NCERT Solutions for Class 6 Science Chapter 8 — Body Movements

Key Concepts

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Intext Questions (Page-by-Page)

Intext Question 1 (Page 82)

Q: Fill in the blanks:

(a) Joints of the bones help in the \_\_\_\_\_ of the body.

Joints are the places where two bones meet. They allow bones to move in different directions, enabling body movement. Without joints, our body would be stiff and unable to move.

(b) A combination of bones and cartilage forms the \_\_\_\_\_ of the body.

The skeleton is the framework of the body, made up of bones and cartilage. Bones provide rigidity and support, while cartilage provides flexibility at certain places like the nose tip and ear pinna.

(c) The bones at the elbow are joined by a \_\_\_\_\_ joint.

The elbow joint is a hinge joint that allows movement in only one direction — the forearm can bend towards the upper arm and straighten back, just like a door hinge.

(d) The contraction of the \_\_\_\_\_ pulls the bones during movement.

Muscles are attached to bones by tendons. When a muscle contracts, it becomes shorter and pulls the bone, causing movement at the joint.

Intext Question 2 (Page 84)

Q: Indicate true (T) or false (F) for the following statements:

(a) The movement and locomotion of all animals is exactly the same.

Different animals have different methods of movement. Fish swim, birds fly, snakes crawl, humans walk, and earthworms move by contracting and expanding their bodies. The type of movement depends on the animal's body structure and habitat.

(b) The cartilage is harder than the bones.

Cartilage is softer and more flexible than bone. Bone is hard and rigid, while cartilage is soft and elastic. Cartilage is found in the nose tip, ear pinna, and between joints where flexibility is needed.

(c) The finger bones do not have joints.

Finger bones have many joints. Each finger (except the thumb) has three bones and two joints. The thumb has two bones and one joint. These joints allow our fingers to bend and perform various tasks.

(d) The forearm has two bones.

The forearm has two bones — the radius and the ulna. These two bones run parallel to each other and allow the forearm to rotate.

(e) Cockroaches have an outer skeleton.

Cockroaches have an exoskeleton (outer skeleton) made of a hard substance called chitin. This outer covering protects the soft internal organs and provides shape to the body.

Intext Question 3 (Page 86)

Q: Match the following:

• (i) Upper jaw — (c) Fixed joint: The upper jaw is fixed to the skull and does not move.
• (ii) Fish — (d) Streamlined body: Fish have a streamlined body shape that helps them swim efficiently in water.
• (iii) Snail — (e) Shell: Snails have a hard shell on their back for protection.
• (iv) Hip joint — (b) Ball and socket joint: The hip joint allows the leg to move in all directions.
• (v) Elbow joint — (a) Hinge joint: The elbow allows movement in one direction, like a hinge.

Intext Question 4 (Page 88)

Q: How many bones does an adult human have?

An adult human has 206 bones.

• At birth, a baby has about 300 bones.
• As the child grows, some of these bones fuse (join) together.
• By adulthood, the number reduces to 206 bones.
• The skeleton provides the framework for the body, protects internal organs, and helps in movement.

Distribution of major bones:

• Skull: 22 bones (including cranial and facial bones)
• Backbone (vertebral column): 33 vertebrae (some fused)
• Rib cage: 12 pairs of ribs (24 ribs) + 1 sternum
• Arms and legs: Multiple bones including humerus, radius, ulna, femur, tibia, fibula, and bones of hands and feet

Intext Question 5 (Page 89)

Q: List the different types of joints in the human body with one example each.

The different types of joints in the human body are:

Intext Question 6 (Page 90)

Q: What is the role of cartilage in the skeletal system?

Cartilage is a soft, elastic, and flexible connective tissue that plays several important roles in the skeletal system:

1. Cushioning at joints: Cartilage is present between bones at joints. It acts as a cushion and prevents the bones from rubbing against each other. This reduces friction and allows smooth movement.

2. Flexibility at specific locations: Cartilage provides flexibility at places where rigidity is not needed. For example:

• The nose tip is made of cartilage, allowing it to bend slightly.
• The ear pinna (external ear) is made of cartilage, making it flexible.
• The trachea (windpipe) has cartilage rings that keep it open but flexible.

3. Shape and structure: Cartilage gives shape to certain body parts like the nose and ears. Without cartilage, these parts would collapse.

4. Growth: In growing children, cartilage at the ends of long bones (growth plates) allows bones to grow in length. As the person matures, this cartilage is replaced by bone.

5. Shock absorption: Cartilage in joints absorbs shocks and impacts, protecting the bones from damage during activities like running and jumping.

