We will use newtonsfor the unit of force (and weight) almost always in the discussionsthat follow.Дensity: There are two kinds of density, 'weight density'and 'mass density'. (You will probably never learn anywhere that on thesurface of the earth, one slug weighs 32.2 pounds-don't worry aboutit, very few people know this!) The pound is the US unit of forcehence the US unit of weight is also the pound. You will learnlater that on the surface of the earth, a mass of 1 kilogram weighs9.8 newtons. It would be correct to say, no matter where youmight be in the universe that 'the weight of a mass is the forceof gravity on the mass.' In the metric system force is measuredin newtons hence weight is also measured in newtons. If you lived on Mars, the above definition would probablychange to: 'The weight of a mass is the force that Mars pulls on themass.' The whole idea of weight is related to the force of gravity(and we hate just to use the word 'gravity' since it can bring upeven more confusion). If we get furtheraway from the earth, the force the earth pulls on us is less and weweigh less. Usually we spend most of our time on Earth so ourweight is the force that the earth pulls on us. We hope youhave a feeling for what force means (and we will discuss it later).The entire idea of weight can be understood as the force of gravityon something. Weight: If you can finally accept the concept mass even ifwe have been unable to define it, weight is easy: The weight of amass is the force that the earth pulls on the mass. The more masssomething has, the harder it is to move or, the more sluggish it is.The correct US unit of mass is called the 'slug'-short forsluggishness- but, as we said, almost no one uses this unit today.) The pound is a unit ofweight-more about weight in the next paragraph. (In the United States today, almost no one knows what the unitof mass is called-it's not the pound. This definition is properly confusing and you can workon the meaning of 'stuff'! In the metric system mass is measured inkilograms and grams and these will be the units we will most oftenuse. Our way of giving up on the impossible task of definingmass is to say: mass is the measure of the amount of 'stuff' insomething. Betterminds than ours have been confused over the meaning of the concept'mass' and even today, better minds than ours contemplate what massreally means. To addto the confusion, mass is related to an object's inertia but it alsois related to how hard objects are attracted to the earth. Others say mass is the measure of an object's inertia(which assumes we understand the elusive property of inertia). Some say massis the amount of matter in something (and hope that no one asks: Whatis matter?).

We can talk all around itbut we will finally have to admit that our words fail. Isaac Newtoncalled mass the quantity of matter. Mass: This concept is so basic that, likelength and time, it is really impossible to define. We hope we can explain the difference between mass,weight and density so clearly that you will have no troubleexplaining the difference to your students. Like many confusing concepts, after they are finallyunderstood, they are still difficult to explain to others who don'tunderstand. Which of us hasn't fallen for the old riddle: What weighsmore, a pound of lead or a pound of feathers? Now that Astronautsregularly seem to be demonstrating 'weightlessness', ( yet we knowthey still have all of the matter or mass they had when they were onearth,) more of us are beginning appreciate the weight - massconfusion. I Words: Most people hardly think that there is adifference between 'weight' and 'mass' and it wasn't until we startedour exploration of space that is was possible for the average personto experience, even indirectly, what it must mean to be 'weightless'.Everyone has been confused over the difference between 'weight' and'density'. (Remember, when we make a statement and enclose it in parenthesis,nothing will be lost if you ignore it.)

Background on Mass, Weight and Density Mass,Weight and, Density

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