Activity 5.13 — Floating, Sinking and Upthrust — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, page 76, Activity 5.13

   Describe Activity 5.13 with the bottle and the bucket of water. What do you feel, and what does it show?

   Hint. There are two separate observations — one while you push, one when you let go.

   The activity. Take an empty bottle with its lid closed tightly and a bucket full of water. Push the bottle down into the water (Fig. 5.15). Do you feel an upward push? Now release the bottle. Does it bounce up?

   What you observe. You would have felt an upward push and the bottle bounces back to the surface of the water.

   Observation	What it shows
   An upward push against your hand while you press down	The water is pushing up on the bottle the whole time
   The bottle springs back to the surface when released	With your hand gone, that upward push is enough to drive it up

   The conclusion. This indicates that water applies a force on the bottle in the upward direction. In fact, all liquids apply a similar force.

   ✦ Why the lid must be closed tightly. So that the bottle stays full of air. Let water in and it will not bounce back — which is a hint that whether something floats depends on what is inside it, not just on how big it is. That is exactly the trap in exercise question 4.

   The everyday version, which the chapter puts at the start of section 5.6: While taking out water from a bucket filled with water using a mug, do you notice that the mug feels lighter when it is inside water? The same upward force is helping you hold it up.

2. 23 marksCuriosity Grade 8, Chapter 5, page 76

   Define upthrust. What decides whether an object floats or sinks?

   Hint. Two forces on the object, pointing opposite ways — compare them.

   Definition. The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.

   The two forces on an object in a liquid. When an object is placed in a liquid, the gravitational force due to the Earth acts on it downwards. But a buoyant force is applied on it by the liquid in the upward direction.

   The rule.

   Comparison	Result
   Gravitational force more than buoyant force	The object sinks
   The two forces equal	The object floats

   In the chapter's words: If the gravitational force is more than the buoyant force, the object sinks, but if the two forces are equal, the object floats.

   ✦ What the rule does not depend on. Not on size, and not on how heavy the object feels in your hand. A huge wooden block floats and a small coin sinks, because what matters is the comparison between the two forces on that particular object, not the value of either one by itself. Exercise question 4 is built entirely on this point.

   Notice that the buoyant force is a contact force, even though it appears in a section full of gravity. The water is touching the object. The Earth is not.

3. 33 marksCuriosity Grade 8, Chapter 5, page 76

   On what does the buoyant force depend, according to the chapter? How far does the chapter go with this?

   Hint. The chapter names one factor and then deliberately stops.

   What the chapter says. One of the factors on which the buoyant force depends upon, is the density of the liquid. You will learn about density in a later chapter of this book.

   Read that sentence carefully — it is doing two things.

   1. It names one factor: the density of the liquid. The phrase one of the factors admits openly that there are others, without naming them.
   2. It defers the explanation, because density has not been defined yet. It is introduced later in this same book, in The Amazing World of Solutes, Solvents, and Solutions.

   ✦ How to answer a question on this without over-reaching. Say that the buoyant force depends on the density of the liquid, note that this is one factor among others, and stop. Do not write a formula, do not give the density of water, and do not explain floating in terms of the object's density being less than the liquid's — none of that is available to you yet, and the chapter is careful not to use it.

   What you can say from the chapter. Everything about floating and sinking that this chapter needs comes from comparing two forces — the Earth's downward pull and the liquid's upward push. That is a complete account at this level, and it answers exercise questions 4 and 10 without any mention of density.

   A liquid denser than water gives a larger upward push, which is why it is easier to float in very salty water. The chapter does not say this, so offer it as an expectation, not a fact from the book.

4. 44 marksCuriosity Grade 8, Chapter 5, pages 76-77, A step further

   State Archimedes' Principle and use it to say when an object will sink and when it will float.

   Hint. The principle is about the liquid the object pushes out of the way.

   Archimedes' Principle. Archimedes, a famous Greek scientist, discovered that when an object is fully or partially immersed in a liquid, it experiences an upward force which is equal to the weight of the liquid it displaces.

   Sinking and floating, in the chapter's words:

   Comparison	Result
   Weight of liquid displaced less than the weight of the object	The object will sink
   Weight of liquid displaced equal to the weight of the object	The object will float

   ✦ This is the same rule as section 5.6, said more precisely. Section 5.6 compared the buoyant force with the gravitational force. Archimedes tells you what the buoyant force actually equals — the weight of the liquid pushed out of the way. So instead of comparing a force you cannot see with another force you cannot see, you can compare two weights you could in principle measure.

   A useful way to picture it. A floating object sinks into the water until it has shoved aside a quantity of water weighing exactly as much as itself, and then it stops sinking. A heavy object goes right under and still has not displaced its own weight in water — so it keeps going down.

   Note the words fully or partially. A floating object is only partly immersed, and the principle still applies — it is the water actually pushed aside that counts. That is precisely what makes exercise question 10 answerable.

5. 54 marksCuriosity Grade 8, Chapter 5, page 76

   Why does a mug feel lighter when it is inside water than when you lift it out?

   Hint. Two forces are acting on the mug, and your hand only has to supply the difference.

   Because the water is helping you hold it up.

