Activities 5.9 to 5.12 — Weight and the Spring Balance — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, page 73, Activity 5.9

   Describe Activity 5.9 with the spring and different objects. What does it establish?

   Hint. The question being tested is written just above the activity.

   The question being tested: Does the Earth pull every object with equal force?

   The activity. Take a spring and a few objects of different masses — a pencil box, a tiffin box, a small stone. Hang one end of the spring from a nail (Fig. 5.12a) and hang an object from the other end (Fig. 5.12b). Does the spring stretch? Now hang the other objects one by one and notice the stretch each time. Is the stretch caused by each object the same?

   What you find. When an object is hung from a spring, the spring stretches due to the force applied on the object by the Earth. We find that the stretch caused in the spring is different for different objects.

   The conclusion. This indicates that the Earth pulls different objects with different forces, that is, the weight of different objects is different.

   ✦ The activity turns a force you cannot see into a length you can measure. You cannot watch the Earth pulling a pencil box, but you can watch a spring stretch, and a bigger stretch means a bigger pull. That single idea is the whole basis of the spring balance, which the next four activities are about — Can we use the spring to measure the weight of an object?

   Use the same spring throughout. A stiffer spring would stretch less for the same object, and the comparison between objects would mean nothing.

2. 23 marksCuriosity Grade 8, Chapter 5, page 72, section 5.5

   Define the weight of an object and state its SI unit. Why is it measured in that unit?

   Hint. The reason for the unit follows from what weight *is*.

   Definition. The force with which the Earth pulls an object towards itself is called the weight of the object. The weight measures how strongly an object is pulled by the Earth.

   SI unit. Since the weight is a force, it is measured in the same unit as that of force. Therefore, SI unit of weight is also newton (N).

   ✦ The reasoning is one short chain and it is worth stating in full: weight is a force — specifically the gravitational force with which the Earth pulls the object. Forces are measured in newton. So weight is measured in newton. Not in kilogram, not in gram.

   Why this matters more than it looks. Almost everyone in everyday life says a bag of wheat 'weighs 10 kg', and the chapter has a whole box about it: though while the units of mass are used, instead of the term mass, the term weight is typically used ... But in scientific use, this is not correct. Writing a weight in kilograms is the single commonest error in this section.

   A weight of 10 N is roughly what the Earth's pull on a 1 kg object amounts to, according to the table on page 75. That is a useful figure for checking whether an answer is sensible.

3. 33 marksCuriosity Grade 8, Chapter 5, page 73, A step further

   How does a spring balance work? Describe its parts and its scales.

   Hint. Two scales, and only one of them is really being measured.

   How it works. A spring balance is a simple device used to measure weight (force). It consists of a spring fixed at one end, with a hook attached at the other end. When we hang an object from the hook, the spring stretches, and the amount of stretching gives the weight of the object.

   The scales. There is a scale on the balance which is marked to show the weight (force) in newton. Usually, there is also another scale to show the corresponding values of mass in gram (g).

   In Fig. 5.13 the two scales run side by side: GRAMS on the left, reading 0 to 1000 g, and NEWTONS on the right, reading 0 to 10 N. So 1000 g lines up with 10 N — the same figure as the planet table on page 75, where a 1 kg object weighs 10 N on Earth.

   ✦ The important sentence is the last one in the box: These values have been marked with the assumption that the spring balance is used on the Earth, with the Earth's gravitational force attracting the object.

   The spring only ever measures how hard something is pulling on it — a force. The gram scale is not a second measurement; it is the newton reading relabelled, using the Earth's gravity to convert. Take the same balance to the Moon and the newton scale would still be right, while the gram scale would read wrong — a 1 kg object would show about 1.6 N, and the gram scale beside it would claim about 160 g.

   That is a consequence of what the chapter says, not an extra fact: mass does not change, weight does.

4. 42 marksCuriosity Grade 8, Chapter 5, page 73, Activity 5.10

   Look at the spring balance in Fig. 5.13. What is the maximum weight it can measure, and what is its range?

