“Gravity is a bitch” is the nonchalant, comical quote by Sly Stallone from the film Cliffhanger, following a death fall of one of the movie's bad guys during a mountain climbing scene. As riggers, gravity is far from comical – it is the reason we are employed.
Gravity is a naturally occurring force caused by the attraction of two physical bodies. While it has different meanings and consequences to physicists; astrophysicist; and engineers; as riggers, our interest is limited to the attractive force the Earth exerts on all objects. In our case, the two physical bodies involved are the Earth and the load we are lifting. As it turns out, weight is the result of gravity and mass. But what are weight, gravity, and mass anyway?
The Difference in Mass and Weight
Weight is the force created when a mass is acted upon by gravity. To a scientist, mass means the amount of matter an object contains, whereas weight refers to the force experienced by that object due to gravitational attraction.
The Bottom Line
While mass and weight are technically different concepts and quantities, for all practical rigging purposes mass and weight are the same thing. This is because our lifts and transports of heavy loads are done at the Earth's surface where a constant gravitational force field prevails.
The fact that both metric and English forms of measurement coexist in today's world can give rise to much confusion when the terms “mass” and “weight” are used. Very broadly speaking, statements or listings of metric units of weight measurement is a mass concept while English units of weight measurement is the resulting force counterpart. The basic unit for mass is the kilogram and for weight, the unit is the pound. We very often encounter a project for which the object to be handled will have a stated load in both metric and English values.
Is It Ton or Tonne?
Without going into a lot of detail regarding the conversion of metric to English units of weight, and vice versa, here are some interesting observations about weight units that you might come across:
- The common U.S. unit weight is the ton (2,000 lb). Most other countries refer to this as a short ton
- In English-speaking countries that are predominantly metric, the tonne (2,240 lb) is common. In the U.S. this is referred to as a long ton or a metric ton.
- Engineers often refer to loads in kips. One kip is equal to 1,000 lb.
Weights are easily converted from one system of units to the other.