In-text — Contact Forces and Muscular Force — Exploring Forces

1. 13 marksCuriosity Grade 8, Chapter 5, page 66, section 5.4.1

   What is a contact force? The chapter says the contact can be direct or indirect — explain the difference with an example of each.

   Hint. Pushing a door with your hand and pushing it with a stick are both contact forces.

   Definition. Forces of this type which act only when there is physical contact between the objects are called contact forces.

   Direct and indirect contact:

   	What it means	Example
   Direct	using our hands or other body parts	Pushing a table with your hand; kicking a football
   Indirect	using a stick or rope	Hitting a ball with a bat; dragging a box with a rope; pushing a door open with a stick

   ✦ Why indirect contact still counts as contact. The chain from you to the object is unbroken — your hand touches the bat and the bat touches the ball. There is no gap anywhere along it. What makes a force non-contact is that it works across a gap with nothing in between, the way a magnet attracts a pin it is not touching.

   A quick test. Ask whether the force would still act if you took everything out of the space between the two objects. Remove the bat and the ball is not struck — so that is a contact force. Remove everything between a magnet and a pin and the pin is still attracted — so that is not.

   The chapter names two contact forces: muscular force and friction.

2. 23 marksCuriosity Grade 8, Chapter 5, page 66

   What is muscular force? How does it come about?

   Hint. The definition is about muscles, not about hands.

   Definition. The force resulting due to the action of muscles is known as muscular force.

   How it comes about. Muscular force occurs when muscles contract and elongate while doing any activity. Whenever we perform a physical activity — walking, running, lifting, pushing, jumping, or stretching — the force is caused by the action of the muscles in our body.

   Why it is a contact force. Your muscles move a part of your body, and that part has to touch the object — directly, or through something you are holding — for the force to reach it. No touch, no effect.

   ✦ It is not only human, and not only obvious movement. Animals, birds, fish, and insects use muscular forces for movement and survival (Fig. 5.3), and humans have long used the muscular force of some animals to carry out many tasks — bullocks drawing a plough or a cart, horses and camels carrying loads (Fig. 5.4).

   And a great deal of it happens inside you without your knowing. See the Ever heard of box on the same page — chewing, digestion and the beating of the heart are all muscular force at work.

3. 33 marksCuriosity Grade 8, Chapter 5, page 67, Ever heard of ...

   Describe the roles muscular force plays inside the human body.

   Hint. Two systems are named — one for food, one for blood.

   The chapter's account. Muscular force plays an important role in many functions inside our body too. This force helps us chew food and push it through the alimentary canal during the process of digestion. The expansion and contraction of our heart muscles allows the blood to circulate in our body — a process essential for survival.

   Where	What the muscles do
   Mouth	Chewing food
   Alimentary canal	Pushing food along through the digestive tract
   Heart	Expanding and contracting so that blood circulates

   ✦ Why the chapter bothers to include this. Everything else in section 5.4.1 is about muscular force you can see and choose — lifting, running, pushing a cart. This box points out that the same kind of force is at work in processes you never decide to carry out and cannot stop. Muscular force is not just what you use to do things; it is part of staying alive.

   A link back to Chapter 3. Health: The Ultimate Treasure treated the heart and digestion from the point of view of health. Here they reappear as examples of a force. The same process looks different depending on which chapter's question you bring to it.

4. 43 marksCuriosity Grade 8, Chapter 5, pages 66-67, Figs. 5.3 and 5.4

   Give examples of muscular force used by (a) humans, (b) other living beings, and (c) animals working for humans.

   Hint. Figures 5.3 and 5.4 are there precisely to be described.

   (a) Humans. The chapter's own list of activities: walking, running, lifting, pushing, jumping, or stretching. Add anything you do with your body — cycling, swimming, writing, opening a drawer, rolling a chapati, pumping air into a bicycle tyre.

   (b) Other living beings. Animals, birds, fish, and insects use muscular forces for movement and survival (Fig. 5.3) — a bird flapping its wings, a fish beating its tail and fins, an insect crawling or jumping, an animal running or digging.

   The words and survival matter. Muscular force is not only how these animals get about; it is how they catch food, escape a predator, build a nest and feed their young.

   (c) Animals working for humans (Fig. 5.4). Humans used the muscular force of some animals to carry out many tasks for a long time — bullocks pulling a plough or a cart, horses and camels carrying people and loads, animals turning a water wheel or a grinding stone.

   ✦ Notice the tense the chapter uses: used ... for a long time. Much of this work is now done by machines running on electricity or fuel, so this is partly a description of history. The chapter states the fact without comment, and so should you — how much animal labour is still used varies enormously from place to place, and the book gives no figures.

5. 53 marksCuriosity Grade 8, Chapter 5, pages 66-67

   Is every force applied by a human being a muscular force? Is every muscular force applied by a human being?

   Hint. Two questions, and the honest answer to each is different from the other.

   Is every force applied by a human a muscular force? In the situations of this chapter, essentially yes — when a person makes something move, the force comes from muscles contracting and elongating, whether the person is pushing directly or using a bat, a rope or a spanner.

   But be careful, because a person can set a non-muscular force to work. Rubbing a plastic scale on polythene takes muscular force, yet the force that then picks up the paper pieces is electrostatic, not muscular. Switching on the electromagnet of Chapter 4 takes a flick of a finger; the force that lifts the clips is magnetic. In both cases a muscular force started the process, but the force doing the job is a different one.

   Is every muscular force applied by a human? No, clearly not. Animals, birds, fish and insects all use muscular force, and so do the muscles inside your own body over which you have no control — the heart and the alimentary canal.

   ✦ The useful habit here is asking what is directly touching the object. That is how you tell which force is acting, rather than tracing the chain back to whoever started it. The paper pieces are not touched by anything, so a muscular force cannot be what lifted them.

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.