Class 8 Science Chapter- 5 Exploring Forces Questions and Answers
Keep the curiosity alive Page no: 77
Question: 1. Match items in Column A with the items in Column B.
Answer: Here is the correct matching of the types of forces with their respective examples:
| 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 |
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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.
Answer: True.
Explanation: A force is necessary to change the speed of an object, whether it is to accelerate it (increase speed) or decelerate it (decrease speed). Without the application of force, an object’s speed remains constant according to Newton’s first law of motion.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
Answer: False.
Explanation: Friction acts in the opposite direction to the motion of the ball. Therefore, it opposes the motion and causes the ball to slow down, rather than increasing its speed.
(iii) There is no force between two charged objects placed at a small distance apart.
Answer: False.
Explanation: Charged objects exert electrostatic forces on each other, even when they are not in direct contact. These forces can be attractive or repulsive depending on the nature of the charges (like charges repel and unlike charges attract).
Question: 3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Answer: When two balloons that have been rubbed with a woollen cloth are brought close to each other, they will repel each other. This occurs because both balloons acquire the same type of electric charge (either positive or negative) when they are rubbed with the cloth. According to the principle of electrostatics, like charges repel each other. Therefore, as the balloons approach, they push away from one another.
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 difference in behavior between the coin and the wooden block in water can be explained by the concept of density and buoyant force:
1. Coin: The coin is denser than water, meaning its weight is greater than the buoyant force acting on it. When dropped into the water, the gravitational force pulls it down, and it sinks.
2. Wooden Block: The wooden block, on the other hand, is less dense than water. When placed in water, the buoyant force (the upward force exerted by the water) is greater than the weight of the block. As a result, the block floats.
This phenomenon illustrates Archimedes’ Principle, which states that an object will float if the weight of the liquid it displaces is greater than or equal to its own weight.
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.
(i) During its upward motion
Answer: 1. Force Acting: Gravitational Force
2. Direction: Downward (towards the Earth)
3. Explanation: As the ball moves upward, the gravitational force acts downward, opposing the upward motion.
(ii) During its downward motion
Answer: 1. Force Acting: Gravitational Force
2. Direction: Downward (towards the Earth)
3. Explanation: The gravitational force continues to act downward, causing the ball to accelerate towards the ground.
(iii) At its topmost position
Answer: 1. Force Acting: Gravitational Force
2. Direction: Downward (towards the Earth)
3. Explanation: At the highest point, the ball momentarily stops, but the gravitational force is still acting downward, pulling it back towards the ground.
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
Answer: Stopping Before Point A
To make the ball stop before point A, we can:
1. Increase Friction: Introduce a rough surface on the horizontal section before point A. This increased friction will slow down the ball more quickly, causing it to stop before reaching point A.
2. Use a Barrier: Place a small barrier or obstacle on the horizontal surface before point A. When the ball reaches this barrier, it will stop upon collision.
3. Reduce the Initial Speed: Release the ball from a lower height on the inclined plane. This will decrease its speed as it reaches the horizontal surface, allowing it to stop before point A.
(ii) after crossing the point A.
Answer: Stopping After Crossing Point A
To ensure the ball stops after crossing point A, we can:
1. Reduce Friction: Use a smoother surface on the horizontal section after point A. This will allow the ball to maintain its speed longer, causing it to travel past point A before stopping.
2. Add a Downward Slope: Create a slight downward slope after point A. This will provide additional gravitational force acting on the ball, propelling it further along the horizontal surface.
3. Increase the Initial Speed: Release the ball from a higher point on the inclined plane. This will give the ball more kinetic energy, allowing it to travel further and stop after crossing point A.
Question: 7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Answer: When we walk on smooth surfaces such as ice or polished floors, we often experience slipping due to reduced friction. Here’s a detailed explanation:
- Frictional Force: Friction is the force that opposes the motion of an object. It is essential for walking, as it allows our feet to push against the ground without slipping.
- Smooth Surfaces: On smooth surfaces, the irregularities that typically provide grip are minimal. This results in lower frictional force.
- Ice and Polish: Ice is particularly slippery because it has a very low coefficient of friction. Similarly, polished floors, while not as slippery as ice, still have significantly less friction compared to rough surfaces.
- Effect of Weight: When we exert our weight on these surfaces, the reduced frictional force may not be enough to counteract the force of our movement, leading to slipping.
Question: 8. Is any force being applied to an object in a non-uniform motion?
Answer: Yes, when an object is in non-uniform motion, it is experiencing a force. Here’s why:
- Definition of Non-Uniform Motion: Non-uniform motion refers to the motion of an object when its speed or direction changes over time. This can be due to acceleration or deceleration.
- Application of Force: According to Newton’s laws of motion, a force is required to change the state of motion of an object. Therefore, if an object is accelerating (speeding up) or decelerating (slowing down), a net force must be acting on it.
- Examples:
- A car speeding up requires a force from the engine.
- A ball thrown upwards slows down due to the gravitational force acting against its motion.
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 change in weight of an object on the Moon compared to its weight on Earth is due to the difference in gravitational pull between the two celestial bodies. Here’s a breakdown:
1. Weight vs. Mass:
– Weight: The force with which the Earth (or Moon) attracts an object is called its weight. Weight is dependent on the gravitational force acting on an object.
– Mass: Mass is the amount of matter in an object and remains constant regardless of location. It is measured in kilograms (kg).
2. Gravitational Force: The Moon’s gravitational force is about one-sixth that of Earth’s. This means that an object will weigh significantly less on the Moon due to the weaker gravitational pull.
3. Effect on Weight:
– If an object weighs 60 N (newtons) on Earth, it will weigh approximately 10 N on the Moon (60 N / 6).
4. Mass Remains Constant: While the weight changes due to the difference in gravitational force, the mass of the object remains the same. Therefore, the mass does not become one-sixth; it stays constant regardless of whether the object is on Earth or the Moon.
Page no: 79
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: Correct Option: (ii) w1 > w2 > w3
Explanation: The object that sinks the least (dips the least) is the lightest in weight, while the object that sinks the most is the heaviest. Therefore, if the objects are made of different materials, their weights will vary, leading to different depths in water due to the buoyant force acting on them. The heavier the object, the deeper it will sink, assuming they all have the same volume.