   Hint. Read the highest number printed on the newton scale.

   Maximum weight: The maximum weight it can measure is 10 N.

   Range: Thus, this scale has a range of 0 to 10 N.

   Reading the figure. The NEWTONS scale on the right of the close-up is marked 0, 01, 02 and so on up to 10 — that is, 0 N to 10 N in steps of 1 N. The GRAMS scale on the left runs alongside it from 0 to 1000 g.

   ✦ Why the chapter makes you look before you use it. Your school laboratory may have spring balances for which the range and the value of the smallest division may be different. It is, therefore, always necessary to look carefully at the spring balance (or any other instrument) you are about to use.

   This is the same discipline the book taught for the thermometer in Temperature and Its Measurement, Curiosity Grade 6 — and the chapter says so explicitly. An instrument's range and smallest division are properties of that particular instrument, not facts to be memorised once.

   Why the range matters in practice. Activity 5.12 warns that the objects should not be heavier than the maximum value of weight the spring balance can measure, otherwise it may get damaged. Overloading a spring balance can stretch the spring permanently, after which every reading it gives is wrong.

5. 54 marksCuriosity Grade 8, Chapter 5, page 74, Activity 5.11

   Work out the smallest weight the spring balance of Fig. 5.13 can measure. Show the three steps.

   Hint. Find the value between two big marks, count the small divisions, and divide.

   Step 1 — the weight difference between two bigger marks. The weight difference indicated between 0 and 01 N or between 01 N and 02 N is 1 N.

   Step 2 — the number of divisions between them. There are 5 divisions between these marks.

   Step 3 — divide.

   One small division = 1 N ÷ 5 = 0.2 N

   So the smallest value that the spring balance can read is 0.2 N.

   ✦ The method, not the number, is what you are being taught. Now using this method, calculate the smallest value of weight that can be measured with the spring balance given to you. Your laboratory's balance may have a different range and a different number of divisions, and then the answer will not be 0.2 N. The three steps stay the same:

   1. What weight lies between two large marks?
   2. How many small divisions is that split into?
   3. Divide the first by the second.

   A worked variation to check you have the method. If a balance had a range of 0–5 N with big marks every 1 N and only 2 divisions between them, one division would read 1 ÷ 2 = 0.5 N — a coarser instrument than the one in the figure, since it cannot distinguish anything finer than half a newton.

6. 63 marksCuriosity Grade 8, Chapter 5, page 74, Activity 5.12

   Describe how to measure weight with a spring balance. What precaution does the activity give, and why?

   Hint. The precaution is about the instrument, not about the object.

   The procedure.

   1. Take a spring balance and a few objects.
   2. Check first that no object is heavier than the maximum the balance can measure.
   3. Suspend the objects one by one from the hook (Fig. 5.14).
   4. Read the scale for weight carefully and record the readings in Table 5.2, whose sample rows are a pencil box and a partially filled water bottle.

   The precaution. Keep in mind that the objects should not be heavier than the maximum value of weight the spring balance can measure, otherwise it may get damaged.

   Why. The balance works because the spring stretches in proportion to the pull on it. Overload it and the spring is stretched too far to return to its original length. After that the instrument is not just unable to read the heavy object — every later reading it gives is wrong, and nothing about its appearance tells you so.

   ✦ Two practical points for reading it accurately. Let the object hang still before reading, because a swinging object makes the pointer move. And read the scale with your eye level with the pointer, so that you are not looking at it from an angle.

   The chapter adds that you can repeat Activities 5.10 to 5.12 for the mass scale shown on the left side of the balance, to measure mass in grams the same way.

7. 74 marksCuriosity Grade 8, Chapter 5, page 75

   Distinguish between mass and weight.

   Hint. Four points of difference: what each is, its unit, whether it changes, and how it is measured.

