A very smart person — lets call him Sir Isaac Newton — once wrote three very important laws that govern the motion of all things around us (that is medium sized things on planet Earth, things that are very very small or very very large or travelling really really fast don’t like to play by some of Newton’s rules).

Newton’s first law describes how forces control the behaviour of objects in motion or at rest. According to Newton’s first law any object will stay doing whatever it’s doing unless a force is applied. How is the application of force related to gravity?

Gravity is a force acting between two masses to bring them together, yes any two but you’ll probably only feel the large one. The mass of Earth (the planet) applies a large force to all masses near / on it (like you and all the medium sized stuff around you). Application of force makes objects change what they’re currently doing (Newton’s first). Anything an object is currently doing is called its velocity, even if it’s at rest (like miles per hour or kilometers per hour), and change in velocity is called acceleration (like miles per hour per second or kilometers per hour per second).

Standard gravity is the measurement of the free fall acceleration that happens on Earth when a smaller mass (like a person or ball) is attracted to it. This acceleration can be measured using any unit of length that is described as changing over time squared (length changing over time is just velocity but acceleration due to force must be a change in velocity over time). Therefore, free fall speeds could be measured in inches per second per second, or inches per second per minute, or meters per second per second.

In an average scenario the measured free fall speed of an object on Earth would be **386.09 inches per second per second**. This applies to an object of any mass and can be easily measured and confirmed at different locations.

The effects of gravity are not uniform across planet Earth and are certainly not the same on a planet of different mass. Considering the variation in gravity we must note that free fall acceleration is only and approximation.

Curious about some of the other units mentioned? The table below describes some other equivalent accelerations. In each case the acceleration is the same (1 standard gravity). What units should you use? Probably you’ll choose customary or metric units depending on what you’re most comfortable with, but the duration of time can easily be changed to make to units more readable. If something is accelerating quickly you can measure it in seconds per second or if the acceleration is slower minutes per second may make more sense.

Standard Gravity | 1 |
---|---|

Meters per Second per Second | 9.81 |

Feet per Second per Second | 32.174 |

Inches per Second per Second | 386.09 |

In this example convert 1 Standard Gravity (free fall acceleration) to Inches per Second per Second (just to talk you through the steps so it makes sense).

From a table of Standard Gravity given above we know 1 Standard Gravity is equal to 386.09 inches per second per second (ips/s), 386.09 is the conversion factor.

Multiply the know value of standard gravity (1) by the conversion factor (386.09) to get the answer in inches per second per second(?).

1 x 386.09

= 386.09 inches per second per second (ips/s)

(I know multiplying by 1 seems silly but so would an example with none standard gravity, where could we be? The moon?..)

]]>A hectare is a measurement that describes the size of an area. Hectares are metric units and staying true to the metric way can be defined by tweaking the decimal place of the base units — square meters — a fact that makes converting between the two rather simple (once you know how of course).

The definition of hectares gives us the conversion factor we need to convert hectares to square meters. If 1 hector is equal 10,000 meters square then multiply a given number of hectares by 10,000 to find meters square, to find hectares from a given measurement in square meters divide by 10,000.

If you’re still unsure about the size of a hectare (maybe you’re familiar with imperial units), or are looking for some other conversion this quick reference table should help out.

Unit | Equivalent to 1 Hectare |
---|---|

Hectare | 1 |

Acre | 2.47105381 |

Square Meter | 10,000 |

Square Foot | 107,639.104 |

In this example convert 10 hectares to square meters.

From a definition of hectare given above we know 1 hectare is equal to 10,000 meters square, 10,000 is the conversion factor.

Multiply the know value of hectares (10) by the conversion factor (10,000) to get the answer in meters square (?).

10 x 10,000

= 100,000 meters square

]]>An atmosphere is a measure of pressure (force) being exerted on an area equal to the force exerted on an object by earth’s atmosphere as gravity pulls down on molecules of air. Although earth’s atmosphere exerts a force varying by altitude and other factors the unit of measure has been defined to a standard of 101.325 kPa (more details on atmospheres available). Force exerted by the atmosphere effects the height of fluids in a column depending on the compressibility of that fluid. Two fluids commonly used in columns to measure pressure are water, as described below, and mercury.

How can 5 atmospheres of pressure be converted to feet of water?

From a lookup table we can retrieve the conversion factor necessary to convert a single atmosphere to feet of water.

In fairly approximate terms the conversion factor is 33.9.

Multiply the know value of atmospheres (5) by the conversion factor (33.9) to get the answer in feet of water (?).

**5 atm x 33.9**

**=169.5 ftH _{2}O**

When converting between gallons and other units it is important to note that the type of gallon being referred to may vary nationally. American (US) and British (Imperial) gallons have different sizes. In addition to the conversion varying depending on location the volume could be fluid gallons or dry gallons. For the purpose of this example lets assume US fluid gallons, used when measuring a volume of wine, for instance.

**How can 10000 cubic meters be converted to gallons?**

From a lookup table we can retrieve the conversion factor necessary to convert a single cubic meter to gallons.

In fairly detailed terms the conversion factor is 264.172052

Multiply the know value of cubic meters (10,000) by the conversion factor (264.172052) to get the answer in gallons (?).

*10000 x 264.172052*

* *

*=2641720.52*

How much does a stone weigh? When someone talks about a stone in terms of weight they are referring to a unit of measure still commonly used used in the UK that is equal to 14 pounds.

- Use the conversion factor we already have: 1 stone (of mass) is equal to 14 pounds.
- Apply the conversion factor by multiplying the mass given in stones (
*st*) by 14, simple right?

See this example:

- Setup: Convert 13 stones to pounds
- Conversion factor: 14 (as in 14 pounts in one stone)
- Therefore: 13 x 14 = 182 pounds in 13 stone!