20
Question Question 20
Why a block of plastic when released under water come up to the surface of water?
When plastic is immersed in water, two forces act on it:
- Gravitational force or its weight in vertically downward direction
- Buoyant force in vertically upward direction.
As the density of plastic is less than that of water, the buoyant force is greater than its weight. Hence, the plastic comes up to the surface.
BRIGHT TUTORIALS
BRIGHT TUTORIALS
CBSE Class IX | Academic Year 2026-2027
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Science | Chapter 9: GravitationWeb Content — Quick Reference
Chapter 9: Gravitation — Quick Reference
Quick Revision Points
- F = Gm₁m₂/r² (G = 6.674 × 10⁻¹¹ N·m²/kg²)
- g = GM/R² = 9.8 m/s² (Earth); g on Moon ≈ g/6
- Mass (kg, constant) vs Weight (N = mg, varies with location)
- Pressure P = F/A (Pa = N/m²); Fluid pressure P = hρg
- Archimedes: buoyant force = weight of displaced fluid
- Float if density < fluid density; Sink if density > fluid density
- Relative density = density of substance / density of water (no unit)
Exam Tips for Chapter 9
- Read the detailed chapter notes for complete coverage of all NCERT topics.
- Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
- Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
- For numericals (if applicable), practice at least 20 problems of varying difficulty.
- Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
- Detailed Notes: ch09-gravitation.html
- Practice Questions: 100+ questions with answers in 05-practice-questions/
- Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
- Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html