Science, Tech, Math › Science What Is Mass? Why are feathers lighter than bricks? Share Flipboard Email Print oska25/Getty Images Science Physics Physics Laws, Concepts, and Principles Quantum Physics Important Physicists Thermodynamics Cosmology & Astrophysics Chemistry Biology Geology Astronomy Weather & Climate By Andrew Zimmerman Jones Math and Physics Expert M.S., Mathematics Education, Indiana University B.A., Physics, Wabash College Andrew Zimmerman Jones is a science writer, educator, and researcher. He is the co-author of "String Theory for Dummies." our editorial process Andrew Zimmerman Jones Updated March 28, 2019 Mass is a scientific term used to describe the density and type of atoms in any given object. The SI unit of mass is the kilogram (kg), though mass can also be measured in pounds (lb). To quickly understand the concept of mass, think of a pillowcase filled with feathers and a similar pillowcase filled with bricks. Which has a greater mass? Because the atoms in the bricks are heavier and denser, the bricks have a greater mass. Thus, even though the pillowcases are the same size, and both are filled to the same degree, one has much greater mass than the other. Scientific Definition of Mass Mass is the quantity of inertia (resistance to acceleration) possessed by an object or the proportion between force and acceleration referred to in Newton's Second Law of Motion (force equals mass times acceleration). In other words, the more mass an object has, the more force it takes to get it moving. Weight Versus Mass In most common instances, mass is determined by weighing the object and using the force of gravity to calculate the value automatically. In other words, in most real-world situations, mass is the same thing as weight. In the example of the feathers and the bricks, the difference in mass can be described by the relative weight of the two pillowcases. Obviously, it takes a lot more work to move a bag of bricks than it does to move a bag of feathers. But weight and mass are not really the same thing. Because of the relationship between weight and mass, these concepts are frequently confused. You can, in fact, convert exactly between weight and mass on the Earth's surface. But that's because we live on planet Earth, and while we are on this planet gravity is always the same. If you were to leave the Earth and go into orbit, you would weigh almost nothing. Yet your mass, defined by the density and type of atoms in your body, would remain the same. If you landed on the moon with your scale and weighed yourself there, you'd weigh more than you weighed in space but less than you weighed on Earth. If you continue your journey to the surface of Jupiter, you'd weigh a great deal more. If you weigh 100 pounds on Earth you would weigh 16 pounds on the moon, 37.7 pounds on Mars, and 236.4 pounds on Jupiter. Yet, throughout your trip, your mass would remain essentially the same. Importance of Mass in Daily Life The mass of objects is tremendously important in our daily lives. We work hard to reduce our mass when we are dieting. Less mass translates to less weight.Many manufacturers work to create less massive versions of items ranging from bicycles and running shoes to cars. When an object is less massive it has less inertia and is easier to move.Body mass index (BMI) is a measure of body fat based on your weight in relation to your height. Fat is lighter (less massive) than muscle, so a high BMI suggests that your body contains more fat and less muscle than it should.