The difference between mass and weight is the mass is a measure of the amount of matter in an object, while weight is a measure of the effect of gravity on that mass. In other words, gravity causes a mass to have weight. The relationship between mass and weight is a simple equation:
W = m * g
Here, W is weight, mass is mass, and g is gravity
People often use the words “mass” and “weight” interchangeably because gravity is pretty much constant on Earth, so there isn’t a difference between their values. But, if you compare weight on Earth to a different place, like the Moon, you get different values. Your mass on the Moon remains the same, but your weight is different because the acceleration due to gravity is different there.
The Difference Between Mass and Weight
There are several differences between mass and weight.
Mass is an intrinsic property of matter. It doesn’t change depending on where you measure it. It is a scalar value, which means it has magnitude, but no direction associated with it. The mass of an object is never zero. You measure mass with an ordinary balance on Earth or an inertial balance in space.
Weight depends on the effect of gravity, so it can change depending on where it’s measured. In the absence of gravity, weight can be zero. Because weight is a force, it is a vector quantity. It has both magnitude and direction. You measure weight using a spring balance.
| Mass is a property of matter. The mass of an object is the same everywhere. | Weight depends on the effect of gravity. Weight increases or decreases with higher or lower gravity. |
| Mass of an object can never be zero. | Weight can be zero if no gravity acts upon an object, as in space. |
| Mass does not change according to location. | Weight varies according to location. |
| Mass is a scalar quantity. It has magnitude. | Weight is a vector quantity. It has magnitude and direction. The direction is toward the center of the Earth or other gravity well. |
| Mass may be measured using an ordinary balance. | Weight is measured using a spring balance. |
| Mass is measured in grams (g) and kilograms (kg). | Weight is measured in Newtons (N). |
Units of Mass and Weight
We measure weight in grams, kilograms, ounces, and pounds. Technically, grams (g) and kilograms (kg) are units of mass. The SI unit of force is the Newton (N), with a 1 kg mass having a force of 9.8 N on Earth. The US unit of force is the pound (lb), while the unit of mass is something called a slug. A pound is the force required to move a 1 slug mass at 1 ft/s2. One slug has a weight of 32.2 pounds.
While it’s fine to use pounds and kilograms interchangeably for most practical purposes, in science it’s best to use kilograms for mass and Newtons for force.
Mass vs Weight Activities
Weight in an Elevator
One simple activity to see the difference between mass and weight is weighing yourself in an elevator. A digital scale works best because it’s easier to see the change in weight as the elevator ascends (increasing acceleration, which adds to gravity) and descends (negative acceleration, which decreases the effect of gravity). For a classroom activity, first have students weigh themselves (or an object) on a scale and discuss whether the value they obtain is mass, weight, or whether it matters. Next, have them make predictions about what will happen in an elevator and conduct the experiment to test their hypothesis.
It can be a challenge to explore the difference between mass and weight on Earth because gravity is all around us. Fortunately, the astronauts on the International Space Station (ISS) conducted experiments that complement activities on Earth. Follow along with the video and compare what happens in microgravity compared to Earth.
Measuring Weight With Rubber Bands
You can compare the weights of objects by hanging them from rubber bands. On Earth, gravity affects a heavier object more than a lighter one and stretches the rubber band further. Predict what will happen when heavy and light objects are suspended from rubber bands on the ISS. What shape will the rubber band take? Do you expect there to be a difference between the way the rubber band responds to a heavy object compared to a light object?
Mass Cars
The easiest way to explore mass on Earth is to conduct experiments that move horizontally rather than vertically. This is because objects can’t change their position from the effect of gravity. Build a “mass car” and use an air pump to accelerate the mass across rollers or a low-friction track. Change the mass of the car, make a prediction about how this will change how far the car rolls, and perform an experiment to test the hypothesis. You can graph the distance the car moves compared to its mass. Predict whether the results will be different in space and use the ISS experiment to reach a conclusion.
Accelerating Mass With a Tape Measure
If you can’t build a mass car or get an air pump, you can use a retractable tape measure to apply acceleration to an object. Do this by pulling out the measuring tape one meter or three feet and attaching the end to an object. Secure or hold the tape measure and click the button to retract the tape. Does it take the same amount of time to retract the tape with a heavier object compared to a lighter one? What does this say about the acceleration produced by the tape measure? Ask students to make predictions and explain results. Make a prediction about what will happen on the ISS and see if you’re correct.
References
- Galili, Igal (2001). “Weight versus Gravitational Force: Historical and Educational Perspectives.” International Journal of Science Education. 23(1): 1073-1093.
- Gat, Uri. (1988). “The Weight of Mass and the Mess of Weight.” Standardization of Technical Terminology: Principles and Practice. ASTM. 2: 45-48.
- Hodgman, Charles D., editor. (1961). Handbook of Chemistry and Physics (44th ed.). Chemical Rubber Co. 3480-3485.
- Knight, Randall Dewey (2004). Physics for Scientists and Engineers: a Strategic Approach. Pearson.
- Morrison, Richard C. (1999). “Weight and Gravity—The Need for Consistent Definitions.” The Physics Teacher. 37(1).