Gravitation

Why This Chapter Matters

The same interaction that makes a dropped stone fall also keeps the Moon in orbit. Newton's insight was to treat terrestrial falling and celestial motion as consequences of one universal force. This chapter begins at astronomical scale, then returns to everyday questions about falling bodies, weighing scales, pressure and floating. The unifying idea is force: gravity pulls masses together, while fluids exert pressure and buoyancy.

Bridge from Earlier Learning

• Newton laws
• Mass and force
• Equations of motion
• Area and volume
• Density

These ideas are reused and deepened in this chapter.

1. Universal Gravitation

2. Why Objects Fall Toward Earth

Earth attracts an object and the object attracts Earth with equal force by Newton's third law. The object's acceleration is noticeable because its mass is small compared with Earth. Earth's acceleration toward the object is extremely tiny because its mass is enormous. Near Earth's surface, ignoring air resistance, freely falling objects have approximately the same acceleration g.

3. Free Fall Equations

Free fall is motion under gravity alone. The standard equations of uniformly accelerated motion can be used with acceleration replaced by g. Sign convention is crucial. If upward is positive, acceleration due to gravity is negative; if downward is positive, it is positive. Many mistakes are sign errors rather than conceptual errors.

4. Mass and Weight

Mass is a property of the body and does not depend on location in ordinary school physics. Weight is the gravitational force on the body, so it depends on local g. A person has essentially the same mass on Earth and Moon but lower weight on the Moon because lunar gravity is weaker.

5. Thrust and Pressure

Thrust is force acting normally on a surface. Pressure is thrust per unit area. The same force produces larger pressure when concentrated on a smaller area, explaining sharp knives, nails and wide shoulder straps. In fluids, pressure acts in all directions and generally increases with depth.

6. Buoyancy

An immersed body experiences pressure on all sides. Since fluid pressure is greater at greater depth, the upward pressure force on the lower surface can exceed the downward force on the upper surface. The net upward force is buoyancy or upthrust. Whether an object rises, sinks or remains suspended depends on the balance between buoyant force and weight.

7. Archimedes Principle

Archimedes principle states that an immersed body experiences an upward buoyant force equal to the weight of the fluid displaced. This explains apparent loss of weight in water and is used in density measurements. A floating object displaces enough fluid that buoyant force equals its weight.

8. Density and Relative Density

Density is mass per unit volume. Relative density compares a substance's density with water, so it has no unit. Objects with average density less than the surrounding fluid tend to float. Shape and enclosed air matter, which is why a steel ship can float even though steel itself is denser than water.

Visualise the Concept

Gravity pulls downward while fluid-pressure differences can create upward buoyant force.

Formula / Relationship Sheet

Worked Examples & Scientific Reasoning

Observe Like a Scientist

Exam Thinking Strategy

Common Mistakes

Quick Revision

Focused Practice

Final Chapter Summary