Summary & Quick Quiz പ്ലവക്ഷമബലം
The whole chapter on one page: the eight key boxes of the textbook, every formula and fact you need, the mistakes to avoid, flashcards, a scored quiz and your study checklist.
1. The chapter in one picture
Read it clockwise from the top left: a fluid pushes up on an object in it; we measure the push as the loss of weight on a spring balance; it depends on the density of the fluid and the volume under it; it equals the weight of the fluid displaced (Archimedes); comparing it with the weight tells us whether the object floats or sinks; floating instruments measure relative density; and ships, submarines, life jackets and balloons all use it.
2. The key boxes of the textbook
When an object is fully or partly immersed in a fluid, the upward force exerted on it by the fluid is the buoyant force. Liquids and gases together are called fluids. – page 5
The buoyant force on an object in a fluid equals its apparent loss of weight (weight in air − weight in the fluid). – page 5
The density of the fluid is one factor that influences the buoyant force: the greater the density of the liquid, the greater the buoyant force (glycerine > salt solution > water > kerosene). – page 6
For a fully immersed object the buoyant force depends on (1) the density of the fluid and (2) the volume of the object. Not on the object’s own density, and not on the depth. – page 6
When an object is fully or partly immersed in a fluid, the buoyant force on it equals the weight of the fluid displaced by the object. – page 7
When an object floats in a fluid, its weight equals the weight of the fluid it displaces. Sinking: FB < weight. Floating: FB = weight. – page 8
It tells how many times the density of water the substance’s density is. It has no unit. Kerosene 0.81, copper 8.9, mercury 13.6. – page 9
A hydrometer measures the relative density of liquids; its scale values increase downwards and it floats higher in denser liquids (reads 1.00 in water). A lactometer is a hydrometer for milk. – page 9
3. Formulas & facts
📐 Formulas & facts card
floating: FB = W · share under = ρobject ÷ ρfluid · RD = ρ ÷ ρwater (no unit)
W = m × g · g = 9.8 m/s² · 1 kgwt = 9.8 N · density = mass ÷ volume
| Rule / fact | Remember |
|---|---|
| Weight in air W₁, in a liquid W₂ | FB = W₁ − W₂ (the apparent loss of weight) |
| Fully immersed object | Vunder = volume of the object; FB is the same at every depth |
| Equal volumes, same liquid | equal buoyant force – whatever the objects are made of (copper and iron spheres of the same diameter) |
| Equal masses | the less dense object has more volume → more buoyant force (iron block > copper block) |
| Sinks | FB < W even when fully under; ρobject > ρliquid |
| Floats | FB = W; ρobject < ρliquid; spring balance reads 0 |
| Ship from river to sea / after unloading | it rises: denser water / less weight → less volume needs to be displaced |
| Relative density | RD = ρ ÷ 1000 kg/m³; ρ = RD × 1000 kg/m³; RD > 1 sinks in water, RD < 1 floats |
| Hydrometer | principle of floatation; 1.00 in water; floats higher in denser liquids; values increase downwards |
| Lactometer | pure milk about 1.028–1.032 (approx.); added water lowers the reading; skimmed milk reads higher |
| Fluids | liquids and gases – air gives a buoyant force too (helium and hot-air balloons) |
| Substance | Density (kg/m³) | Relative density | In water it… |
|---|---|---|---|
| Water ജലം | 1000 | 1 | – |
| Saturated salt solution പൂരിത ഉപ്പുലായനി | 1025 | 1.025 | denser – fresh water floats on it |
| Glycerine ഗ്ലിസറിൻ | 1260 | 1.26 | sinks |
| Kerosene മണ്ണെണ്ണ | 810 | 0.81 | floats on top |
| Coconut oil വെളിച്ചെണ്ണ | 920 | 0.92 | floats on top |
| Ice | 917 | 0.917 | floats, about 92 % under (89 % in sea water) |
| Aluminium / iron / copper | 2700 / 7800 / 8900 | 2.7 / 7.8 / 8.9 | sink |
| Mercury | 13 600 | 13.6 | sinks (iron floats on mercury) |
- Thinking the buoyant force grows as a fully immersed object goes deeper. It doesn’t – the volume displaced stays the same (Let us assess Q6: 30 N at both depths).
- Thinking a heavier (denser) object of the same volume gets more buoyant force. Equal volume in the same liquid → equal buoyant force.
- Writing a unit for relative density. It is a ratio of two densities – no unit.
- Saying a floating object has no weight, or that the buoyant force on a floating object is greater than its weight. It is equal; only the spring balance reads 0.
- Confusing “weight of displaced water” with “volume of displaced water”. They are linked by density: mass = density × volume, weight = mass × g.
- Forgetting to convert kgwt ↔ N (1 kgwt = 9.8 N) or g → kg (100 g = 0.1 kg).
- Saying a hydrometer sinks deeper in a denser liquid. It floats higher.
- Thinking the buoyant force exists only in water. It acts in every fluid, including air.
4. Flashcards
Click a card to flip it.
5. Scored quiz
Answer all fifteen, then press Check my answers. Each wrong answer shows the page to revise. For number answers type only the number.
6. Your study checklist
Ticks come from “Mark this page as completed” at the bottom of each page.
Next: the model test paper ▶ – attempt it on paper with a timer first, then check with the solved paper.