   While the mug is under water, two forces act on it besides your hand:

   • The Earth pulls it down — its weight, which is the same wherever it is.
   • The water pushes it up — the buoyant force.

   Your hand only has to make up the difference between them. Lift the mug clear of the water and the upward push from the water is gone, so your hand must now support the whole weight on its own — and the mug feels suddenly heavier.

   ✦ The mug's weight has not changed. This is the same distinction as in the swing question at the start of the chapter: what you feel is not the Earth's pull itself but how much you have to supply. The Earth pulls the mug just as hard under water as above it.

   The same effect, felt from the other side, is Activity 5.13. There you push a bottle down and feel the water pushing back up. Here you hold a mug up and feel the water taking part of the load. One force, two ways of noticing it.

   It is also why it is easier to lift a heavy stone while it is under water in a pond than to lift it out onto the bank — and why the last stretch, as it leaves the water, is the hardest part.

6. 63 marksCuriosity Grade 8, Chapter 5, page 76, Ever heard of ...

   How can a rock float on water? Describe the example the chapter gives.

   Hint. The rock is not solid all the way through.

   The chapter's account. There are some rocks which can float on water. One such rock is Pumice, which is formed during volcanic eruptions. When lava with lots of gas and water vapour cools quickly, it traps tiny bubbles of gas inside. This creates a light, porous rock — filled with air pockets which is less dense than water and floats on it.

   How it forms, step by step:

   1. Lava carrying a great deal of gas and water vapour erupts from a volcano.
   2. It cools quickly, before the gas can escape.
   3. The gas is trapped as tiny bubbles inside the solidifying rock.
   4. The result is a light, porous rock, full of air pockets.

   ✦ Why this box is placed exactly here. Everyone expects rock to sink and wood to float, as though floating were a property of the material's name. Pumice breaks that expectation, and the reason is instructive: a piece of pumice is mostly air. What floats is not 'rock' or 'wood' but a particular object, and what matters is how much the whole thing weighs compared with the water it pushes aside.

   The same idea explains a much larger case. A ship is made of steel, and a lump of steel sinks — but a ship is mostly air inside, so the water it displaces weighs as much as the whole ship does.

   The chapter uses the word less dense than water, so you may use it too, while remembering that density itself is defined in a later chapter.

7. 74 marksCuriosity Grade 8, Chapter 5, pages 76-77

   An iron nail sinks in water but a large iron ship floats. Explain, using the chapter's rule.

   Hint. Compare each object with the water it pushes out of the way, not with the other object.

   The mistake to avoid first. The answer is not that the ship is bigger, and not that iron sometimes floats. Being large does not help an object float — exercise question 4 makes the same point with a small coin and a big wooden block.

   Apply the rule to each separately.

   	Iron nail	Iron ship
   What is it made of?	Solid iron throughout	A steel shell with a great deal of air inside
   Water pushed aside	Only as much as the small nail's own volume	An enormous volume, because the hull is broad and hollow
   Weight of that water	Less than the nail's weight	Equal to the ship's whole weight
   Result	Buoyant force is less than the gravitational force → sinks	The two forces balance → floats

   In the chapter's terms. If the gravitational force is more than the buoyant force, the object sinks, but if the two forces are equal, the object floats. Archimedes' Principle supplies the missing piece: the buoyant force equals the weight of the liquid it displaces.

   ✦ What actually decides it. Not the material, and not the size, but how much water the object pushes aside compared with how much it weighs. Hammer a ship's steel into a solid block and it sinks at once, because the same steel now displaces far less water — nothing about the iron has changed.

   Pumice floats for the same reason as the ship: it is a solid full of trapped air.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 5 'Exploring Forces', book pages 62-79 (hecu105.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Fig. 5.17 (exercise 10) was MEASURED, not eyeballed: the page was rendered at 1200 dpi and the three cylinders and their waterlines located by colour segmentation. Taking the waterline at the ellipse's mid-height gives submerged fractions of about 82%, 59% and 32% for objects 1, 2 and 3; taking it at the ellipse's top edge gives 69%, 46% and 19%. Both methods give the same strict ordering 1 > 2 > 3, so object 1 displaces the most water, has the largest buoyant force and therefore the largest weight - answer (ii), w1 > w2 > w3. Fig. 5.13's scale was also read directly: NEWTONS 0 to 10 N alongside GRAMS 0 to 1000 g, so 1000 g lines up with 10 N, consistent with the planet table on page 75. Three deliberate restraints on what is claimed. (1) The chapter is entirely qualitative and contains no formula. Nothing here uses F = ma, W = mg, F = Gm1m2/r^2, a value of g, or Newton's laws of motion - none of which is in this book. Balanced forces are named once and explicitly deferred by the chapter to higher grades, and this file defers them too. (2) The swing question in Probe and ponder is answered by distinguishing weight from the seat's upward push, with an explicit note that the chapter does not explain it and that the full account needs later ideas. (3) Buoyancy is explained purely by comparing two forces, as the chapter does, because density is not defined until a later chapter of the same book; the chapter's own phrase 'less dense than water' is quoted only where the book itself uses it.. Questions are referenced from the NCERT textbook for identification.