Intext Question 7 (Page 91)

Q: Explain how muscles work in pairs to move bones.

Muscles work in pairs to produce movement at joints. This is how they function:

How muscle pairs work: 1. Muscles can only pull (contract); they cannot push. They become shorter when they contract. 2. To move a bone in one direction, one muscle contracts (becomes shorter) and pulls the bone. 3. To move the bone back in the opposite direction, the first muscle relaxes and the second muscle contracts. 4. The two muscles work as a pair — when one contracts, the other relaxes.

Example — Bending and straightening the arm:

• Bending the arm (flexion): The biceps muscle on the front of the upper arm contracts and becomes shorter. It pulls the forearm upward, bending the arm at the elbow. The triceps muscle on the back of the upper arm relaxes.
• Straightening the arm (extension): The triceps muscle contracts and becomes shorter. It pulls the forearm downward, straightening the arm. The biceps muscle relaxes.

Key point: Because muscles can only pull and not push, we need at least two muscles working as a pair to move a bone in both directions at a joint.

Intext Question 8 (Page 93)

Q: How does an earthworm move?

An earthworm moves by the following process:

1. Body structure: An earthworm has a long, cylindrical body made up of many segments. It has two types of muscles — circular muscles and longitudinal muscles. It also has tiny hair-like structures called bristles (setae) on the underside of its body.

2. Extension: The earthworm first extends the front part of its body by contracting the circular muscles in the front segments. This makes the front part thin and long, reaching forward.

3. Gripping: The bristles (setae) on the front segments grip the ground firmly.

4. Pulling: Then the earthworm contracts the longitudinal muscles in the rear part, making it short and thick. This pulls the rear part forward towards the anchored front part.

5. Repeat: The earthworm repeats this cycle — extend front, grip, pull rear — to move forward in a wave-like motion.

Role of mucus: The earthworm secretes a slimy substance (mucus) that keeps its body moist and helps it slide smoothly through the soil.

Intext Question 9 (Page 94)

Q: How does a snake move?

A snake moves in the following ways:

1. Serpentine movement (lateral undulation): This is the most common method. The snake curves its body into S-shaped loops. It pushes against the ground, rocks, or any rough surface with the curves of its body. The resistance from the ground helps the snake move forward in a zigzag path.

2. Rectilinear movement: The snake uses its belly muscles to grip the ground and pull itself forward in a straight line. This is a slow, straight movement.

3. Concertina movement: The snake coils the rear part of its body, then extends the front part forward, then coils the front part and pulls the rear part forward. This is used in narrow tunnels.

Key points about snake movement:

• Snakes do not have legs, so they use their long backbone and muscles to move.
• A snake's body has many vertebrae and connected muscles that help it curve and move.
• The snake's ventral scales (belly scales) provide grip against the ground.
• Snakes move fastest on rough surfaces where they can push against bumps and grooves.

Intext Question 10 (Page 95)

Q: How does a fish move in water?

A fish moves in water using the following adaptations and methods:

1. Streamlined body: The fish has a streamlined (torpedo-shaped) body that tapers at both ends. This shape reduces water resistance and allows the fish to move smoothly through water.

2. Muscles and backbone: The fish has strong muscles on both sides of its backbone. When these muscles contract alternately on the left and right sides, the body bends in an S-shaped wave that pushes the fish forward.

3. Fins: Fish have several types of fins that help in movement and balance:

• Tail fin (caudal fin): The main fin that propels the fish forward by pushing water backwards.
• Dorsal and ventral fins: Fins on the top and bottom that prevent the fish from rolling.
• Pectoral and pelvic fins: Fins on the sides that help the fish steer, turn, and stop.

4. Swim bladder: Most fish have a gas-filled swim bladder that helps them float at different depths without sinking.

5. Gills: Gills allow the fish to breathe oxygen dissolved in water while it swims.

Movement process: The fish sweeps its tail from side to side. The muscles on one side contract while the other side relaxes, creating a wave-like motion. The tail fin pushes water backwards, and the fish moves forward (Newton's third law of motion).

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Exercise Questions and Answers

Q1. Fill in the blanks:

(a) Joints of the bones help in the \_\_\_\_\_ of the body.

Joints allow bones to move relative to each other, enabling body movement.

(b) A combination of bones and cartilage forms the \_\_\_\_\_ of the body.

The skeleton is the framework of the body made up of bones and cartilage.

(c) The bones at the elbow are joined by a \_\_\_\_\_ joint.

