Chapter 5: Exploring Forces - Solutions
Question 1
Match items in Column A with the items in Column B.
| Column A (Type of force) | Column B (Example) |
|---|---|
| (i) Muscular force | (b) A child lifting a school bag |
| (ii) Magnetic force | (e) A compass needle pointing North |
| (iii) Frictional force | (a) A cricket ball stopping on its own just before touching the boundary line |
| (iv) Gravitational force | (c) A fruit falling from a tree |
| (v) Electrostatic force | (d) Balloon rubbed on woollen cloth attracting hair strands |
Question 2
State whether the following statements are True or False.
- (i) A force is always required to change the speed of motion of an object. True.
- (ii) Due to friction, the speed of the ball rolling on a flat ground increases. False.
- (iii) There is no force between two charged objects placed at a small distance apart. False.
Question 3
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Answer: When the two balloons are rubbed with a woollen cloth, they acquire similar static charges (usually negative). When brought near each other, they repel each other because like charges repel. This is due to electrostatic force, a non-contact force that acts between charged objects even without touching.
F = k × (|q1 × q2| / r2)
Since q1 and q2 have the same sign (negative), the resulting force F is repulsive.
Question 4
When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
Answer: The coin sinks because its weight (gravitational force) is greater than the buoyant force exerted by water. In contrast, the wooden block floats because the buoyant force acting on it is equal to or greater than its weight. Even though the block is larger, it is less dense than water, so it displaces enough water to balance its weight, allowing it to float. The key factors are density and the balance between buoyant force and gravitational force.
For the wooden block: Wblock ≤ Fbuoyant ⇒ Floats
Where Weight (W) = m × g, and Buoyant Force (Fbuoyant) = Volume displaced × Density of fluid × g.
Question 5
If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
Answer: Forces on a ball thrown upwards:
- (i) During its upward motion: Applied force (upward), gravitational force (downward).
- (ii) During its downward motion: Only gravitational force (downward).
- (iii) At its topmost position: Only gravitational force (downward).
Question 6
A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops (i) before the point A (ii) after crossing the point A.
Answer:
- (i) To make the ball stop before point A: Increase the friction on the horizontal surface. You can place a rougher surface like a carpet or cloth on the horizontal part. This will create more friction, which will slow the ball down more quickly and make it stop before point A.
- (ii) To make the ball stop after crossing point A: Decrease the friction on the horizontal surface. Use a smoother surface like polished glass, plastic, or tile. This reduces friction, allowing the ball to roll further and stop after point A.
These changes affect the force of friction, which controls how quickly the ball loses its motion.
Question 7
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Answer: We sometimes slip on smooth surfaces like ice or polished floors because they offer very little friction. Friction is a contact force that helps our feet grip the ground when we walk. On smooth surfaces, the irregularities between our feet and the floor are minimal, so the force of friction is reduced. With less friction, it becomes harder to maintain balance and grip, making it easier to slip.
Where μ is the coefficient of friction and N is the normal force.
On ice or polished floors, μ is extremely low, approaching zero, resulting in very low friction (f ≈ 0).
Question 8
Is any force being applied to an object in a non-uniform motion?
Answer: Yes, a force is being applied to an object in non-uniform motion. Non-uniform motion means the object’s speed or direction is changing, and any change in speed or direction requires the action of a force. According to Newton’s laws of motion, a force is needed to accelerate or decelerate an object or to change its direction. Therefore, the presence of non-uniform motion always indicates that some force is acting on the object.
Fnet = m × a
Non-uniform motion implies acceleration (a ≠ 0). Therefore, a net force (Fnet ≠ 0) must be acting on the object.
Question 9
The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
Answer: The weight of an object becomes one-sixth on the Moon because the gravitational force on the Moon is weaker—about one-sixth of that on Earth. Weight depends on the gravitational pull of the planet or celestial body. However, the mass of an object remains unchanged because mass is the amount of matter in the object and does not depend on gravity. So, only weight changes with location, but mass stays the same everywhere in the universe.
Weight on Moon: Wm = m × gm
Given gm ≈ ge / 6,
Therefore, Wm = m × (ge / 6) = We / 6
Mass (m) is constant.
Question 10
Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in the water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects 1, 2, and 3 are w1, w2, and w3, respectively, then:
- (i) w1 = w2 = w3
- (ii) w1 > w2 > w3
- (iii) w2 > w3 > w1
- (iv) w3 > w1 > w2
Answer: (ii) w1 > w2 > w3.
Explanation: Because they are of the same size and shape, the deeper an object sinks, the heavier it is compared to the buoyant force it experiences. Object 1 sinks the deepest, thus possessing the greatest weight (w1). Object 3 sinks the least, possessing the smallest weight (w3).
