(the units used in the SI, CGS and English Systems)

  Quantity and
  SI Unit  CGS  English
 Length  Meter (m) Centimeter (cm) Foot (ft)
 Mass  Kilogram (kg) Gram (g) pound-mass (lb)
 Time Second (s) Second (s) Second (s)
 distance / time
 Meters / Second
      m / s
 Centimeters / Second
      cm / s
 Feet / Second
      ft / sec
 distance / time2
 Meters / Second2
      m / s2
 Centimeters / Second2
      cm / s2
 Feet / Second2
      ft / s2
 mass • acceleration
      kg • m / s2
      g • cm / s2
 pound-force (lbf)
  lb • ft / s2
 Force • distance
 Newton • meter
      kg • m2 / s2
      g • cm2 / s2
 Foot-pound (ft • lbf)
     lb • ft2 / s2
 Joule / second
 Newton • meter
      kg • m2 / s3
 ergs / Second
      g • cm2 / s3
 Foot-pound / Second
          (ft • lbf)
     lb • ft2 / s3
 'g ' Acceleration
 Due to Gravity
 9.80665 m / s2  980.665 cm / s2  32.17405 ft / s2

Classical Mechanics is that branch of physics that deals with the motion of objects and the energy and forces acting upon them.

One important point to mention is that each of the 3 systems listed (SI, CGS and English) obviously use different units.
Yet another difference occurs when discussing force.
When first learning physics, a great many students (myself included) think that at the surface of the Earth, a mass of 1 kilogram would "weigh" 1 newton and a mass of 1 gram would "weigh" 1 dyne. They do not.
However, a pound of mass does "weigh" one pound of force (lbf) at the surface of the Earth.
What's the reason for this difference?
The English system uses the acceleration due to gravity in its F = m • a equation whereas the SI uses an acceleration of one meter per second2 and the CGS system uses one cm per second2. Notice that neither the SI nor CGS systems use the acceleration due to gravity when defining force, energy or power, but the English system does. This is yet another reason to have one system of units!

Basically, since 1960, the Système International (SI) has become the official system of units used by almost all countries in the world.

Yes, even the United States has agreed to using this system but some American agencies and companies continue to use the English System, which can create problems. Here's a good example.
In December, 1998 the Mars Climate Orbiter was launched from Earth and in September, 1999 after travelling 461 million miles as it neared Mars, it either crashed to the Martian surface or failed to go into orbit and continued traveling into space.
And why did this error occur?
Two companies were involved with putting the satellite into its Martian orbit. The Jet Propulsion Laboratory of Pasadena, California used metric units but Lockheed Martin of Denver Colorado used the English System.

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