# Weight and Mass Facts

• Mass is the amount of matter in an object.
• Weight is not the same as mass. Scientists say weight is the force of gravity pulling on an object. Weight varies with the mass of the object and the strength of gravity.
• Objects weigh more at sea level, which is nearer the center of the Earth, than up a mountain.
• A person on the Moon weighs one sixth of their weight on Earth because the Moon’s gravity is one sixth of the Earth’s gravity.
• Weight varies with gravity but mass is always the same, so scientists use mass when talking about how heavy something is.
• The smallest known mass is that of a photon (see light and atoms). Its mass is 5.3 times 10- 63 (62 zeros and a 1 after the decimal point) kg.
• The mass of the Earth is 6 x 1024 (six trillion trillion) kg. The mass of the Universe may be 10 51 (10 followed by 50 zeros) kg.
• Density is the amount of mass in a certain space. It is measured in grams per cubic centimeter (g/cm3).
• The lightest solids are silica aerogels made for space science, with a density of 0.005 g/cm 3. The lightest gas is hydrogen, at 0.00008989 g/cm 3. The density of air is 0.00128 g/cm3.
• The densest solid is osmium at 22.59 g/cm3. Lead is 11.37 g/cm 3. A neutron star has an incredible density of about one billion trillion g/cm3.