# 1.10: Gravity, Mass, and Density

- Page ID
- 9698

## Gravity, Mass, and Density

**Gravity** is the weak force that attracts a body toward the center of the earth, or toward any other physical body having mass.

**Mass** is the property of matter that measures its resistance to acceleration. Roughly, the mass of an object is a measure of the number of atoms in it. Gravity is the force that holds Earth in orbit around the Sun, and the Moon in orbit around the Earth. Mass is often confused with **weight**. Weight is a measure of an amount of mass under the influence of gravity. For instance, a 150 pound person on Earth would only weigh 25 pounds on the moon because the Moon only has 1/6 the gravity of Earth.

**Density **is the ratio between mass and volume. It is a measure of how much matter an object has in a unit volume (such as cubic meter or cubic centimeter).

• **Density = mass/volume**

• Usually defined in **grams per cubic centimeter** - **gm/cm ^{3}**

**Density Stratification**

• The earth and oceans have layers based upon density differences, they are density stratified.

**Examples of the density of earth materials**:

• Air ~0.1 gm/cm^{3}

• Freshwater 1.0 gm/cm^{3}

• Saltwater ~1.001-1.03 gm/cm^{3}

• Surface rocks ~3 gm/cm^{3}

• Center of earth ~16 gm/cm^{3}

Calculate the change in density when we add 1% salt to freshwater: (.99)(1.0 gm/cm^{3}) + (0.01)(3.0 gm/cm^{3}) = 1.02 gm/cm^{3}

**Seawater** has an average density of **1.027 gm/cm ^{3}**, but this varies with temperature and salinity over a range of about 1.020 to 1.029 gm/cm

^{3}.