In-text — Force as an Interaction Between Objects — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, page 65, section 5.3

   Why does the chapter say that at least two objects must interact for a force to come into play? Test the idea against the rows of Table 5.1.

   Hint. For each row, count the objects. Can you find one with only a single object in it?

   The argument. When you push a table, your hand is one object applying force on another object — the table. Look through Table 5.1 and every row has two objects in it:

   Action	Object 1	Object 2
   Friend holding your moving bicycle to stop it	Friend's hands	Bicycle
   Hitting a moving ball with a bat	Bat	Ball
   Pressing an inflated balloon	Hand	Balloon
   Opening a drawer	Hand	Drawer
   A fruit falling from a tree	The Earth	The fruit

   Do you notice that forces result only when two objects are interacting in some way or the other? From these examples, we can infer that at least two objects must interact for a force to come into play.

   ✦ The last row is the one that tests the rule properly. Nothing is touching the falling fruit, so it is tempting to say only one object is involved. But the pull comes from the Earth, so there are still two objects — they simply are not in contact. The rule survives, and that is what makes section 5.4.2 on non-contact forces possible.

   A test you can apply to any force question: name the two objects. If you cannot name both, you have not identified the force properly. 'The ball has a force' is not an answer; 'the Earth pulls the ball' is.

2. 23 marksCuriosity Grade 8, Chapter 5, page 65

   Give the chapter's full definition of force, and state its SI unit exactly as the book writes it.

   Hint. The book is fussy about the capital letter, and it says why.

   Definition. A force is a push or pull on an object resulting from the object's interaction with another object.

   Compare this with the first, provisional definition on page 63 — the push or pull applied on an object — and you can see what section 5.3 added: the interaction with another object. That is the part that makes the definition complete.

   SI unit. The SI unit of force is newton (written with a small 'n') and its symbol is N.

   ✦ Why the book stops to mention the small 'n'. The unit is named after a person, and the convention is that such a unit is written in lower case when spelled out — newton, not Newton — but its symbol takes a capital: N. So you write 5 newton or 5 N, never 5 Newton and never 5 n. The same convention applies to other units named after people.

   The newton is used again in section 5.5, because weight is a force and SI unit of weight is also newton (N). That is the unit you read off a spring balance.

3. 33 marksCuriosity Grade 8, Chapter 5, page 65, A step further

   Suppose an object is at rest. Does it mean that no force is acting on it?

   Hint. The chapter answers this in one sentence and then deliberately stops.

   No. It means that the forces acting on the object are balancing one another.

   An object sitting still on a table is being pulled down by the Earth — the gravitational force does not switch off because the object is not moving. Yet it stays put. The chapter's explanation is that the forces on it balance, so no change of motion results.

   ✦ Notice what this does to the rule from section 5.2. That rule said a change in speed, direction or shape never happens without a force. It did not say that a force always produces a change — and this box is why. Forces can be present and still produce no change, if they cancel out.

   How far to take this in an answer. The chapter ends the box with You will learn about balanced forces in higher grades, so 'the forces on it are balancing one another' is the complete Class 8 answer. Do not go on to draw force diagrams, name a normal force, or write equations — none of that is in this book.

   The common wrong answer is 'an object at rest has no force on it'. If that were true, letting go of a stationary ball held in mid-air would leave it hanging there.

4. 44 marksCuriosity Grade 8, Chapter 5, page 66, A step further

   When you push a table with your hand, do you feel a force on your hand too? What does the chapter conclude from this?

   Hint. The box answers with a general rule, not just an observation about hands.

   Yes — you feel a force on your hand as well. And the moment you stopped pushing, the force on your hand disappeared.

   The general rule the chapter draws from it: Whenever two objects interact, each object experiences a force from the other. As soon as the interaction ceases, the two objects no longer experience the force.

   Two separate claims in that rule:

   1. Forces come in pairs — the hand pushes the table, and the table pushes the hand. You do not have one object doing the forcing and another merely receiving it.
   2. The pair exists only while the interaction lasts. Stop pushing, and both forces vanish together.

   ✦ This completes the idea that force is an interaction. Section 5.3 said a force needs two objects; this box says that in that interaction both objects feel something. That is why 'force' is better thought of as something happening between two objects than as something one object has.

   Keep the answer at this level. The chapter states the rule and moves on; the formal law about action and reaction, with its equal-and-opposite wording, is not in this book and should not be quoted in a Class 8 answer.

5. 54 marksCuriosity Grade 8, Chapter 5, pages 63-66

   For each of these, name the two interacting objects: (a) a fruit falling from a tree, (b) a magnet attracting a pin from a distance, (c) a rolling ball slowing down on the ground, (d) a bottle bobbing back up when released under water.

   Hint. In two of these nothing is touching. Name both objects anyway.

   	Situation	Object 1	Object 2	Contact?
   (a)	Fruit falling from a tree	The Earth	The fruit	No — non-contact (gravitational)
   (b)	Magnet attracting a pin from a distance	The magnet	The pin	No — non-contact (magnetic)
   (c)	Rolling ball slowing down	The ground	The ball	Yes — contact (friction)
   (d)	Bottle pushed up when released under water	The water	The bottle	Yes — contact (buoyant force)

   ✦ Cases (a) and (b) are the ones worth dwelling on. Nothing is touching, so it feels as though only one object is involved and the force is somehow a property of the falling fruit. It is not. The Earth is the second object, and without it there would be no pull. Naming the Earth is the whole content of the answer.

   Case (c) is the one students most often name wrongly. The two objects are the ball and the ground it is rolling on — friction acts between two surfaces in contact. Saying 'the ball and the air' names a real but much smaller effect and misses the point of Activity 5.3.

   The habit is worth forming now. Every force in this chapter has two named objects behind it, and questions that look hard usually become easy the moment you write both of them down.

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.