Confusion of Mass and Weight

A few further comments should be added about the single force that is a source of much confusion to many students of physics – the force of gravity. As mentioned above, the force of gravity acting upon an object is sometimes referred to as the weight of the object. Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object. Mass is related to how much stuff is there and weight is related to the pull of the Earth (or any other planet) upon that stuff. The mass of an object (measured in kg) will be the same no matter where in the universe that object is located. Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not (at least for purposes of our study); and its mass will be 2 kg whether it is being pushed upon or not.

On the other hand, the weight of an object (measured in Newton) will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g – the gravitational field strength. On earth’s surface g is 9.8 N/kg (often approximated as 10 N/kg). On the moon’s surface, g is 1.7 N/kg. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet. So if we were to measure g at a distance of 400 km above the earth’s surface, then we would find the g value to be less than 9.8 N/kg. (The nature of the force of gravity will be discussed in more detail in a later unit of The Physics Classroom.) Always be cautious of the distinction between mass and weight. It is the source of much confusion for many students of physics.

A 1.0-kg mass is suspended from a spring scale in an effort to determine its weight. The scale reads just short of 10.0 N – close enough to call it 9.8 N. Mass refers to how much stuff is present in the object. Weight refers to the force with which gravity pulls upon the object.

Investigate!

Even on the surface of the Earth, there are local variations in the value of g that have very small effects upon an object’s weight. These variations are due to latitude, altitude and the local geological structure of the region. Use the Gravitational Fields widget below to investigate how location affects the value of g.