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.