Exploring Forces

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force

newton

two

muscular

friction

opposite

magnetic

positive

gravitational

attractive

weight

newton (N)

upthrust

mass

one-sixth

True

False (Friction decreases the speed and brings the ball to a stop.)

True

True

False (Muscular force is a contact force, caused by the action of muscles.)

False (Like charges repel each other; unlike charges attract.)

True

True

True

False (On the Moon it is about 1.6 N, less than the 10 N on Earth.)

True

True

False (Friction is greater on rough surfaces.)

True

False (A spring balance measures weight/force.)

(i)–B, (ii)–C, (iii)–E, (iv)–A, (v)–D

(i)–B, (ii)–D, (iii)–C, (iv)–A, (v)–E

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(c) (Both true; R describes the sinking condition, not directly why floating occurs.)

(c)

Hint: A push or pull on an object resulting from interaction with another object; SI unit is the newton (N).

Hint: Make an object move from rest; change speed; change direction of motion; change shape.

Hint: A force results from an interaction; when you push a table, your hand and the table are two interacting objects; the force disappears when interaction ceases.

Hint: Contact — needs physical contact (muscular force, friction); Non-contact — acts from a distance (magnetic force, gravity).

Hint: Force caused by the action of muscles when they contract and elongate; e.g., lifting a school bag.

Hint: The force that comes into play when an object moves or tries to move over a surface; it acts opposite to the direction of motion.

Hint: Even smooth surfaces have minute irregularities that lock into each other, opposing motion between the surfaces.

Hint: The nature of the surfaces in contact (rough surfaces give more friction) and the materials of the surfaces.

Hint: To reduce the friction due to air or water around them as they move.

Hint: The force exerted by a magnet on another magnet or magnetic material; it is a non-contact force.

Hint: Like poles (N–N, S–S) repel each other; unlike poles (N–S) attract each other.

Hint: Electrical charges that build up on surfaces when two objects of certain materials are rubbed together; they do not move by themselves.

Hint: Like (similar) charges repel each other; unlike (opposite) charges attract each other.

Hint: The force exerted by a charged body on another charged or uncharged body; it is a non-contact force.

Hint: The force with which the Earth attracts objects towards itself; always attractive.

Hint: Mass is the amount of matter, same everywhere, measured in kg; weight is the gravitational force pulling an object, measured in newton and varies from place to place.

Hint: It measures weight (force) in newton; it also has a scale showing mass in gram.

Hint: About 10 N on the Earth and about 1.6 N on the Moon (one-sixth of Earth's weight).

Hint: The upward force applied by a liquid on an object; an object floats when buoyant force equals gravitational force.

Hint: When an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid it displaces.

Introduction: A force is a push or pull arising from the interaction between two objects.
Key Points:
1. Change in speed — A friend pulling a moving bicycle from behind decreases its speed.
2. Change in direction — Hitting a moving ball with a bat changes its direction of motion.
3. Change in shape — Pressing an inflated balloon changes its shape.
Conclusion: Thus, force can change speed, direction, or shape of an object.

Introduction: Forces are classified based on whether physical contact is needed.
Key Points:
1. Contact forces — Act only when objects are in physical contact; e.g., muscular force and friction.
2. Non-contact forces — Act even without contact; e.g., magnetic, electrostatic and gravitational force.
3. Interaction — Both arise from interaction between objects, but contact forces need touching.
Conclusion: Knowing the type helps predict how a force will act on an object.

Introduction: Muscular force is the force caused by the action of our muscles.
Key Points:
1. Definition — Force resulting from muscles contracting and elongating during activities.
2. Examples — Walking, running, lifting, pushing, jumping and stretching all use muscular force.
3. Internal role — It helps chew food, push it through the alimentary canal, and circulate blood via the heart.
Conclusion: Muscular force is essential for movement and many vital body functions.

Introduction: Friction is the force that opposes motion between two surfaces in contact.
Key Points:
1. Origin — It arises from minute irregularities on surfaces that lock into each other.
2. Direction — It always acts opposite to the direction of motion or attempted motion.
3. Surface dependence — Friction is greater on rough surfaces and less on smooth surfaces.
Conclusion: The nature of the surfaces in contact decides how strong friction will be.

