Probe and Ponder — The Opening Questions — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, page 62

   Why does it feel harder to pedal a bicycle when going uphill than on flat ground?

   Hint. Going uphill, something is pulling you the other way the whole time.

   Because going uphill you are working against the Earth's pull.

   The Earth attracts every object towards itself — that is the gravitational force, and it acts straight downwards. On flat ground it pulls you down onto the road, but it does not oppose your forward motion, so your pedalling only has to overcome friction and the air. On a slope, part of that downward pull acts along the road, backwards down the slope, so every turn of the pedals has to overcome it as well as everything else.

   ✦ The chapter's own story makes the same point from the other side. Coming back down the hill, Sonali and Ragini find their bicycles moving down at a great speed even though they were not pedalling, and Sonali asks, "It seems something is pulling us downhill, what could it be?" Uphill and downhill are the same force, helping in one direction and hindering in the other.

   Two other things also make it harder, both named in the story:

   • The wind. "We are riding against the wind. We must push our pedals harder to move faster." Air exerts a force of friction on objects moving through it.
   • A rough road. Some parts of the road were rough where they found it hard to pedal, while other parts were smoother. Friction is greater on rough surfaces.
2. 23 marksCuriosity Grade 8, Chapter 5, page 62

   Why is it easier to slip on a wet surface?

   Hint. What is it about two dry surfaces that normally stops them sliding?

   Because water reduces the friction between your foot and the ground.

   The chapter explains where friction comes from: Friction arises due to the irregularities in the two surfaces in contact. Even surfaces which appear smooth, have a large number of minute irregularities. When placed in contact, the irregularities of two surfaces lock into each other and oppose any effort to move one surface over the other.

   A film of water gets between the two surfaces and keeps those irregularities from locking into each other properly. With less interlocking there is less friction, so your foot slides instead of gripping.

   ✦ Why that makes you fall. Walking depends on friction. When you walk, your foot pushes backwards against the ground, and it is friction that stops the foot from simply sliding backwards — which is what pushes you forward. Take the friction away and the push does nothing except slide your foot out from under you.

   The same reasoning answers exercise question 7 about ice and polished floors: those surfaces have very few irregularities to begin with, so friction is small even when they are dry.

3. 34 marksCuriosity Grade 8, Chapter 5, page 62

   Why do we feel 'light' or like we are 'floating' just after our swing reaches its highest point and begins to come down?

   Hint. Ask what you actually feel when you sit on something — is it gravity you feel, or the seat?

   Your weight has not changed at all. The Earth pulls you with the same force at every point of the swing. What changes is how hard the seat presses up on you — and that push is what you actually feel.

   Where the feeling of weight comes from. Sitting still on a chair, you feel the chair pressing up against you. That upward push is what your body registers as feeling heavy. You never feel the Earth's pull directly; you feel whatever is holding you up against it.

   On a swing. At the highest point of the arc the seat is tilted and you are momentarily not moving, and the seat pushes up on you less than it does when you are sitting still at the bottom. Less push means less of the sensation you read as weight — so you feel light, as though you were floating. As you swing back down through the lowest point the seat presses hardest, and you feel heaviest there.

   ✦ A caution about how far this answer goes. The chapter does not explain this question — it is put at the start to make you curious, not to be answered from the text. What the chapter does give you is enough to rule out the wrong answer: gravity has not weakened, and your weight has not changed, because the force with which the Earth pulls an object towards itself is called the weight of the object and nothing about you or the Earth changed during the swing. Working out exactly how the seat's push varies around the arc needs ideas that come in a later grade.

4. 43 marksCuriosity Grade 8, Chapter 5, page 63

   In the opening story, Sonali and Ragini find their bicycles racing downhill without pedalling, and Sonali asks what could be pulling them. Answer her, and list every other force mentioned in the story.

   Hint. The whole chapter is the answer to Sonali's question. The rest of the story is a list of forces in disguise.

   Sonali's question answered. It is the gravitational force — the force with which the Earth attracts objects towards itself, also called the force of gravity or simply gravity. It acts on the cyclists and their bicycles all the time, and on a slope part of that pull acts along the road, so they speed up without pedalling.

   The other forces hiding in the story:

   In the story	The force
   Pumping air into their bicycle tyres	Muscular force — a contact force
   "The wind is pushing me hard!"	Air exerting a force of friction on them as they move through it
   We must push our pedals harder	Muscular force on the pedals
   Some parts of the road were rough where they found it hard to pedal	Friction between tyres and road, greater on the rough parts
   They pressed their bicycle bells	Muscular force — a push
   Turned the handles to change direction	A force changing the direction of motion
   Applying brakes (from the chapter's list of examples)	Friction, decreasing the speed

   ✦ Notice that the story is organised, not accidental. It contains a push and a pull, a contact force and a non-contact force, and at least three of the effects a force can have — starting motion, changing speed, changing direction. The chapter is already teaching before section 5.1 begins.

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.