Compare masses
Predict how changing either mass changes the gravitational attraction.
AP Physics 1 · Unit 2, Part 2 · Circular Motion and Gravity
Newton’s Law of Gravitation and Gravitational Fields
Change two masses and their separation to test gravitational force, then distinguish the force on an object from gravitational field strength.
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Learn by doing
Three ideas you’ll put into motion
Test the distance rule
Compare center-to-center separations to investigate the inverse-square relationship.
Separate force and field
Connect gravitational force with the force per unit test mass.
The big idea
The key ideas, at a glance.
Gravity depends on both masses
Newton’s law gives F = GMm/r2, where r is the separation between centers for point or spherically symmetric masses.
A field describes the source
Field strength is g = F/m = GM/r2. Changing the test mass changes force but not the source’s field.
A quick comparison
Double the separation while keeping both masses fixed.
Gravitational force becomes one quarter as large.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Newton’s laws, ratios, and squared quantities. Distinguish source mass from test mass.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 2: gravitational interactions, fields, and proportional reasoning.
- Check for understanding
- Ask what happens to force and field strength when only the test mass doubles.
- Optional extension
- Investigate the setup in the Newton’s Cannon experiment.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.