Earth's Gravitational Field
Physical Science,
Earth and Space Science
Recommended for grades 9-12.
Teacher Prep time: 15 min
Student Activity: 30 min
ViewPhenomenon
What can you tell about the distribution of mass on Earth by looking at a relief map?
Essential Question
How can variations in Earth’s gravitational force be explained and predicted?
Activity Objectives
- Using GRACE satellite data, explain variations (anomalies) and patterns in Earth’s gravitational field.
- Explain how the gravity field anomalies could be addressed by Newton’s law of universal gravitation.
Next Generation Science Standards* (NGSS)
HS-PS2-4. Use mathematical representations of Newton’s law of gravitation and Coulomb’s law to describe and predict the gravitational and electrostatic forces between objects.
Science and Engineering Practices
Using Mathematics and Computational Thinking
- Use mathematical representations of phenomena to describe explanations.
Disciplinary Core Ideas
PS2.B: Types of Interactions
- Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects.
Crosscutting Concepts
Patterns
- Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.
Safety Procedures and Precautions
No PPE is required for the activity.
Teacher Preparation and Disposal
Prepare student guide by printing or uploading to a class site. Print or upload a relief map for student use. You may wish to print and laminate the magnetic field maps to reuse with other classes or activities.
- Carefully examine the 3 gravity field maps of the different regions of the world. Notice that the key is for gravity anomalies, a difference in values (milligal, or mgal, used as the unit for acceleration in the science of gravimetry) above or below the expected value for a featureless, uniform surface on Earth.
- Review with students what a map showing anomalies means.
- Record any variations or patterns you notice.
- Go over the key of the gravity field map so students understand how to interpret it.
- Compare the gravity field maps to a world relief map.
- Record any variations or patterns you notice.
Data and Observations
1. Gravity field map observations
Student answers may vary, but students should see the obvious differences between ocean and terrestrial gravity field strength. The Atlantic and Pacific tend to be below average gravity field strength, and continents tend to be above average strength. The exceptions are central Asia and the Amazon basin, which are also below average in gravity field strength.
2. Gravity field map and relief map comparisons
Student answers may vary. Positive anomalies are associated with mountain ranges. Note the Rockies, Andes, Alps, and Himalayans. Large river basins like the Mississippi, Amazon, and Nile appear slightly below average, which makes sense because the rivers and large, extended flood plains do not have a buildup of mass. The Arctic and Antarctic have positive anomalies.
Analysis & Discussion
How can Newton’s law of universal gravitation be used to explain the anomalies in the global gravity field map? Use the table of densities to help support your claims.
Newton’s law of universal gravitation has the mass of 2 objects in the numerator. Based on the global gravity field maps and topographic or relief maps, it is apparent that mass is not evenly distributed across the globe. Water is less dense than the rocks that make up continents, which means that for the same amount of volume, water has less mass than rock.
As rock builds up or erodes away, mass is redistributed. As mass is redistributed, the gravity field changes. For example, as mass increases, like along a mountain chain, the magnetic field strength should increase. This is supported by the red color on the gravity field map indicating a positive anomaly. A decrease in mass or high volume of water, like in the Amazon basin, shows a negative anomaly since water is less dense than rock.
*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.
