Chapter 5 · Section 1Textbook pages 95–97

Buoyant Force പ്ലവക്ഷമബലം

An iron nail sinks, yet a steel ship weighing thousands of tonnes floats. An empty bottle shoots back up when you push it under water. A bucket feels light while it is under water and suddenly heavy when it comes out. One hidden upward push explains all of these – and on this page you learn to see it, feel it and measure it with a spring balance.

📖 What you need (pages 1–4 of this guide)

Weight W = mg, with g = 9.8 m/s². A 100 g mass weighs 0.1 × 9.8 = 0.98 N. Also 1 kgwt = 9.8 N.
A spring balance (സ്പ്രിങ് ബാലൻസ്) shows the pull on its hook in newtons.
Forces are drawn as arrows; when the upward and downward forces are equal, the object stays still.
Page 4: water pushes harder on the deeper bottom face of an object than on its top face – that is where the upward push comes from.

1. A ship floats, a nail sinks

The chapter opens with India's aircraft carrier INS Vikrant on the sea, with Kochi's Chinese fishing nets on the shore. A boy wonders:

വളരെ കുറഞ്ഞ ഭാരമുള്ള ഇരുമ്പാണി പോലും ജലത്തിൽ മുങ്ങി പോകുന്നു. എന്നാൽ ടൺ കണക്കിന് ഭാരമുള്ള കപ്പൽ എങ്ങനെയാണ് സമുദ്രജലത്തിൽ പൊങ്ങിക്കിടക്കുന്നത്?Even a very light iron nail sinks in water. So how does a ship weighing thousands of tonnes float on the sea?

🎬 Nail and ship – both made of iron and steel

Hold on to this puzzle. You will solve it completely on page 8 (principle of floatation). The first clue is right here: water pushes up on everything put into it.

🌴 Kerala connection – INS Vikrant, built at Cochin Shipyard

INS Vikrant, India's first home-built aircraft carrier, was built at Cochin Shipyard in Kochi and joined the Navy in 2022. It is about 262 m long and, fully loaded, has a mass of about 43 000 tonnes – that is about 43 000 × 1000 × 9.8 ≈ 4.2 × 10⁸ N of weight. Yet it floats. By the end of page 8 you will be able to explain exactly how.

2. Activity – an empty bottle in a bucket (Fig 5.1)

Take a transparent bucket about three-quarters (¾) full of water. Put an empty plastic bottle with its cap closed on the water surface (ജലോപരിതലം). Now try to push it down into the water with your hand. Share what you feel with your friends.

⚠ Never lean over a well or play near deep water; do water experiments in a bucket or tub, with an adult nearby.

🧪 Virtual lab: push the bottle down, then let go

push the bottle down %

Sample bottle: 1 litre (1000 cm³), mass 25 g (approx.). Up to 100 % the slider says how much of the bottle is under water; beyond 100 % you push the whole bottle deeper. Watch the blue arrow: it grows while more of the bottle goes under, then stops growing once the bottle is completely under water.

Textbook questions (page 95) – think first, then check.

ഈ കുപ്പി ജലത്തിൽ പൂർണ്ണമായി താഴ്ത്തിയ ശേഷം കുപ്പിയിൽ നിന്ന് കൈ എടുക്കുക. എന്ത് നിരീക്ഷിക്കുന്നു? Push the bottle fully under, then take your hand away. What do you observe?
പ്ലാസ്റ്റിക് കുപ്പി ജലോപരിതലത്തിലേക്ക് ഉയർന്നുവരാനുള്ള കാരണം എന്തായിരിക്കും? Why does the bottle come up to the surface?
ഭൂഗുരുത്വാകർഷണ ബലം ഈ കുപ്പിയിൽ അനുഭവപ്പെടുന്നുണ്ടോ? ഉണ്ടെങ്കിൽ ഏത് ദിശയിൽ? Does the Earth's gravitational force act on the bottle? If so, in which direction?
എന്തുകൊണ്ടാണ് കുപ്പി ജലത്തിൽ മുങ്ങിക്കിടക്കാത്തത്? Why doesn't the bottle stay under water?

1. The bottle shoots up to the surface (it may even jump out a little) and then floats. Why: nothing holds it down any more, and an upward force is acting on it.
2. The water pushes it upwards with a force. Why: you had to push hard to keep it under – the water was pushing back up on it all the time.
3. Yes – downwards, towards the centre of the Earth. That pull is the bottle's weight (here only about 0.25 N).
4. Because the upward force of the water on the fully immersed bottle (about 9.8 N for a 1 litre bottle) is much greater than its weight (about 0.25 N). The net force is upwards, so the bottle rises until only a small part of it is under water.