The elbow is a hinge joint that allows movement in one direction (bending and straightening).

(d) The contraction of the \_\_\_\_\_ pulls the bones during movement.

Muscles contract and become shorter, pulling the bones they are attached to, causing movement.

Q2. Match the items in Column I with one or more items in Column II:

• (a) Upper jaw — (iii) Fixed joint: The upper jaw is firmly attached to the skull and cannot move.
• (b) Fish — (iv) Streamlined body: Fish have a streamlined body shape to move easily in water.
• (c) Snail — (v) Shell: Snails carry a hard shell on their back for protection.
• (d) Hip joint — (ii) Ball-and-socket joint: The hip joint allows the leg to move in all directions.
• (e) Elbow joint — (i) Hinge joint: The elbow allows movement in one direction, like a door hinge.

Q3. True or False:

(a) The movement and locomotion of all animals is exactly the same.

Different animals move in different ways depending on their body structure and habitat. For example, fish swim, birds fly, snakes crawl, and humans walk on two legs.

(b) The cartilage is harder than the bones.

Cartilage is softer and more flexible than bone. Bone is hard and rigid, while cartilage is elastic and can bend. Cartilage is found in places where flexibility is needed (nose tip, ears).

(c) The finger bones do not have joints.

Finger bones have several joints. Each finger has three bones (phalanges) with two joints between them, plus a joint connecting the finger to the hand. These joints allow the fingers to bend and perform various tasks.

(d) The forearm has two bones.

The forearm has two bones — the radius and the ulna. These two bones allow the forearm to rotate, enabling movements like turning a doorknob.

(e) Cockroaches have an outer skeleton.

Cockroaches have an exoskeleton (outer skeleton) made of a hard substance called chitin. This outer covering provides protection and support for the body.

Q4. How many types of joints are there? Name them with one example each.

There are four main types of joints in the human body:

Q5. Explain how muscles help in the movement of bones.

Muscles play a crucial role in the movement of bones. Here is how they work:

How muscles cause movement:

1. Muscles are attached to bones by tendons: Tendons are strong, fibrous cords that connect muscles to bones. Each muscle is attached to two different bones across a joint.

2. Muscles can only pull, not push: When a muscle contracts, it becomes shorter and pulls the bone it is attached to. Muscles cannot push bones back.

3. Muscles work in pairs: Because muscles can only pull, two muscles are needed to move a bone in both directions. One muscle bends the joint (flexor) and the other straightens it (extensor).

4. Contraction and relaxation: When one muscle contracts, it pulls the bone in one direction. At the same time, the paired muscle relaxes. To move the bone in the opposite direction, the first muscle relaxes and the second muscle contracts.

Example — Movement of the forearm at the elbow:

• Bending the arm: The biceps muscle (on the front of the upper arm) contracts. It pulls the forearm upward, bending the arm at the elbow. The triceps muscle (on the back of the upper arm) relaxes.
• Straightening the arm: The triceps muscle contracts. It pulls the forearm downward, straightening the arm. The biceps muscle relaxes.

Q6. How does a snail move?

A snail moves in the following way:

1. Body structure: A snail has a soft, muscular body and a hard shell on its back for protection. The part of the body that touches the ground is called the foot.

2. Muscular foot: The snail has a large, flat muscular foot on the underside of its body. This foot is the main organ of locomotion.

3. Wave-like contractions: The foot produces wave-like muscular contractions that travel from the front to the back. These contractions push the snail forward slowly.

4. Mucus secretion: The snail secretes a thick, slimy mucus from its foot. This mucus serves several purposes:

• It reduces friction between the foot and the ground.
• It helps the snail slide smoothly over rough surfaces.
• It protects the soft foot from getting injured.
• It allows the snail to move even on sharp surfaces.

5. Slow movement: Snails move very slowly. A garden snail moves at about 1 millimetre per second.

Note: The shell does not help in movement. It only provides protection from predators and from drying out.

Q7. Describe the movement of a bird.

Birds move in the following ways:

1. Flight (flying): This is the most common and important method of movement for birds.

• Birds have lightweight bones that are hollow, making it easier to fly.
• Their front limbs are modified into wings.
• The wings have large feathers that provide a large surface area to push against the air.
• Upstroke: The wings move upward — the feathers fold to reduce air resistance.
• Downstroke: The wings push downward and backward — the feathers spread out to push against the air and generate lift.
• The breast muscles (flight muscles) are very strong and power the wing movement.

2. Walking/Hopping: Birds can walk or hop on their two legs. Their legs have strong muscles and claws that grip surfaces.

