The standard unit of mass is the kilogram. (The British system equivalent is the slug, which is perhaps another reason to go metric.)

Physicists define mass as the property of an object that measures its resistance to a change in motion. A car has more mass than a bicycle. The three people shown straining at the car above will cause it to accelerate slowly; if they were pushing a bicycle instead, they could increase its speed much more quickly. Once they do set the car in motion, if they are not careful, its mass might prevent them from stopping it.

The official kilogram, the International Prototype Kilogram, is a cylinder of platinum-iridium alloy that resides at France’s International Bureau of Weights and Measures. Copies of this kilogram reside in other secure facilities in different countries and are occasionally brought back for comparison to the original.

A liter of water has a mass of about one kilogram. A typical can of soda contains about 354 milliliters and has a mass of 0.354 kilograms.

It is tempting to write that one kilogram equals about 2.2 pounds, but this is wrong. The pound is a unit of weight; kilograms and slugs are units for mass. Weight measures the force of gravity that a planet exerts on an object, while mass reflects that object’s resistance to change in motion. A classic example illustrates the difference: Your mass is the same on the Earth and the Moon, but you weigh less on the Moon because it exerts less gravitational force on you. On Earth, the force of gravity on one kilogram is 2.2 pounds but the force of gravity on a kilogram is only 0.36 pounds on the Moon.

Kilogram is abbreviated as kg. We typically use kilograms in this book, not grams (which are abbreviated as g).

The three units you need in order to understand motion, force and energy, the topics that start a physics textbook, are meters, kilograms and seconds. Other units used in studying these topics are derived from these fundamental units.