   	Mass	Weight
   What it is	The amount of matter in an object	The gravitational force with which the Earth (or another planet) pulls an object
   Unit	Gram (g) or kilogram (kg)	Newton (N) — because it is a force
   Does it change?	Its value remains the same at every place	Weight can change — gravitational force varies slightly from place to place on the Earth, and can be very different on other planets
   Measured with	A beam balance, by comparison with a known mass — or indirectly from the weight	A spring balance

   The chapter recalls the definition of mass from Materials Around Us, Curiosity Grade 6, and sums the difference up in one line: weight can change, but mass does not.

   ✦ Why we get away with confusing them in daily life. Since the weight of an object remains almost the same everywhere on the Earth, so for all practical purposes it is acceptable to weigh an object to find its mass. The shopkeeper's balance works because you and the shopkeeper are standing on the same planet. The distinction only bites when you leave it.

   The mass of an object can be found in two ways, as the chapter's box says: by measuring its weight (using a spring balance) or by comparing its weight with the weight of an object of a known mass (using a beam balance).

   This distinction is exactly what exercise question 9 tests.

8. 84 marksCuriosity Grade 8, Chapter 5, page 75, A step further

   The chapter gives a table of the weight of a 1 kg object on different bodies. Reproduce it and say what it demonstrates.

   Hint. Read across the two rows and notice which one changes.

   The table, exactly as the chapter gives it:

   	Earth	Moon	Mars	Venus	Jupiter
   Mass of the object	1 kg	1 kg	1 kg	1 kg	1 kg
   Weight of the object	10 N	1.6 N	3.8 N	9 N	25.4 N

   What it demonstrates. Although the mass of an object remains the same, its weight is different on the Earth, the Moon, and other planets.

   The top row never changes: the object is the same object, with the same amount of matter in it, wherever it is taken. The bottom row changes by a factor of more than fifteen between the Moon and Jupiter — because each body pulls with a different strength.

   ✦ Two things the table lets you check.

   • The Moon figure matches exercise question 9. That question says the weight on the Moon is one-sixth of that on Earth; the table gives 1.6 N against 10 N, and 10 ÷ 6 ≈ 1.7. The two agree, so the exercise is consistent with the table rather than a separate claim to be taken on trust.
   • Venus is nearly the same as Earth — 9 N against 10 N — which is a useful reminder that a different planet does not automatically mean a dramatically different weight.

   Use the table's numbers and do not add your own. The chapter gives no figures for the Sun, Mercury, Saturn or any other body, and quoting extra values from memory is how a wrong number gets into an answer.

9. 93 marksCuriosity Grade 8, Chapter 5, page 75, A step further

   Why is it wrong, scientifically, to say 'the weight of the wheat bag is 10 kg'? Why does everyone say it anyway?

   Hint. The chapter is unusually direct about the gap between everyday and scientific language.

   Why everyone says it. In everyday life, particularly for the goods we commonly use, we are more interested in the amount of matter in an object (its mass), rather than the force applied by the Earth upon it (its weight). When you buy wheat you want to know how much wheat there is — that is its mass.

   What goes wrong. Though while the units of mass are used, instead of the term mass, the term weight is typically used. So the sentence borrows the unit of mass and the word for weight, and mixes the two.

   The correct versions:

   Statement	Verdict
   The mass of the wheat bag is 10 kg	✅ Correct — mass, in a unit of mass
   The weight of the wheat bag is about 100 N	✅ Correct — weight, in newton (using 1 kg ≈ 10 N from the table)
   The weight of the wheat bag is 10 kg	❌ Wrong — weight is a force and is never measured in kilograms

   ✦ The chapter's verdict, in its own words: But in scientific use, this is not correct and it is important to use the correct terms with their correct units, even if every day language is more casual.

   Notice what is not being said. The chapter does not claim that ordinary speech is foolish or that shopkeepers should change how they talk. It says that in scientific use the terms must be used properly. Knowing which register you are writing in is part of the skill — and in a science answer, the scientific one applies.

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.