3. The bucket from the well and the mug in the bucket (Fig 5.2)

The textbook (page 96) asks: കിണറ്റിൽ നിന്ന് ജലം ഉയർത്തുമ്പോൾ ബക്കറ്റ് ജലോപരിതലത്തിൽ എത്തുന്നത് വരെ ഭാരക്കുറവ് അനുഭവപ്പെടാറുണ്ട്. എന്തായിരിക്കും ഈ ഭാരക്കുറവിന് കാരണം? – When we draw water from a well, the bucket feels lighter until it reaches the water surface. What is the reason for this loss of weight?

The activity (Fig 5.2): take a bucket ¾ full of water. Lift a mug full of water from the bottom of the bucket up to the surface, and then out of the water. Compare what your hand feels.

⚠ Never lean over a well or play near deep water; do water experiments in a bucket or tub, with an adult nearby. Drawing water from a well is an adult's job – use the bucket-and-mug version yourself.

🧪 Virtual lab: lift it slowly – what does your hand feel?

lift it %

Sample values (approx.): mug 0.1 kg of plastic (density ≈ 1200 kg/m³) holding 500 mL of water; well bucket 0.5 kg of plastic holding 10 L of water. Force felt = weight − buoyant force. The water inside the container is held up by the water outside as long as both are under the surface.

Q (Fig 5.2). ബക്കറ്റിലെ ജലത്തിനുള്ളിലും ജലനിരപ്പിന് മുകളിലും കപ്പിന്റെ ഭാരത്തിൽ എന്ത് വ്യത്യാസമാണ് അനുഭവപ്പെട്ടത്? എന്തായിരിക്കും കാരണം? – What difference did you feel in the weight of the mug inside the water and above the water surface? What is the reason?

Inside the water the mug feels much lighter (in our lab about 0.16 N); above the surface it feels heavy (about 5.9 N). Why: the water exerts an upward force on the mug and on the water inside it. Out of the water this support is gone, so your hand has to hold the full weight.

Q (page 96). Why does the well bucket feel lighter until it reaches the water surface?

While the bucket is under water, the water of the well pushes it upwards (buoyant force), so the rope has to pull only weight − buoyant force – less than 1 N in our lab. As the bucket rises out of the water this upward push shrinks to zero and we must lift its full weight (about 103 N ≈ 10.5 kgwt). Why it changes gradually: the buoyant force depends on how much of the bucket is still under water.

ദ്രാവകത്തിൽ സ്ഥിതി ചെയ്യുന്ന വസ്തുക്കളിൽ ആ ദ്രാവകം മുകളിലേക്ക് ബലം പ്രയോഗിക്കുന്നതായി അനുഭവപ്പെട്ടല്ലോ? – So a liquid exerts an upward force on objects in it.

4. Only in liquids? – the helium balloon

അന്തരീക്ഷവായുവിൽ സ്വതന്ത്രമാക്കുന്ന ഹീലിയം നിറച്ച ബലൂണിന് എന്ത് സംഭവിക്കും? – What happens to a helium-filled balloon released in air?

🎈 Let the balloon go

Sample balloon (approx.): 10 litres of helium; rubber 3 g + helium 1.8 g → weight ≈ 0.047 N. The air it pushes aside has a mass of about 12.9 g, so the air pushes it up with ≈ 0.126 N.

Q. What happens to the helium balloon, and why?

It rises. Why: the balloon has weight, but the air pushes it upwards with a force greater than its weight (അതിനെക്കാൾ കൂടിയ ബലം അന്തരീക്ഷവായു മുകളിലേക്ക് പ്രയോഗിച്ചതുമൂലമല്ലേ ബലൂൺ ഉയർന്നു പൊങ്ങിയത്). So gases push objects upwards too.

So the upward push is felt not only in liquids (ദ്രാവകങ്ങൾ) but also in gases (വാതകങ്ങൾ). Liquids and gases together are called fluids (ദ്രവങ്ങൾ) – they can flow.

From these activities, two forces act on an object in a fluid:

the force acting downwards on the object – its weight (ഭാരം);
the force the fluid exerts upwards on the object.

⭐ Key idea (textbook page 96) – Buoyant force

ഒരു വസ്തു പൂർണ്ണമായോ ഭാഗികമായോ ഒരു ദ്രവത്തിൽ മുങ്ങിയിരിക്കുമ്പോൾ ആ വസ്തുവിൽ ദ്രവം മുകളിലേക്ക് പ്രയോഗിക്കുന്ന ബലമാണ് പ്ലവക്ഷമബലം.

When an object is fully or partly immersed in a fluid, the upward force exerted on it by the fluid is called the buoyant force (പ്ലവക്ഷമബലം).

📒 Science diary (സയൻസ് ഡയറി) – where do we feel buoyant force?

The textbook gives the first one; try to add more yourself before opening the list.

ജലത്തിനുള്ളിൽ നിന്ന് വായു കുമിളകൾ ഉയരുന്നത്. – Air bubbles rise up through water.

