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In physics, force is that which can cause a mass to accelerate. It may be experienced as a lift, a push, or a pull. The acceleration of the body is proportional to the vector sum of all forces acting on it (known as net force or resultant force). In an extended body, force may also cause rotation, deformation, or an increase in pressure for the body. Rotational effects are determined by the torques, while deformation and pressure are determined by the stresses that the forces create.
Net force is mathematically equal to the time rate of change of the momentum of the body on which it acts. Since momentum is a vector quantity (has both a magnitude and direction), force also is a vector quantity.
The concept of force has formed part of statics and dynamics since ancient times. Ancient contributions to statics culminated in the work of Archimedes in the 3rd century BC, which still forms part of modern physics. In contrast, Aristotle's dynamics incorporated intuitive misunderstandings of the role of force which were eventually corrected in the 17th century, culminating in the work of Isaac Newton. Following the development of quantum mechanics it is now understood that particles influence each another through fundamental interactions and therefore the standard model of particle physics demands that everything experienced fundamentally as a "force" is actually mediated by gauge bosons. Only four fundamental interactions are known: strong, electromagnetic, weak (unified into one electroweak interaction in 1970s), and gravitational.

In physics, force is what causes a mass to accelerate. It may be experienced as a twist, a push, or a pull. The acceleration of a body is proportional to the vector sum of all forces acting on it (known as the net force or resultant force). In an extended body, force may also cause rotation, deformation, or an increase in pressure for the body. Rotational effects are determined by the torques, while deformation and pressure are determined by the stresses that the forces create.[1][2]
Net force is mathematically equal to the time rate of change of the momentum of the body on which it acts.[3] Since momentum is a vector quantity (has both a magnitude and direction), force also is a vector quantity.
The concept of force has been used in statics and dynamics since ancient times. Ancient contributions to statics culminated in the work of Archimedes in the 3rd century BC, which still forms part of modern physics.[4] In contrast, Aristotle's dynamics incorporated intuitive misunderstandings of the role of force which were eventually corrected in the 17th century, culminating in the work of Isaac Newton.[2] Following the development of quantum mechanics, it is now understood that particles influence each other through fundamental interactions, and therefore the standard model of particle physics demands that everything experienced fundamentally as a "force" is actually mediated by gauge bosons. Only four fundamental interactions are known; in order of decreasing strength, they are: strong, electromagnetic, weak (unified into one electroweak interaction in 1970s), and gravitational.[1]

Gravitation

Building on the work of Galileo and Kepler, Isaac Newton formulated the theory of gravitation in the 1680s. The story goes that Newton was sitting under a tree when an apple fell and hit him on the head. This made him curious and inspired him to determine that there was a force called gravity that pulled the apple down from the tree.

Universal Gravitation

This force on Earth pulls everything toward the ground. Newton determined that all masses of matter attract each other through the force of gravity. This is called the Theory of Universal Gravitation. What this means is that not only does the Earth pull the apple toward it by the force of gravity, but the apple also pulls the Earth toward it by that same force.
(See Newton's Universal Gravity Equation for more information.)

Modern theories

It wasn't until the early 1900s that Albert Einstein gave another interpretation of the force gravity in his General Theory of Relativity. He stated that gravitation was the result of the curvature of space around matter and not due to some force. Recently there have been new theories that the force of gravity is caused by graviton particles or by waves. Those theories satisfy rules of Quantum Mechanics that Einstein's concepts didn't.
(See //Modern Views of the Force of Gravity// for more information. )

Gravity on Earth

The force of gravity from the mass of the Earth pulls your mass toward the ground and prevents you from floating off into space. Of course, your mass pulls the Earth and contributes to the total force.
The equation for the force of gravity for all objects relatively close to the Earth is:
F = mg
where
  • F is the force pulling objects toward the Earth; it is also the weight of the object
  • m is the mass of the object
  • g is the acceleration due to gravity; this number is a constant for all masses of matter
  • mg is the product of m times g
This acceleration due to the force of gravity on Earth g equals 9.8 m/s² in the metric system and 32 ft/s² in the English system.

Note: g is often called the acceleration of gravity. That is incorrect and misleading, since gravity does not accelerate. The expression should be the acceleration due to the force of gravity, which is a more accurate definition for g.

Weight

The weight of an object is the measurement of the force of gravity on that object. You weigh something on a scale, according to the force that the Earth pulls it down. Thus the weight is actually the force of gravity on that object:
w = mg
where
  • w is weight
  • m is mass
  • g is acceleration due to gravity
Your weight can actually be slightly less than w = mg because of the effect of the gravity from the Moon pulling you upward. Although this effect is very small, the gravity from the Moon is responsible for the tides. The attraction is pulling water in the ocean toward it, causing the water to rise when the Moon is overhead.
The acceleration due to gravity on the Moon (gm) is 1/6 of the value on the Earth (g). Thus, if you put the same object on the Moon and weighed it, its weight would be 1/6 the weight on Earth. In other words, a 180-pound man would only weigh 30 pounds on the Moon.

Objects fall at the same rate

Since the acceleration due to gravity is a constant for all objects, no matter what their mass, that means that all objects fall at the same rate—assuming the effect of air resistance is negligible. This is counterintuitive, since you would expect a heavy object to fall faster than an object that weighed less. But it is a fact. Try dropping two objects at the same time, from the same height, making sure they are heavy enough not to be affected by air resistance. You will see they hit the ground at the same time.
(See //Gravity Equations for Falling Objects// for more information.)

Gravity in the Universe

The gravitational attraction of amounts of matter towards each other was responsible for the formation of the stars and planets in the early days of the Universe. In the case of planets, the hot gases collected and cooled into relatively solid spheres. Since so much matter collected in the various suns, they are kept hot by thermonuclear reactions.
The pull of gravity resulted in the Moon rotating in orbit around the Earth. Otherwise, the Moon would fly off in a straight line, according to the Law of Inertia. Likewise, the planets are held in orbit around the Sun by the gravitational attraction between the Sun and each planet. The speed of the planets prevent them from falling into the Sun. It is an exact balance.
(See //Influence of Gravity in the Universe// for more information.)

In conclusion

Gravity is a force that acts at a distance and attracts bodies of matter toward each other. Isaac Newton formulated the Theory of Universal Gravitation. Our experience with gravity is the that it pulls all objects toward the Earth. The force of gravity on an object is called its weight. Not only does gravity pull objects toward the Earth, it also is responsible for keeping the Moon in orbit around the Earth, as well as the Earth and planets in orbit around the Sun.