force

Force is a quantitative description of the interaction between two physical bodies, such as an object and its environment. Force is proportional to acceleration. In calculus terms, force is the derivative of momentum with respect to time.

Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Other forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.

The concept of force was originally defined by Sir Isaac Newton in his three laws of motion. He explained gravity as an attractive force between bodies that possessed mass (gravity within Einstein's general relativity doesn't require force).

In physics, force is what changes or tends to change a state of rest or motion in an object. Force causes objects to accelerate, add to the object's overall pressure, change direction, or change shape. Force is measured in Newtons. ('N').

According to Newton's Second Law of Motion, the formula for finding force is:

'''F = ma'''

where F is the force,
m is the mass of an object,
and a is the acceleration of the object.

If one sets a to the standard gravity g, then another formula can be found:

'''W = mg'''

where W is the weight of an object,
m is the mass of an object,
and g is the acceleration due to gravity at sea level. It is about 9.8m / s².

Force is a vector, so it has both a magnitude and a direction.

Another equation that is useful is:

F = G(m1)(m2) / d²

F is force; G is the gravitational constant, which is used to show how gravity accelarates an object; m1 is the mass of one object; m2 is the mass of the second object; and d² is the distance between the objects.

A force is always a push, pull, or a twist, and it affects objects by pushing them up, pulling them down, pushing them to a side, or by changing their motion or shape in some other way.

motion

In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time .^[1] An object's velocity cannot change unless it is acted upon by a force, as described by Newton's first law. An object's momentum is directly related to the object's mass and velocity, and the total momentum of all objects in a closed system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum.

A body which does not move is said to be at rest, motionless, immobile, stationary, or to have constant (time-invariant) position.

Motion is always observed and measured relative to a frame of reference. As there is no absolute frame of reference, absolute motion cannot be determined; this is emphasised by the term relative motion.^[2] A body which is motionless relative to a given reference frame, is still moving relative to infinitely many other frames. Thus, everything in the universe is moving.^[3]

More generally, the term motion signifies any temporal change in a physical system. For example, one can talk about motion of a wave or a quantum particle (or any other field) where the concept location does not apply.

Laws of Motion

Main article: Mechanics

In physics, motion in the universe is described through two sets of apparently contradictory laws of mechanics. Motions of all large scale and familiar objects in the universe (such as projectiles, planets, cells, and humans) are described by classical mechanics. Whereas the motion of very small atomic and sub-atomic objects is described by quantum mechanics.

[edit] Classical mechanics

Main article: Classical mechanics

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies. It produces very accurate results within these domains, and is one of the oldest and largest subjects in science, engineering and technology.

Classical mechanics is fundamentally based on Newton's Laws of Motion. These laws describe the relationship between the forces acting on a body and the motion of that body. They were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. His three laws are:

1. In the absence of a net external force, a body either is at rest or moves with constant velocity.
2. The net external force on a body is equal to the mass of that body times its acceleration; F = ma. Alternatively, force is proportional to the time derivative of momentum.
3. Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction.^[4]

Newton's three laws of motion, along with his law of universal gravitation, explain Kepler's laws of planetary motion, which were the first to accurately provide a mathematical model or understanding orbiting bodies in outer space. This explanation unified the motion of celestial bodies and motion of objects on earth.

Classical mechanics was later further enhanced by Albert Einstein's special relativity and general relativity. Special relativity explains the motion of objects with a high velocity, approaching the speed of light; general relativity is employed to handle gravitation motion at a deeper level.

[edit] Quantum mechanics

Main article: Quantum mechanics

Quantum mechanics is a set of principles describing physical reality at the atomic level of matter (molecules and atoms) and the subatomic (electrons, protons, and even smaller particles). These descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation energy, this described in the wave–particle duality.

In contrast to classical mechanics, where accurate measurements and predictions can be calculated about location and velocity, in the quantum mechanics of a subatomic particle, one can never specify its state, such as its simultaneous location and velocity, with complete certainty (this is called the Heisenberg uncertainty principle).

In addition to describing the motion of atomic level phenomena, quantum mechanics is useful in understanding some large scale phenomenon such as superfluidity, superconductivity, and biological systems, including the function of smell receptors and the structures of proteins.

[edit] Kinematics

Main article: Kinematics

Kinematics is a branch of classical mechanics devoted to the study of motion, but not the cause of the motion. As such it is concerned with the various types of motions.

Two classes of motion covered by kinematics are uniform motion and non-uniform motion. A body is said to be in uniform motion when it travels equal distances in equal intervals of time (i.e. at a constant speed). For example, a body travels 5 km in 1 hour and another 5 km in the next hour, and so on continuously. Uniform motion is closely associated with inertia as described in Newton's first law of motion. However, most familiar types of motion would be non-uniform motion, as most bodies are constantly being acted upon by many different force simultaneously, as such they do not travel equal distances in equal intervals of time. For example, a body travels 2 km in 25 minutes but takes 30 minutes to travel the next 2 km.

Types of motion

• Simple harmonic motion – (e.g. pendulum).
• Linear motion – motion which follows a straight linear path, and whose displacement is exactly the same as its trajectory.
• Reciprocating (i.e. vibration)
• Brownian motion (i.e. the random movement of particles)
• Circular motion (e.g. the orbits of planets)
• Rotary motion – a motion about a fixed point ex. the wheel of a bicycle
• Curvilinear motion – It is defined as the motion along a curved path that may be planar or in three dimensions.
• Rotational motion - (e.g. Ferris wheel)