A log, a coconut or a country boat floats on a river or on Vembanad lake.
We feel lighter in a pond or a swimming pool, and can lift a friend easily in water.
A beach ball or a float is very hard to hold under water.
A mug of water or a bucket feels light under water and heavy above it.
Hot-air balloons and helium balloons rise in air (buoyant force of air).
Ships, boats and the house boats of Alappuzha float.
Smoke and steam rise; oil drops rise to the top of water.

5. Can we measure the buoyant force? (Fig 5.3, Table 5.1)

The thinking boy asks: പ്ലവക്ഷമബലം കണക്കാക്കാനാകുമോ? – Can the buoyant force be calculated? The textbook's experiment: take a spring balance marked in newtons, a trough about ¾ full of water, a stone, an iron block and a glass slab. Weigh each in air (W₁, വായുവിലെ ഭാരം), then lower it completely into the water and weigh it again (W₂, ജലത്തിലെ ഭാരം).

🎬 Fig 5.3 step by step – the stone in air and in water

🧪 Your own trials – complete Table 5.1

object lower the balance %

Table 5.1 – sample readings (your real readings will differ slightly)
Object വസ്തു	Weight in air W₁ (N)	Weight in water W₂ (N)	Loss of weight in water = buoyant force W₁ − W₂ (N)

Sample objects (approx.): stone 300 g (density ≈ 2600 kg/m³), iron block 500 g (7800 kg/m³), glass slab 200 g (≈ 2500 kg/m³). The lab works out the buoyant force from F = ρ_water × V_{under water} × g – you will meet this formula on page 7.

Q. ജലത്തിൽ വസ്തുക്കൾക്ക് ഭാരക്കുറവ് അനുഭവപ്പെട്ടത് പ്ലവക്ഷമബലം മൂലമല്ലേ? – Isn't the loss of weight felt by the objects in water due to the buoyant force?

Yes. The mass of the stone did not change and the Earth still pulls it with W₁. The balance reads less only because the water pushes the stone up and takes part of its weight. So the apparent loss of weight (ഭാരക്കുറവ്) W₁ − W₂ is exactly the buoyant force.

⭐ Key idea (textbook page 97)

ഒരു വസ്തു ദ്രവത്തിൽ സ്ഥിതി ചെയ്യുമ്പോൾ അതിൽ അനുഭവപ്പെടുന്ന പ്ലവക്ഷമ ബലം വസ്തുവിന്റെ ഭാരക്കുറവിന് തുല്യമായിരിക്കും.

When an object is in a fluid, the buoyant force on it is equal to its apparent loss of weight.

Buoyant force = W₁ − W₂ (weight in air − weight in the liquid)

💡 “Apparent” loss – the weight is not really lost

The Earth's pull on the stone (mg) is the same in air and in water. What changes is the reading: two upward forces now share the job of holding the stone – the spring balance (W₂) and the water (buoyant force). So W₂ + buoyant force = W₁.

🧪 The textbook's simulation – PhET → Buoyant Force (KITE Ubuntu)

The textbook's box “PhET → Buoyant Force (KITE Ubuntu)” refers to the copy of the PhET buoyancy simulation installed offline on the KITE Ubuntu school laptops – open it from the Education/Science menu if your school computer has it. In it you drop blocks of different materials into different liquids and read the forces on a scale – exactly what this page's labs do. Online, the matching PhET simulations are called “Buoyancy” and “Buoyancy: Basics” (and “Density”) – see the Explore box below.

🌐 Explore online needs internet · opens in a new tab

🧪 Buoyancy: Basics (PhET simulation) PhET, University of Colorado Boulder
Drop blocks into water and watch the upward buoyant force balance their weight.
🧪 Buoyancy (PhET simulation) PhET, University of Colorado Boulder
The fuller version (the textbook's 'Buoyant Force' sim): change fluid density and see force arrows for weight and buoyancy.
🧪 Interactive - Buoyancy Simulation The Physics Classroom
Lower an object into a fluid and read off the buoyant force and the apparent loss of weight.
📄 Will it sink or float? (Vigyan Pratibha learning unit, PDF) HBCSE-TIFR (Vigyan Pratibha), India
Indian hands-on lab unit: measure upthrust with a spring balance and find what it depends on.

6. Check yourself

🧭 Summary

Liquids and gases are fluids. A fluid pushes upwards on any object fully or partly immersed in it: the buoyant force.
Two forces act on an object in a fluid: its weight (down) and the buoyant force (up). If the buoyant force is bigger, the object rises (bottle, balloon).
Buoyant force = apparent loss of weight = W₁ − W₂. Sample: stone 2.94 N in air, 1.81 N in water → buoyant force 1.13 N.
Once an object is fully under water, pushing it deeper does not increase the buoyant force.

Next: What decides the buoyant force? ▶