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# What is Force?

## WHAT DOES FORCE MEAN IN PHYSICS?

Forces in physics always come in pairs, and the sum of the pair is zero. This is Newton's third law. In other words, for every action - there is an equal and opposite reaction.

#### Force

Forces act on an object or objects and cause a change in the state of rest, or motion, of the object. For example, a weight of a tractor towing a plough exerts force on the ground as it moves over a paddock; it also exerts force on the plough. The plough, in turn, exerts force on the soil.

Force is described by Newton’s Second Law of Motion
F = ma = kg-m/s2

Where m = mass
a = acceleration

Force is measured in Newtons (N).  One Newton is the force necessary to accelerate 1kg at a rate of 1m/s2.

Keep in mind the weight is a force (gravitational force acting on an object), however it may be expressed different in different system.  The above example is in metric, in the British system the unit of force may be expressed as pounds.

Additionally Mass refers to the amount of matter and object is made of and it can also be a measure of inertia.    Weight and mass are related.

Weight = mass x acceleration (due to gravity), (g is explained further in the lesson)

w = mg

#### Force of Gravity

Newton’s Law of Gravitation
Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

F = Gm1m2/r2

Where G = Gravitational constant

Note that G is not the same as g which is the acceleration due to gravity.  G is actually very small and is equal to 6.67 x 10 -11 kg-m/s2.

Force of Gravity is exerted by the earth on all matter. It is considered to act vertically at a single point – this point is known as the Centre of Gravity.

Gravitational forces are actually very small, they do exist between everyday objects, but it is not strong enough to notice it.  It is only noticeable with very large objects such as a planet.

Acceleration due to gravity is the same for all objects regardless of their mass.  This can be shown in general by:
g = GM/R2

Where G = gravitational constant
M = Mass of any uniform object
R2 = Radius, or distance to the centre
We can see from the above that g is independent from m (mass).

Specifically to Earth:
g= GME/RE2

Where G = gravitational constant
ME = Mass of the Earth
R2E = Radius of the Earth

Also, with regards to Newton's third law:
The gravitational force of the earth on the moon is the same as the gravitational force of the moon on the earth.
For any interaction between bodies 1 and 2:
F1, 2 = -F1,2

[31/10/2020 17:58:11]