Introduction: Rubbing objects produces static charges whose behaviour reveals two charge types.
Key Points:
1. Charged balloons — Two balloons rubbed with woollen cloth repel each other, being similarly charged.
2. Balloon and cloth — A charged balloon and the rubbing cloth attract each other, having opposite charges.
3. Inference — Like charges repel; unlike charges attract; the two kinds are positive and negative.
Conclusion: These activities prove that there are two kinds of static charges.

Introduction: Gravitational force is the attraction the Earth exerts on all objects.
Key Points:
1. Definition — The force with which the Earth attracts objects towards itself; also called gravity.
2. Nature — It is always attractive, never repulsive, and acts without contact.
3. Example — A ball thrown upwards slows, stops momentarily, and falls back down due to gravity.
Conclusion: Gravitational force is a non-contact, always-attractive force of the Earth.

Introduction: Mass and weight are related but distinct quantities.
Key Points:
1. Mass — The amount of matter in an object; measured in grams/kilograms; same everywhere.
2. Weight — The gravitational force with which the Earth pulls an object; measured in newton; varies.
3. Example — A 1 kg object has mass 1 kg everywhere but weighs 10 N on Earth and 1.6 N on the Moon.
Conclusion: Mass stays constant while weight changes with gravitational force.

Introduction: A spring balance uses a stretching spring to measure weight (force).
Key Points:
1. Working — Hanging an object stretches the spring; the stretch indicates the weight in newton.
2. Range — The balance shown measures a maximum of 10 N (range 0–10 N).
3. Least count — Between two big marks (1 N) there are 5 divisions, so least count = 1/5 = 0.2 N.
Conclusion: The least count is the smallest weight the spring balance can read.

Introduction: Weight depends on the gravitational pull of the celestial body.
Key Points:
1. On Earth — 1 kg weighs about 10 N, so 2 kg weighs 2 × 10 = 20 N.
2. On Moon — Weight becomes one-sixth of Earth's weight, so 20/6 ≈ 3.3 N.
3. Mass unchanged — The mass remains 2 kg on both Earth and Moon.
Conclusion: Weight changes with location even though mass stays constant.

Introduction: Whether an object floats or sinks depends on two opposing forces.
Key Points:
1. Buoyant force — The liquid applies an upward force called upthrust or buoyant force.
2. Gravitational force — The Earth pulls the object downward.
3. Condition — If gravity > buoyant force, the object sinks; if they are equal, it floats.
Conclusion: The balance between downward gravity and upward buoyant force decides floating or sinking.

Introduction: Archimedes discovered how liquids exert an upward force on immersed objects.
Key Points:
1. Principle — An object fully or partially immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces.
2. Sink condition — If the weight of displaced liquid is less than the object's weight, it sinks.
3. Float condition — If the weight of displaced liquid equals the object's weight, it floats.
Conclusion: This principle explains floating and sinking and the working of ships and boats.

Introduction: A ball thrown up moves under the influence of gravitational force.
Key Points:
1. Upward motion — Gravity acts downward, so the ball slows down as it rises.
2. Topmost point — The ball stops momentarily at the top, then changes direction.
3. Downward motion — Gravity now acts in the direction of motion, so speed increases as it falls.
Conclusion: Gravitational force causes the change in speed and direction throughout the motion.

Introduction: Friction acts not only on solids but also in fluids like air and water.
Key Points:
1. Fluid friction — Air, water and other liquids exert friction on objects moving through them.
2. Example 1 — Aeroplanes and high-speed trains are shaped to reduce air friction.
2. Example 2 — Ships and boats are shaped to reduce water friction.
Conclusion: Streamlined shapes reduce fluid friction and make motion efficient.

Introduction: Some rocks, like pumice, can float on water despite being rock.
Key Points:
1. Formation — Pumice forms during volcanic eruptions when lava with gas and water vapour cools quickly.
2. Trapped bubbles — Tiny gas bubbles are trapped inside, creating a light, porous rock.
3. Low density — Being less dense than water and full of air pockets, it floats.
Conclusion: Pumice floats because it is less dense than the water it displaces.

Introduction: The depth to which an object dips in water relates to its weight and buoyant force.
Key Points:
1. Floating objects — Each floating object displaces water whose weight equals its own weight.
2. Different depths — Objects that dip deeper displace more water, indicating greater weight.
3. Order — The object dipping deepest has the greatest weight; the shallowest has the least.
Conclusion: Greater weight means deeper dipping, since more water must be displaced to balance it.

Hint: (a) Greater friction on rough surfaces opposes motion. (b) Gravitational force (gravity). (c) Non-contact force. (d) Friction acts opposite to the direction of motion, reducing speed.

Hint: (a) Friction. (b) Make the horizontal surface rougher (e.g., spread sand or cloth) to increase friction. (c) Make the surface smoother (e.g., glass) or oil it to reduce friction. (d) Contact force.

Hint: (a) Magnetic force (repulsion between like poles). (b) Non-contact force. (c) The magnets attract and stick together. (d) A magnet can exert force on another magnet without being in contact.

Hint: (a) Static charge. (b) A charged object attracts uncharged objects made of certain materials. (c) Non-contact (electrostatic) force. (d) They would repel each other.

Hint: (a) Gravitational force (gravity). (b) Always attractive. (c) Downward (opposite to motion), slowing the ball. (d) Speed increases because gravity now acts in the direction of motion.

Hint: (a) W = 3 × 10 = 30 N. (b) W = 30/6 = 5 N. (c) No — mass is the amount of matter and remains the same everywhere. (d) Newton (N).

Hint: (a) 0 to 10 N. (b) 1 N / 5 = 0.2 N. (c) Weight (force), in newton. (d) 3 N + 2 × 0.2 N = 3.4 N.

Hint: (a) Upthrust or buoyant force. (b) Gravitational force downward and buoyant force upward. (c) The buoyant force exceeds the gravitational force, pushing it up. (d) The density of the liquid.

Hint: (a) Gravitational force on the coin is more than the buoyant force. (b) The buoyant force equals the gravitational force, so it floats. (c) An immersed object experiences an upward force equal to the weight of liquid displaced. (d) The balance between gravitational force and buoyant force.

Hint: (a) Object 1 (deepest). (b) w₁ > w₂ > w₃. (c) A heavier object must displace more water to balance its weight, so it dips deeper. (d) Archimedes' principle (floating object displaces liquid equal to its own weight).

Introduction: A force is a push or pull that arises when two objects interact with each other.
Key Points:
1. Move from rest — A force can make a stationary object start moving, like kicking a football.
2. Change speed — It can increase or decrease the speed of a moving object, like applying brakes.
3. Change direction — A force can change the direction of motion, like a bat hitting a ball.
4. Change shape — It can alter the shape of an object, like pressing a balloon or rolling a chapati.
5. Necessity — None of these changes take place without the action of a force.
Conclusion: Force is essential for any change in the state of motion or shape of an object.

Introduction: Forces are classified based on whether physical contact is required.
Key Points:
1. Contact forces — Act only when objects are in physical contact, directly or indirectly.
2. Muscular force — A contact force caused by muscles; used in walking, lifting and pushing.
3. Friction — A contact force that opposes motion between two surfaces in contact.
4. Non-contact forces — Act even without contact; e.g., magnetic force between magnets.
5. Other non-contact — Electrostatic force and gravitational force also act from a distance.
Conclusion: Understanding contact and non-contact forces helps us analyse forces in daily life.

Introduction: Friction is the force that opposes the relative motion between two surfaces in contact.
Key Points:
1. Cause — Minute irregularities on surfaces lock into each other, opposing motion.
2. Direction — Friction always acts opposite to the direction of motion or attempted motion.
3. Surface dependence — It is greater on rough surfaces and less on smooth surfaces.
4. Effect — A ball rolling on flat ground slows and stops due to friction.
5. In fluids — Air, water and other liquids also exert friction on moving objects like ships and planes.
Conclusion: Friction is a contact force that both helps and hinders motion in everyday life.

Introduction: Magnetic and electrostatic forces are important non-contact forces.
Key Points:
1. Magnetic force — A magnet attracts magnetic materials and exerts force on another magnet without contact.
2. Ring magnets activity — Like poles facing each other make the second magnet float, showing repulsion.
3. Electrostatic force — A charged body attracts uncharged objects like paper pieces.
4. Charged scale activity — A plastic scale rubbed with polythene attracts small paper pieces.
5. Charges — Like charges repel and unlike charges attract; the two kinds are positive and negative.
Conclusion: Both forces act from a distance and reveal the nature of charges and magnetism.

Introduction: Gravitational force is the Earth's attraction on all objects, giving them weight.
Key Points:
1. Gravitational force — The force with which the Earth attracts objects; always attractive and non-contact.
2. Weight — The force with which the Earth pulls an object; SI unit is the newton (N).
3. Mass — The amount of matter in an object; same everywhere, measured in kg.
4. Variation — Weight varies from place to place (and planet to planet), but mass does not.
5. Example — A 1 kg object weighs 10 N on Earth, 1.6 N on the Moon, and 25.4 N on Jupiter.
Conclusion: Mass is constant, while weight depends on the gravitational force at a location.

Introduction: A spring balance is a simple device used to measure weight (force).
Key Points:
1. Construction — A spring fixed at one end with a hook at the other; hanging an object stretches the spring.
2. Working — The amount of stretching gives the weight, read on a scale marked in newton.
3. Range — The balance shown measures up to 10 N, so its range is 0 to 10 N.
4. Least count — There are 5 divisions between two big marks spaced 1 N apart; least count = 1/5 = 0.2 N.
5. Mass scale — It also has a mass scale in grams, assuming use on Earth.
Conclusion: Always check the range and least count of an instrument before using it.

Introduction: Whether an object floats or sinks depends on buoyant force and gravity.
Key Points:
1. Buoyant force — The upward force a liquid applies on an object placed in it.
2. Gravitational force — The Earth pulls the object downward while in the liquid.
3. Sink condition — If gravity is more than buoyant force, the object sinks (e.g., a coin).
4. Float condition — If the two forces are equal, the object floats (e.g., a wooden block).
5. Archimedes' principle — The upward force equals the weight of the liquid displaced; it depends on density.
Conclusion: Archimedes' principle explains why some objects float and others sink.

Introduction: The weight of the same object differs across celestial bodies.
Key Points:
1. Earth — A 1 kg object weighs about 10 N, used as the reference.
2. Moon — Weight is about 1.6 N, roughly one-sixth of Earth's, due to weaker gravity.
3. Mars and Venus — Weights are about 3.8 N and 9 N respectively, depending on their gravity.
4. Jupiter — Weight is about 25.4 N, much greater due to Jupiter's strong gravity.
5. Mass unchanged — The mass remains 1 kg everywhere; only weight changes with gravity.
Conclusion: Weight varies with the gravitational force of the celestial body, while mass stays constant.

Introduction: Non-contact forces act on objects even without physical contact.
Key Points:
1. Magnetic force — Two ring magnets with like poles facing make one float above the other.
2. Repulsion — Like poles repel, pushing the second magnet upward without contact.
3. Electrostatic force — A rubbed plastic scale attracts small paper pieces from a distance.
4. Charges — Two charged balloons repel, while a charged balloon attracts the rubbing cloth.
5. Gravitational force — A thrown ball falls back to the ground without anything touching it.
Conclusion: Magnetic, electrostatic and gravitational forces all act from a distance.

Introduction: Friction is useful in many situations but also causes problems.
Key Points:
1. Necessity — Friction allows us to walk, write, and grip objects without slipping.
2. Stopping motion — Brakes on a bicycle and a ball stopping rely on friction.
3. Problem — Friction wears out surfaces, heats moving parts, and wastes energy.
4. Fluid friction — Air and water friction slow down planes, ships and trains.
5. Reducing friction — Use smooth/polished surfaces, oils, lubricants and streamlined shapes.
Conclusion: Friction is essential for many activities but must be reduced where it hinders motion.
NCERT "Curiosity" Textbook | Chapters 6–7
Student: Shipan Das
Chapters Covered:
- Chapter 6: Pressure, Winds, Storms, and Cyclones
- Chapter 7: Particulate Nature of Matter
Total Questions: 316 (158 per chapter)