Gravitation applies the force ideas from force and motion to masses at a distance. The lessons start with Newton’s law and end with the difference between mass, weight and field strength.
This cluster is part of SPM Physics. It works best after force and motion I, which introduces forces and weight.
What does one set of numbers show?
Here is a worked example with G = 6.67 × 10⁻¹¹ N m² kg⁻², Earth’s mass 5.97 × 10²⁴ kg and Earth’s radius 6.37 × 10⁶ m. A 60 kg person stands on the surface.
Newton’s law gives F = GMm ÷ r² = (6.67 × 10⁻¹¹ × 5.97 × 10²⁴ × 60) ÷ (6.37 × 10⁶)² ≈ 589 N. The same person’s weight from mg with g = 9.81 is 60 × 9.81 = 589 N.
The two methods agree. The “g” in weight is a short form of GM ÷ r² for the Earth’s surface, which is why g is about 10 N kg⁻¹ and falls with distance.
How do the lessons connect?
- Applying Newton’s law of gravitation uses F = GMm ÷ r² and the inverse-square idea.
- Explaining centripetal force in orbital motion links gravity to circular paths.
- Using orbital relationships with consistent units keeps radii and powers of ten correct.
- Distinguishing mass, weight and gravitational field strength separates three related ideas.
The chapter practice set mixes all four with explained answers.
Who should start where?
If you are unsure why force drops with distance, begin with Newton’s law. If orbits seem magical, go to the centripetal force lesson. If answers are wrong by a power of ten, use the units lesson. You can follow the year order in the Form 4 learning route.
A teacher can work from your own mistakes if you want that. See online one-to-one Physics tuition for how lessons start.