·m).

In fluid dynamics, drag (sometimes called air resistance or fluid resistance) refers to forces acting opposite to the relative motion of any substance moving in a fluid. This can exist between two fluid layers(or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.

Drag forces always decrease fluid velocity relative to the solid object in the fluid's path.

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system. The force applied on a surface in a direction perpendicular or normal to the surface is called thrust.

In mechanical engineering, force orthogonal to the main load (such as in parallel helical gears) is referred to as thrust.

In physics, mass is a property of a physical body which determines the body's resistance to being accelerated by a forceand the strength of its mutual gravitational attraction with other bodies. The SI unit of mass is the kilogram (kg). As mass is difficult to measure directly, usually balances or scales are used to measure the weight of an object, and the weight is used calculate the object's mass. For everyday objects and energies well-described by Newtonian physics, mass describes the amount of matter in an object. However, at very high speeds or for subatomic particles, general relativity shows that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction.

The mass of an object determines its acceleration in the presence of an applied force. This phenomenon is called inertia. According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by ^F/_m. A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass m_A is placed at a distance r (center of mass to center of mass) from a second body of mass m_B, each body experiences an attractive force F_g = Gm_Am_B/r², where G = 6.67×10⁻¹¹ N kg⁻²m² is the "universal gravitational constant". This is sometimes referred to as gravitational mass. Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical; since 1915, this observation has been entailed a priori in the equivalence principle of general relativity.

• Inertial mass measures an object's resistance to changes in velocity. (the object's acceleration)

• Active gravitational mass measures the gravitational force exerted by an object.

• Passive gravitational mass measures the gravitational force experienced by an object in a known gravitational field.

• Mass-Energy measures the total amount of energy contained within a body, using E=mc²

• The tonne (t) (or "metric ton") is equal to 1000 kg.

• The electronvolt (eV) is a unit of energy, but because of the mass–energy equivalence it can easily be converted to a unit of mass, and is often used like one. In this context, the mass has units of eV/c². The electronvolt is common in particle physics.