Exploring Forces

Given: Five types of forces are to be matched with their examples.

Concept used: Each type of force is identified by its nature — muscular force involves body muscles, magnetic force involves magnets, frictional force opposes motion, gravitational force is Earth's pull, and electrostatic force involves charged objects.

Matching:

- (i) Muscular force → (b) A child lifting a school bag
A child uses the muscles of the body to lift the bag, which is an example of muscular force.

- (ii) Magnetic force → (e) A compass needle pointing North
The Earth acts like a giant magnet and exerts magnetic force on the compass needle, making it point North.

- (iii) Frictional force → (a) A cricket ball stopping on its own just before touching the boundary line
The ball slows down and stops due to the force of friction acting between the ball and the ground, opposing its motion.

- (iv) Gravitational force → (c) A fruit falling from a tree
The Earth exerts gravitational force (pull) on the fruit, causing it to fall downward.

- (v) Electrostatic force → (d) Balloon rubbed on woollen cloth attracting hair strands
Rubbing the balloon on woollen cloth creates static charges on it; the charged balloon then attracts hair strands through electrostatic force.

Final Answer:
(i) → (b), (ii) → (e), (iii) → (a), (iv) → (c), (v) → (d)

Concept used: Properties of force, friction, and electrostatic force.

(i) A force is always required to change the speed of motion of an object.

True.

*Explanation:* According to the concept of force, any change in the speed (or direction) of a moving object requires a force to be applied on it. Without a net force, the speed of an object remains unchanged.

(ii) Due to friction, the speed of the ball rolling on a flat ground increases.

False.

*Explanation:* Friction always acts in a direction opposite to the direction of motion of the object. Therefore, friction decreases the speed of the rolling ball; it does not increase it.

(iii) There is no force between two charged objects placed at a small distance apart.

False.

*Explanation:* Electrostatic force is a non-contact force. Two charged objects exert electrostatic force on each other even without being in contact. Like charges repel and unlike charges attract each other even at a distance.

Given: Two balloons are rubbed with a woollen cloth and then brought near each other.

Concept used: Electrostatic force — like charges repel each other.

What happens: The two balloons will move away from each other, i.e., they will repel each other.

Why:
When a balloon is rubbed with a woollen cloth, static electric charges are produced on its surface. Since both balloons are rubbed with the same material (woollen cloth) in the same way, they acquire similar (like) charges.

According to the law of electrostatic force:
$$\text{Like charges repel each other.}$$

Therefore, when the two similarly charged balloons are brought near each other, they experience a repulsive electrostatic force and move away from each other.

Conclusion: The two balloons repel each other because they carry similar static electric charges acquired by rubbing with woollen cloth.

Given: A coin sinks in water; a wooden block floats in water.

Concept used: Floating and sinking depend on the balance between gravitational force (weight) and buoyant force (upthrust).

Explanation:

When any object is placed in water, 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 water on the object.

The buoyant force depends on the volume of water displaced by the object and the density of the liquid.

- Coin (sinks): A coin is made of metal, which is very dense. The weight of the coin is greater than the buoyant force exerted by the water on it. Since gravitational force > buoyant force, the coin sinks to the bottom.

W_{\text{coin}} > F_{\text{buoyant}} \Rightarrow \text{Coin sinks}

- Wooden block (floats): Wood is less dense than water. When the wooden block is placed in water, it displaces a volume of water whose weight equals the weight of the wooden block. The buoyant force becomes equal to the weight of the wooden block, so it floats.

$$W_{\text{wood}} = F_{\text{buoyant}} \Rightarrow \text{Wood floats}$$

Conclusion: The coin sinks because its weight exceeds the buoyant force, while the wooden block floats because the buoyant force equals its weight. This difference is due to the difference in their densities relative to water.

Given: A ball is thrown vertically upward; it slows down, stops momentarily at the top, and then falls back.

Concept used: Gravitational force always acts downward (towards the Earth). Air resistance (friction due to air) always acts opposite to the direction of motion.

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(i) During its upward motion:

Forces acting on the ball:
- Gravitational force — acts downward (towards the Earth). This force decelerates the ball and is responsible for slowing it down.
- Air resistance (friction due to air) — acts downward (opposite to the direction of motion, which is upward). This also contributes to slowing the ball.

Both forces act in the downward direction during upward motion, which is why the ball slows down.

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(ii) During its downward motion:

Forces acting on the ball:
- Gravitational force — acts downward (towards the Earth). This accelerates the ball as it falls.
- Air resistance (friction due to air) — acts upward (opposite to the direction of motion, which is now downward). This opposes the fall.

The net force is downward (gravitational force is greater than air resistance), so the ball accelerates downward.

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(iii) At its topmost position:

Forces acting on the ball:
- Gravitational force — acts downward (towards the Earth).
- Air resistance — At the topmost point, the ball is momentarily at rest (speed = 0), so air resistance is effectively zero at that instant.

Only gravitational force acts on the ball at the topmost point, directed downward, which then causes the ball to start falling back to the ground.

Given: A ball released from point P travels down an inclined plane and stops at point A on the horizontal surface due to friction.

