Activity 5.8 — Gravitational Force — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, pages 71-72, Activity 5.8

   Describe Activity 5.8. What is the point of throwing the ball a second time, harder?

   Hint. The second throw is closing off an objection to the conclusion.

   The activity. Take a ball and throw it vertically upwards. Does it come down? Now throw it again, harder. Does it still fall back down to the ground? Then think of other situations where an object thrown up in any direction finally falls or comes back to the ground (Fig. 5.10).

   What you find. It comes down every time, however hard you throw it and whichever direction you throw it in.

   ✦ Why the harder throw matters. After one gentle throw you might think the ball came back because you did not throw it hard enough — that a stronger throw would send it away for good. Throwing harder tests that idea and it fails: the ball goes higher and still returns. The pull is not something you can overcome by trying harder, which is what makes it worth explaining.

   And 'in any direction' matters too. Objects thrown sideways, or dropped, or knocked off a table all end up on the ground. Whatever the force is, it always brings things to the same place.

   The conclusion. Since all the objects fall towards the Earth, it means the Earth attracts (pulls) them.

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

   Define gravitational force. What other names does the chapter give it, and why is it a non-contact force?

   Hint. Three names for the same thing.

   Definition. The force with which the Earth attracts objects towards itself is called the gravitational force.

   Other names. The gravitational force exerted by the Earth is also called force of gravity or simply gravity.

   Why non-contact. Since the gravitational force acts without contact with the object it attracts, it is a non-contact force. A falling fruit is touched by nothing, and the Earth is far away, yet the pull acts the whole time.

   ✦ The two interacting objects here are the object and the Earth. This is worth stating explicitly, because gravity is the force students most often describe as though it belonged to the falling object alone. The fruit does not 'have' gravity; the Earth pulls the fruit. Section 5.3's rule — name both objects — applies here as much as anywhere.

   The property that sets it apart from the other two non-contact forces: Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.

   The chapter is careful to speak of the gravitational force exerted by the Earth. Other bodies exert it too — the Moon and the planets, as the table in section 5.5 shows — and how it works in general belongs to a later grade.

3. 34 marksCuriosity Grade 8, Chapter 5, page 72, Fig. 5.11

   Describe what happens when an object is (a) dropped from a height and (b) thrown vertically upwards. What does the chapter mean by vertical motion?

   Hint. In (b) there are four stages, not two.

   (a) Dropped from a height (Fig. 5.11a). When an object is dropped from a height, it takes a straight vertical path downwards before touching the ground. Its speed goes on increasing as it falls.

   (b) Thrown vertically upwards (Fig. 5.11b). The object moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical path downwards.

   Stage	What the object does
   Going up	Moves straight up, speed goes on decreasing
   At the top	Stops momentarily
   Direction change	Its direction of motion changes
   Coming down	Moves straight down, speed goes on increasing

   Vertical motion. We say that the object undergoes a vertical motion when it moves in a vertical direction under the influence of the gravitational force.

   ✦ Count the effects of force in stage (b) and you find three of the four from section 5.2 — the speed decreases, the direction of motion changes, and then the speed increases. One force, acting steadily downwards the whole time, produces all three. That is why this figure is placed where it is.

   The force does not stop at the top. The object stops; the force does not. If it did, the object would stay up there. Exercise question 5(iii) tests exactly this.

4. 43 marksCuriosity Grade 8, Chapter 5, pages 69-72

   Why can gravitational force not be used to push something away, while magnetic and electrostatic forces can?

   Hint. Count the kinds of pole and the kinds of charge — then count the kinds of mass.

   Because gravity has no opposite kind.

   Force	The two kinds involved	Result
   Magnetic	North pole and South pole	Like poles repel, unlike attract — so it can push or pull
   Electrostatic	Positive charge and negative charge	Like charges repel, unlike attract — so it can push or pull
   Gravitational	Nothing corresponding	Only ever attracts — it can only pull

   The chapter states the conclusion without the explanation: Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.

   ✦ How you could have noticed this yourself. Turning one ring magnet over in Activity 5.5 changes repulsion into attraction. Rubbing a second balloon gives you repulsion where the cloth gave attraction. There is nothing you can turn over or swap in Activity 5.8 — every object thrown up, in any direction, comes back down.

   Be careful what you claim here. The chapter reports that gravity is always attractive; it does not explain why, and neither should a Class 8 answer. What you can say confidently is that there is no observed repulsive gravity, and no gravitational counterpart to a north pole or a negative charge.

   This is why the project imagine a scenario where the gravity disappears is set as a story and a cartoon strip, not as an experiment.

5. 54 marksCuriosity Grade 8, Chapter 5, pages 66-72

   A student claims: 'A book resting on a table has no gravitational force on it, because it is not falling.' What is wrong with this, and which part of the chapter answers it?

   Hint. The chapter has already dealt with objects at rest.

   The claim is wrong, and it confuses a force acting with a force producing motion.

   The Earth attracts every object towards itself. The book on the table is being pulled downwards just as surely as a fruit on a branch — and the fruit is not falling either, until the stalk gives way.

   The part of the chapter that answers it is the A step further box on page 65: Suppose an object is at rest. Does it mean that no force is acting on this object? It means that the forces acting on the object are balancing one another. The book stays put because the forces on it balance, not because gravity has stopped.

   ✦ A test that settles it in one move. Slide the book off the edge of the table. It falls at once. Nothing about the book or the Earth changed in that instant — only the support was removed. So the pull must have been there all along, held in check by the table.

   Where the confusion comes from. Section 5.2's rule is easy to run backwards carelessly: no change of motion happens without a force does not mean no force acts without a change of motion. Forces can be present and still produce no change, if they balance.

   Keep the answer at this level. You will learn about balanced forces in higher grades, as the chapter says.

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.