3. Swimming: Some birds like ducks, penguins, and swans can swim. They have webbed feet that act like paddles to push through water.

Adaptations for flight:

• Streamlined body to reduce air resistance
• Hollow bones to reduce body weight
• Strong flight muscles attached to the breastbone
• Large wings covered with feathers
• Light body weight relative to wing size

Q8. How does the backbone help us?

The backbone (vertebral column or spine) helps us in the following ways:

1. Support: The backbone provides the main support for the body. It holds the body upright and gives it a proper shape.

2. Protection: The backbone protects the spinal cord, which is a bundle of nerves running from the brain to the rest of the body. The spinal cord is enclosed within the vertebrae.

3. Flexibility and movement: The backbone is not a single bone. It is made up of 33 small bones called vertebrae, joined by cartilage discs. This allows the backbone to bend forward, backward, and sideways, giving flexibility to the body.

4. Attachment point: The backbone provides attachment points for ribs (forming the rib cage) and for many muscles that enable body movement.

5. Weight distribution: The backbone helps distribute the weight of the upper body to the lower body, especially when standing, sitting, or lifting objects.

Structure of the backbone:

• 33 vertebrae: 7 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 5 sacral (fused into sacrum), and 4 coccygeal (fused into tailbone).
• Cartilage discs between vertebrae act as shock absorbers and allow flexibility.

Q9. Why do we need a skeleton?

We need a skeleton for the following reasons:

1. Support and shape: The skeleton gives the body its shape and supports all the soft organs. Without a skeleton, the body would be like a shapeless mass.

2. Protection of organs: The skeleton protects vital internal organs:

• The skull protects the brain.
• The rib cage protects the heart and lungs.
• The backbone protects the spinal cord.

3. Movement: Bones and muscles work together to produce movement. Muscles pull on bones, and the joints between bones allow the body to move in different directions.

4. Blood cell production: The bone marrow inside certain bones (like the femur) produces red blood cells, white blood cells, and platelets.

5. Mineral storage: Bones store minerals like calcium and phosphorus, which are essential for the body.

6. Attachment for muscles: Bones provide surfaces for muscles to attach. Without bones, muscles would have nothing to pull against to produce movement.

Q10. How do animals move? Give one example of movement for each:

(a) Human beings — Walking and Running

Humans move on two legs (bipedal locomotion). The leg muscles contract and pull the bones at the hip, knee, and ankle joints. When one leg swings forward, the other supports the body. The arms swing alternately for balance.

(b) Fish — Swimming

Fish have a streamlined body and fins. The tail fin moves from side to side, pushing water backwards and propelling the fish forward. The other fins help in balance and steering.

(c) Snake — Crawling (Serpentine movement)

Snakes crawl by curving their body into S-shaped loops. They push against the ground with the curves of their body and the belly scales, moving forward in a zigzag path.

(d) Bird — Flying

Birds fly by flapping their wings. The strong breast muscles pull the wings downward (downstroke) to generate lift and forward thrust, then raise them upward (upstroke). The streamlined body and hollow bones help in flight.

(e) Earthworm — Crawling

The earthworm extends its front end by contracting circular muscles, grips the ground with bristles, then pulls its rear end forward by contracting longitudinal muscles. This wave-like motion moves it forward.

(f) Snail — Crawling

The snail moves using its muscular foot. Wave-like contractions of the foot push the snail forward. A trail of mucus helps it slide smoothly.

(g) Insect (cockroach) — Walking and Flying

Cockroaches have three pairs of legs for walking and two pairs of wings for flying. They walk by moving their legs in a coordinated pattern. The hard exoskeleton provides support and the muscles inside help in movement.

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Multiple Choice Questions (MCQs)

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Common Mistakes to Avoid

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Exam Checklist

• [ ] Can you name and describe the four types of joints with examples?
• [ ] Can you explain how muscles work in pairs to move bones?
• [ ] Can you describe the movement of an earthworm, snail, snake, fish, and bird?
• [ ] Can you state the number of bones in an adult human body (206)?
• [ ] Can you explain the role of cartilage in the skeletal system?
• [ ] Can you list the functions of the skeleton (support, protection, movement)?
• [ ] Can you distinguish between ball-and-socket, hinge, pivot, and fixed joints?
• [ ] Can you explain how the backbone helps us (support, protection, flexibility)?
• [ ] Can you describe the streamlined body of a fish and its advantage?
• [ ] Can you explain why we need both bones and muscles for movement?

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Quick Reference: Types of Joints

Quick Reference: Animal Movements