Mass vs. Weight

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Though the terms weight and mass are used interchangeably in common language, in science there is distinct difference between the two terms. The weight of an object = force of gravity felt by that object but the mass of an object is the amount of matter the object has. Mass is a measure of the object's resistance to acceleration: a push on a skateboard will make it roll away quickly but the same push on a more massive car will barely budge it.

An object's weight depends on the pull of the gravitating object but the object's mass is independent of the gravity. For example, Joe Average weighs himself on the Earth's surface and then on the Moon's surface. His weight on the Moon will be about six times less than on the Earth but the number of atoms in his body has not changed so his mass is the same at the two places. In the old English unit system, there is a ``pound'' of force and ``pound'' of mass. On only the Earth's surface, an object's pound of mass = the number of pounds of force felt by the object due to the Earth's gravity.

In the metric system there is no confusion of terms. A kilogram is a quantity of mass and a newton is a quantity of force. One kilogram (kg) = 2.205 pounds of mass and 4.45 newtons (N) = 1 pound of force. If someone uses ``pounds'', be sure you understand if s/he means force or mass!

How do you do that?

To find something's weight in newtons, you multiply the mass in kilograms by the acceleration of gravity in the units of meters/seconds2. For example: Joe Average has a mass of 63.5 kg and he feels a force of gravity on the Earth = 63.5 kg × 9.8 m/s2 = 623 kg m/s2 = 623 N. His weight is 623 N. The other value in the preceding equation, 9.8 m/s2, is the acceleration due to gravity close to the Earth's surface. Joe Average's weight at other places in the universe will be different but his mass will remain the same.

Vocabulary

kilogram mass newton
weight

Review Questions

  1. What is the difference between mass and weight?
  2. When the astronauts landed on the Moon, how were they able to stay on the ground?
  3. On the Moon, the astronauts weighed about six times less than they did on the Earth. Compare the amount of gravity on the Moon's surface with that on the Earth's surface. If objects fall with an acceleration of about 10 m/s2 on the Earth, how much would the acceleration be on the Moon's surface? Explain your answer.
  4. If Joe Astronaut has a mass of 40 kilograms on the Earth, how much mass would he have on an asteroid with 10 times less surface gravity than the Earth's surface gravity? Explain your answer.

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last updated: 11 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel