Chapter 5 Important Questions: Exploring Forces

✔ Correct Answer: (b) Newton

The SI unit of force is newton (written with a small 'n'), symbol $\textbf{N}$. It is named after Sir Isaac Newton who studied the effects of forces on objects.

✔ Correct Answer: (c) Opposite to the direction of motion

Friction always opposes motion, acting in the direction opposite to the direction in which the object is moving or trying to move. $\vec{f}_{\text{friction}} \text{ is opposite to } \vec{v}_{\text{object}}$

✔ Correct Answer: (c) Gravitational force

Gravitational force acts between the Earth and objects without any physical contact. Muscular force and friction are contact forces as they require physical contact between objects.

✔ Correct Answer: (b) Repel each other

Both balloons acquire the same (like) charge when rubbed with the same woollen cloth. Since like charges repel, the two charged balloons move away from each other. $\text{Like charges} \Rightarrow \text{Repulsion}$

✔ Correct Answer: (c) Weight (force) of an object

A spring balance measures the weight of an object in newtons (N), as the spring stretches in proportion to the gravitational pull (weight) on the object. It may also have a secondary scale showing mass in grams.

✔ Correct Answer: (c) One-sixth of its weight on Earth

The Moon's gravitational pull is much weaker than Earth's. $W_{\text{Moon}} = \frac{1}{6} \times W_{\text{Earth}}$ However, the mass of the object remains the same everywhere.

✔ Correct Answer: (d) Buoyant force (Upthrust)

Upthrust or buoyant force is the upward force exerted by a liquid on an immersed object. According to Archimedes' Principle, this force equals the weight of the liquid displaced by the object.

✔ Correct Answer: (d) An ox pulling a plough

Muscular force is the force produced by the action of muscles. An ox pulling a plough uses its muscular strength. Fruit falling involves gravity, compass involves magnetic force, and charged balloon involves electrostatic force.

✔ Correct Answer: (b) Irregularities in the surfaces that interlock

Even apparently smooth surfaces have tiny irregularities that interlock with each other when two surfaces are in contact. These interlocked irregularities resist motion, giving rise to friction.

✔ Correct Answer: (b) $W_1 > W_2 > W_3$

The object that dips deepest (Object 1) is heaviest, as gravity pulls it down more than the buoyant force can support. Since Object 1 dips most and Object 3 the least: $W_1 > W_2 > W_3$

Weight. Weight is the gravitational force exerted by the Earth on an object, measured in newtons (N). Unlike mass, weight can vary from place to place depending on the local gravitational pull. $\text{Weight} = \text{Gravitational force on the object}$

Static charges. When certain materials are rubbed together, static charges accumulate on their surfaces. They are called 'static' because these charges do not flow or move on their own, unlike current electricity.

Gravity (Gravitational force). The gravitational force exerted by the Earth always pulls objects towards itself; it never pushes objects away. This makes it uniquely and exclusively an attractive force among the forces discussed in this chapter.

Displaced. Archimedes' Principle states that the buoyant (upthrust) force on a fully or partially immersed object equals the weight of the liquid it displaces. If the weight of displaced liquid $\geq$ weight of the object, the object floats.

Friction. The force of friction is a contact force that opposes motion between two surfaces in contact. It arises due to the interlocking of tiny irregularities on the surfaces. $\text{Friction} \Rightarrow \text{Opposes motion}$

True. Any change in speed (increase or decrease) of a moving object requires a net force to act on it. Without a force, an object in motion would continue moving at the same speed in the same direction.

False. Friction opposes motion, so it causes the ball to slow down (decrease in speed), not speed up. A ball rolling on flat ground eventually comes to rest because of friction between its surface and the ground.

False. Two charged objects exert electrostatic force on each other even without contact. Like charges repel and unlike charges attract — this force can act across a distance without physical contact. $\text{Charged objects} \Rightarrow \text{Electrostatic force at a distance}$

False. Mass is the amount of matter in an object and remains the same everywhere — on Earth, Moon, or any other planet. Only weight changes because gravitational pull is different at different locations. $m_{\text{Moon}} = m_{\text{Earth}}$

True. A magnet attracts or repels another magnet or magnetic material from a distance, without any physical contact. This is why magnetic force is classified as a non-contact force.

The labeled part X is the Newtons scale on the spring balance. It is the scale (ranging from $0$ to $10\,\text{N}$ in the example shown) that directly reads the weight (force) in newtons (N) when an object is suspended from the hook of the spring balance.

The gap between the two ring magnets demonstrates magnetic repulsion. When like poles (e.g., North facing North) of two magnets face each other, they exert a repulsive magnetic force on each other, causing the upper magnet to float without touching the lower one. This shows magnetic force is a non-contact force.

Force is a push or pull on an object resulting from the object's interaction with another object. It can change the speed, direction of motion, or shape of an object. The SI unit of force is newton (N). $\text{Force} = \text{Push or Pull} \quad [\text{SI unit: } N]$

Muscular force is the force produced by the contraction and elongation of muscles during physical activities such as walking, running, lifting, or pushing. Example: An ox pulling a plough uses its muscular force — this is a contact force since physical contact is needed.

Key Points:

• Contact forces act only when there is physical contact between objects. Example: Friction (between a sliding box and the floor).

• Non-contact forces act even without physical contact. Example: Gravitational force (Earth pulling a falling fruit from a distance).

Electrostatic force is the force exerted by a charged body on another charged body or an uncharged body. It can be attractive (unlike charges) or repulsive (like charges). It is a non-contact force because a charged object can attract or repel another object without being in physical contact with it. $\text{Like charges} \Rightarrow \text{Repulsion} \quad | \quad \text{Unlike charges} \Rightarrow \text{Attraction}$

Key Points:

• Mass is the amount of matter in an object, measured in grams (g) or kilograms (kg); it remains the same everywhere.

• Weight is the gravitational force with which the Earth pulls an object, measured in newtons (N); it can vary from place to place.

$W_{\text{Moon}} = \frac{1}{6} W_{\text{Earth}} \quad \text{but} \quad m_{\text{Moon}} = m_{\text{Earth}}$

Upthrust (Buoyant force) is the upward force exerted by a liquid on an object placed in it. An object floats when the upthrust exerted by the liquid equals the weight of the object (i.e., the weight of liquid displaced equals the object's weight). $\text{Object floats} \Rightarrow \text{Upthrust} \geq \text{Weight of object}$

Key Points: During the upward journey of the ball, two forces act:

• Gravity (Gravitational force): Acts downward (towards the Earth), slowing the ball.

• Air friction (Frictional force): Also acts downward (opposite to the upward motion of the ball), further slowing it.

Both forces act downward, which is why the ball decelerates and eventually stops.

A ball rolling on a flat surface experiences friction between its surface and the ground. Friction acts opposite to the direction of motion, gradually reducing the ball's speed. The ball eventually comes to rest because friction continuously removes energy from the ball's motion. This shows that friction is a contact force.

Static charges are electrical charges that build up on the surface of certain objects when two different materials are rubbed together. They are called 'static' because they do not move by themselves. Example: Rubbing a plastic scale vigorously with polythene causes static charges to build up on the scale, enabling it to attract small paper pieces.

Archimedes' Principle states that when an object is fully or partially immersed in a liquid, it experiences an upward (buoyant) force equal to the weight of the liquid it displaces. $F_{\text{upthrust}} = \text{Weight of liquid displaced}$ If this upthrust equals the object's weight, the object floats; if less, the object sinks.

Activity: Place an object (like an empty lunch box) on different surfaces — glass, cloth, wood, ceramic tile, and sand. Push it with the same force each time and observe the distance it travels before stopping.

Observation: The object travels different distances on different surfaces. On rough surfaces (like sand or cloth), it stops sooner; on smoother surfaces (like glass), it travels farther.

Conclusion: The force of friction depends on the nature of the surfaces in contact — rougher surfaces produce greater friction because their irregularities interlock more. $\text{Rougher surface} \Rightarrow \text{Greater friction} \Rightarrow \text{Shorter distance}$

Observation 1: When two balloons rubbed with the same woollen cloth are brought near each other, they repel each other (move apart). Both balloons acquire similar (like) charges through the same rubbing process, and like charges repel. $\text{Like charges} \Rightarrow \text{Repulsion}$

Observation 2: When the woollen cloth is brought near one rubbed balloon, they attract each other. The rubbed object (balloon) and the rubbing object (cloth) acquire opposite (unlike) charges, and unlike charges attract. $\text{Unlike charges} \Rightarrow \text{Attraction}$

This demonstrates two types of electrostatic charges: positive and negative.

When any object is placed in water, two forces act on it:

• Gravitational force (Weight): Pulls it downward.

• Buoyant force (Upthrust): Pushes it upward, equal to the weight of water displaced.

Coin: The coin is dense and heavy relative to the water it can displace. Its weight > upthrust, so it sinks.

Wooden block: Wood is less dense; the weight of water displaced by the large block equals (or exceeds) the block's weight. So upthrust ≥ weight and the block floats.

$\text{If Weight} > \text{Upthrust} \Rightarrow \text{Sinks} \quad | \quad \text{If Upthrust} \geq \text{Weight} \Rightarrow \text{Floats}$

Friction arises due to the interlocking of irregularities on two surfaces in contact. On smooth surfaces (like polished floors or ice), the irregularities are very small, so the surfaces interlock very little, producing very low friction.

When friction is very low, our feet cannot grip the surface firmly. Any slight push or movement causes us to slide, resulting in slipping.

On wet surfaces, water acts as a lubricant, filling in the tiny gaps between irregularities, further reducing friction. This is why walking on wet or polished surfaces is more dangerous than on rough, dry surfaces. $\text{Smooth/Wet surface} \Rightarrow \text{Less friction} \Rightarrow \text{More likely to slip}$

A force can have the following effects on an object:

Note: A force may cause one or more of these effects simultaneously. $\text{Force} \Rightarrow \text{Change in speed, direction, shape, or motion}$

Types of Forces

Forces are broadly classified into contact forces and non-contact forces.

$\text{Forces} \rightarrow \begin{cases} \text{Contact Forces} \\ \text{Non-Contact Forces} \end{cases}$

Contact Forces

These act only when objects are in physical contact.

1. Muscular Force

• Produced by the action of muscles.

• Examples: A horse pulling a cart; a child lifting a school bag.

• Also works inside the body (e.g., heart muscles pumping blood).

2. Frictional Force (Friction)

• Acts when an object moves or tries to move over another surface.

• Always acts opposite to the direction of motion.

• Caused by interlocking of surface irregularities.

• Examples: A ball rolling on the ground slowing down; a sliding box coming to rest.

$\vec{f}_{\text{friction}} \text{ opposes } \vec{v}_{\text{motion}}$

Non-Contact Forces

These act without physical contact between objects.

3. Magnetic Force

• Force exerted by a magnet on another magnet or magnetic material.

• Like poles repel; unlike poles attract.

• Examples: A compass needle pointing North; a floating ring magnet repelled by another.

4. Electrostatic Force

• Force exerted by a charged body on another charged or uncharged body.

• Like charges repel; unlike charges attract.

• Examples: A charged plastic scale attracting paper pieces; two charged balloons repelling each other.

$\text{Like charges} \Rightarrow \text{Repulsion} \quad ; \quad \text{Unlike charges} \Rightarrow \text{Attraction}$

5. Gravitational Force

• Force with which the Earth attracts objects towards itself.

• Always attractive — never repulsive.

• Examples: A fruit falling from a tree; a ball thrown upward falling back to the ground.

Weight, Mass, and Gravitational Force

Gravitational Force

The gravitational force is the force with which the Earth attracts all objects towards itself. It is a non-contact, always-attractive force.

$\text{Gravitational Force} \Rightarrow \text{Earth attracts objects downward}$

Weight

Weight is the force with which the Earth pulls a specific object towards itself. Since it is a force, its SI unit is newton (N).

• An object thrown upward slows down, stops, and returns — all due to gravity (weight acting downward).

• Weight is not the same everywhere — it depends on the local gravitational pull.

$W_{\text{Moon}} = \frac{1}{6} \times W_{\text{Earth}}$

Mass

Mass is the amount of matter in an object, measured in grams (g) or kilograms (kg). It does not change from place to place.

$m_{\text{Earth}} = m_{\text{Moon}} = m_{\text{Mars}} \quad (\text{same object})$

Measuring Weight Using a Spring Balance

A spring balance works on the principle that a spring stretches proportionally to the force (weight) applied on it.

Steps:

1. Hang the spring balance from a fixed support.

2. Note the zero reading (pointer at $0\,\text{N}$).

3. Suspend the object from the hook at the bottom.

4. The spring stretches due to the weight of the object.

5. Read the value on the Newtons scale — this is the weight of the object.

Example from textbook: Maximum weight measurable $= 10\,\text{N}$; smallest division $= 0.2\,\text{N}$.

$\text{Least count} = \frac{\text{Difference between two large marks}}{\text{Number of small divisions}} = \frac{1\,\text{N}}{5} = 0.2\,\text{N}$

Why weight varies but mass doesn't: Weight depends on gravitational force $g$, which varies slightly on Earth and significantly on other planets (e.g., Moon's $g \approx \frac{1}{6}$ of Earth's). Mass, being the amount of matter, is an intrinsic property and remains constant regardless of location.

Part 1: Spring Balance — Diagram and Working

Draw the following labeled diagram:

• A vertical rectangular casing (the body of the spring balance)

• A spring coiled inside the casing, fixed at the top

• A hook at the bottom of the spring (outside the casing) to hang objects

• A pointer attached to the spring

• Two parallel scales on the front:

  • Left scale: Grams (g) — mass scale, range $0$ to $1000\,\text{g}$

  • Right scale (Newtons): Weight scale, range $0$ to $10\,\text{N}$

• A nail or support at the top to hang the spring balance

• An object (e.g., a rock) hanging from the hook

Labels: Spring, Hook, Pointer, Grams scale, Newtons scale, Object, Support/Nail

Working:

When an object is hung from the hook, the spring stretches due to the weight of the object. The heavier the object, the more the spring stretches. The pointer moves along the scale, and the reading on the Newtons scale gives the weight of the object.

$\text{Greater weight} \Rightarrow \text{Greater stretch} \Rightarrow \text{Higher reading on scale}$

Part 2: Activity — Friction Depends on Nature of Surface

Materials: Empty lunch box, different surfaces (glass, cloth, wood, ceramic tile, sand)

Steps:

1. Place the lunch box on a smooth glass surface.

2. Push it gently with a measured force and note the distance it travels before stopping.

3. Repeat on cloth, wood, ceramic tile, and sand using the same force.

4. Record distances in each case.

Observation:

• On glass (very smooth): box travels the farthest.

• On sand (very rough): box stops almost immediately.

Conclusion: Friction depends on the nature of surfaces in contact — rougher surfaces have more irregularities that interlock, causing greater friction.

Part 3: Stopping Before/After Point A

A ball released from Point P on an inclined plane normally stops at Point A on the horizontal surface due to friction.

To stop BEFORE Point A:

• Increase friction on the horizontal surface by placing a rough material (like a cloth or sandpaper) between P and A.

• This increases friction $\Rightarrow$ ball decelerates faster $\Rightarrow$ stops earlier.

To stop AFTER Point A (beyond A):

• Reduce friction by making the horizontal surface smoother (e.g., polish it or add a smooth sheet).

• Reduced friction $\Rightarrow$ ball travels farther before stopping.

$\text{More friction} \Rightarrow \text{Stops before A} \quad | \quad \text{Less friction} \Rightarrow \text{Stops after A}$

Part 1: Floating and Sinking — Buoyant Force

When an object is placed in a liquid, two forces act on it:

1. Gravitational force (Weight): Acts downward, pulling the object towards the Earth.

2. Buoyant force (Upthrust): Acts upward, exerted by the liquid on the object.

$\text{If } W_{\text{object}} > F_{\text{upthrust}} \Rightarrow \text{Object Sinks}$

$\text{If } W_{\text{object}} \leq F_{\text{upthrust}} \Rightarrow \text{Object Floats}$

Example: A coin (dense, heavy relative to water displaced) sinks. A wooden block (less dense, displaces water equal to its weight) floats.

Part 2: Archimedes' Principle

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

$F_{\text{upthrust}} = \text{Weight of liquid displaced by the object}$

• If weight of displaced liquid $<$ weight of object $\Rightarrow$ object sinks.

• If weight of displaced liquid $=$ weight of object $\Rightarrow$ object floats.

Interesting fact: Pumice rock (formed from lava) is so porous (full of air pockets) that it is less dense than water, so it floats — a rare example of a rock that floats.

Part 3: Activity to Demonstrate Buoyant Force

Materials: Empty plastic bottle (lid tightly closed), bucket full of water.

Steps:

1. Take an empty bottle and push it into the bucket of water.

2. Observe: You feel an upward push on your hand — this is the buoyant force from the water.

3. Release the bottle inside the water.

4. Observe: The bottle bounces back up to the surface.

Conclusion: Water exerts an upward force (upthrust/buoyant force) on the submerged bottle. Since the bottle is light (air inside), the upthrust exceeds its weight, causing it to float.

Part 4: Weight on Moon vs Earth — Mass vs Weight

Weight depends on the gravitational force of the planet or celestial body. The Moon's gravity is much weaker than Earth's:

$W_{\text{Moon}} = \frac{1}{6} \times W_{\text{Earth}}$

This means the Earth pulls an object 6 times more strongly than the Moon does. So the same object feels much lighter on the Moon.

Does mass change? No. Mass is the amount of matter in the object, which remains the same everywhere — on Earth, Moon, or any planet. Only weight changes because gravitational pull varies.

$\text{Mass = constant everywhere} \quad ; \quad \text{Weight = varies with gravity}$

What Is Friction?

Friction is the force that comes into play when an object moves or tries to move over another surface. It always acts in a direction opposite to the direction of motion.

$\text{Force of Friction} \Rightarrow \text{Opposes motion between surfaces in contact}$

Friction is a contact force — it arises only when two surfaces are in contact.

How Friction Arises

All surfaces, even apparently smooth ones, have tiny irregularities (bumps and grooves) at the microscopic level. When two surfaces are placed in contact, these irregularities interlock with each other.

When one surface tries to move over the other, the interlocked irregularities resist the motion, creating friction.

$\text{More irregularities} \Rightarrow \text{More interlocking} \Rightarrow \text{Greater friction}$

This is why: Rough surfaces (like sand) have greater friction than smooth surfaces (like glass).

Friction as a Necessity

Friction is essential in many situations:

Friction as a Problem

Friction can also be harmful in some situations:

Special Shapes for Aeroplanes and Ships

Air, water, and other liquids also exert frictional force on objects moving through them. This friction (called drag) slows down fast-moving objects.

To reduce drag from air and water:

• Aeroplanes are given a streamlined (tapered/pointed) shape so that air flows smoothly around them with minimum resistance.

• Ships and boats are given hull shapes that cut through water efficiently.

• High-speed trains are also shaped to minimise air friction.

$\text{Streamlined shape} \Rightarrow \text{Less fluid friction} \Rightarrow \text{Greater speed, less fuel consumed}$

Conclusion: Friction is both a necessity and a problem — we need it for safety and movement but also need to reduce it to save energy and reduce wear.