Chapter Exercises Solutions

Questions and Answers

1. Match Items (Column A to Column B)

2. True or False

(i) A force is always required to change the speed of motion of an object. Answer: True. (Newton’s First Law: An object stays in uniform motion unless acted upon by an external force).

(ii) Due to friction, the speed of the ball rolling on a flat ground increases. Answer: False. (Friction opposes motion, so the speed decreases).

(iii) There is no force between two charged objects placed at a small distance apart. Answer: False. (Electrostatic force is a non-contact force; they will attract or repel even at a distance).

3. Charged Balloons

Question: Two balloons rubbed with a woollen cloth are brought near each other. What happens? Answer: They will repel each other.

• Reason: Rubbing both balloons with the same material (wool) gives them the same type of charge (e.g., both negative). Like charges repel each other.

4. Sinking Coin vs. Floating Wood

Question: Why does a coin sink but a big wooden block float? Answer: This depends on Density and Buoyancy.

• The coin is denser than water. The upthrust (buoyant force) provided by the water is less than the coin’s weight, so it sinks.
• The wooden block is less dense than water. It displaces enough water to create an upthrust equal to its weight, so it floats.

5. Ball Thrown Upwards

(i) During upward motion:

• Forces: Gravity (Downwards), Air Friction (Downwards).
• Result: Speed decreases.

(ii) During downward motion:

• Forces: Gravity (Downwards), Air Friction (Upwards - opposing motion).
• Result: Speed increases.

(iii) At topmost position:

• Forces: Gravity is still acting downwards. Even though velocity is zero for an instant, the force of gravity never stops.

6. Ball on Inclined Plane (Diagram Analysis)

The ball starts at P, goes down, and stops at A due to friction on the horizontal surface.

(i) How to stop BEFORE point A?

• Increase Friction: Make the horizontal surface rougher (e.g., spread sand or carpet).
• Lower Height P: Release the ball from a lower height so it has less energy.

(ii) How to stop AFTER crossing point A?

• Decrease Friction: Make the surface smoother (e.g., apply oil, use glass or ice).
• Increase Height P: Release the ball from a higher point.

7. Slipping on Smooth Surfaces

Answer: We walk by pushing the ground backward; friction pushes us forward (action-reaction). On smooth surfaces (ice/polish), the force of friction is extremely low. Without sufficient friction to grip our feet, we cannot push effectively, leading to a slip.

8. Non-uniform Motion

Answer: Yes. Non-uniform motion means the speed or direction is changing. According to physics, any change in speed or direction requires an unbalanced external force.

9. Weight on Moon

Answer:

• Cause: The Moon has less mass than Earth, so its gravitational pull is weaker (approx 1/6th of Earth’s). Hence, Weight ($W=mg$) decreases.
• Mass: No, the mass does not change. Mass is the amount of matter in the object, which remains constant regardless of location.

10. Floating Objects Depth (Fig 5.17)

• Object 1 dips very little (Floats high).
• Object 2 dips halfway.
• Object 3 dips almost completely (Floats low).

Analysis: To float, Upthrust must equal Weight ($U = W$). Upthrust depends on the volume submerged.

• Object 3 is submerged the most $\rightarrow$ It needs the most upthrust to balance its weight $\rightarrow$ It is the heaviest.
• Object 1 is submerged the least $\rightarrow$ It is the lightest.

Correct Option: (iv) $w_3 > w_2 > w_1$ (Note: The question options in the text might have typos, but logically, deepest sinker = heaviest among same-sized floating objects). Checking options provided in PDF: (i) Equal (ii) $w_1 > w_2 > w_3$ (iii) $w_2 > w_3 > w_1$ (iv) $w_3 > w_1 > w_2$ (This implies 3 is heaviest, then 1, then 2. Wait, looking at diagram 5.17:

• 1 is barely in water.
• 2 is somewhat in.
• 3 is mostly in.
• Therefore $Weight_3 > Weight_2 > Weight_1$.
• None of the provided options exactly match $3 > 2 > 1$.
• Let’s re-read the options carefully.
• (iv) says $w_3 > w_1 > w_2$. This claims 1 is heavier than 2. But 1 is floating higher than 2.
• Correction: Let’s look at the diagram again. 1 is high, 2 is middle, 3 is low. Weight corresponds to submerged volume. So $W_3 > W_2 > W_1$.
• If the options in the book are rigid, there may be a typo in the book or my reading of the diagram depth. Assuming standard physics: Deepest = Heaviest.