Concept used: The force of friction depends on the nature (roughness) of the surfaces in contact. Greater roughness → greater friction → ball stops sooner. Lesser roughness → lesser friction → ball travels farther.

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(i) To make the ball stop BEFORE point A:

We need to increase the force of friction on the ball so that it loses its speed faster and stops before reaching point A.

Method: Make the horizontal surface (and/or the inclined plane) rougher — for example, by spreading sand, placing a rough cloth (like a jute mat or sandpaper) on the surface between the bottom of the incline and point A. The increased friction will slow the ball down more quickly, and it will stop before point A.

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(ii) To make the ball stop AFTER crossing point A:

We need to decrease the force of friction on the ball so that it retains its speed for longer and travels beyond point A.

Method: Make the horizontal surface smoother — for example, by polishing it, applying oil or wax, or placing a smooth glass sheet on the surface. The reduced friction will allow the ball to travel a greater distance and stop after crossing point A.

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Conclusion: By increasing friction (rougher surface), the ball stops before A. By decreasing friction (smoother surface), the ball stops after A.

Given: People slip on smooth surfaces like ice or polished floors.

Concept used: Friction arises due to the irregularities (interlocking) between two surfaces in contact. The smoother the surface, the lesser the friction.

Explanation:

When we walk, we push the ground backward with our feet. The friction between our feet (or shoes) and the ground acts in the forward direction, which allows us to move forward without slipping. Friction thus provides the necessary grip.

On smooth surfaces like ice or polished floors, the surfaces are very smooth — they have very few irregularities. As a result, the interlocking between the surfaces of our feet/shoes and the floor is very poor, leading to very little friction.

Since the frictional force is very small, our feet cannot get a proper grip on the surface. When we try to push backward, the surface offers little resistance, and our feet slide, causing us to slip.

Conclusion: We slip on smooth surfaces because such surfaces have very little friction due to fewer surface irregularities, which reduces the grip between our feet and the surface.

Given: An object is in non-uniform motion (its speed is changing).

Concept used: A force is required to change the speed or direction of motion of an object. Non-uniform motion means the speed of the object is not constant — it is either increasing or decreasing.

Answer: Yes, a force is being applied to an object that is in non-uniform motion.

Explanation:

Non-uniform motion means the object's speed changes over time — it either speeds up (accelerates) or slows down (decelerates).

Since a force is needed to change the speed of an object, a net force must be acting on the object whenever it is in non-uniform motion.

Examples:
- A ball rolling on the ground slows down due to the force of friction (non-uniform motion).
- A ball thrown upward slows down due to gravitational force (non-uniform motion).
- A car speeding up has an engine force acting on it (non-uniform motion).

Conclusion: Yes, whenever an object is in non-uniform motion, a net force is acting on it, causing its speed to change.

Given: Weight of an object on the Moon $= \dfrac{1}{6}$ of its weight on the Earth.

Concept used:
- Weight is the force with which a planet/celestial body pulls an object towards itself. It depends on the gravitational force of that body.
- Mass is the amount of matter contained in an object. It does not depend on location.

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What causes the change in weight?

The weight of an object depends on the gravitational force of the celestial body on which it is placed. The Moon is much smaller than the Earth and has much less mass. As a result, the gravitational force of the Moon is about one-sixth of the gravitational force of the Earth.

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

Since weight = gravitational pull on the object, and the Moon's gravity is $\dfrac{1}{6}$th of Earth's gravity, the weight of the object on the Moon is one-sixth of its weight on the Earth.

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Does the mass also become one-sixth?

No. The mass of the object does not change. Mass is the amount of matter in the object and remains the same everywhere — on Earth, on the Moon, or anywhere else in the universe. It is an intrinsic property of the object and does not depend on the gravitational force of the location.

$$m_{\text{Moon}} = m_{\text{Earth}}$$

Conclusion: The change in weight is caused by the weaker gravitational force of the Moon compared to the Earth. However, the mass of the object remains unchanged — it does not become one-sixth on the Moon.

Correct Option: (ii) w_1 > w_2 > w_3

Given: Three objects of the same size and shape but different materials are placed in water. They dip to different depths (from the figure, object 1 dips the deepest, object 2 dips to an intermediate depth, and object 3 dips the least / floats highest).

Concept used (Archimedes' Principle): When an object floats in a liquid, the buoyant force equals the weight of the object. The buoyant force equals the weight of the liquid displaced, which depends on the volume of the object submerged.

Reasoning:

Since all three objects are floating (not fully submerged), for each object:
$$\text{Weight of object} = \text{Buoyant force} = \text{Weight of water displaced}$$

The weight of water displaced depends on the volume of the object submerged (since all objects have the same shape and size, more depth submerged = more volume submerged = more water displaced = greater buoyant force needed = greater weight of object).

From Fig. 5.17 (as described): Object 1 dips deepest → displaces the most water → has the greatest weight. Object 3 dips the least → displaces the least water → has the smallest weight.

Therefore:
w_1 > w_2 > w_3

Answer: Option (ii) w_1 > w_2 > w_3