NCERT Solutions for Class 8 Science Curiosity Chapter 5 Exploring Forces

1. Match items in Column A with the items in Column B.

See AnswerHere are the correct matches with reasoning:
(a) A cricket ball stopping on its own just before touching the boundary line matches with (iii) Frictional force. The grass rubbing against the ball creates friction, which opposes the ball’s motion and slows it down to a stop.
(b) A child lifting a school bag matches with (i) Muscular force. The child uses the force generated by their muscles to lift the bag against gravity.
(c) A fruit falling from a tree matches with (iv) Gravitational force. The Earth pulls the fruit towards its center. We call this pull gravity!
(d) Balloon rubbed on woollen cloth attracting hair strands matches with (v) Electrostatic force. Rubbing the balloon builds up static electric charge, which can then attract uncharged or oppositely charged objects like hair without touching them.
(e) A compass needle pointing North matches with (ii) Magnetic force. The Earth itself is a giant magnet, and its magnetic force pulls on the small magnet inside the compass, making it align with the Earth’s magnetic field lines.

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.
See AnswerTrue. An object will keep moving at a constant speed (or stay at rest) unless a net force acts on it. To make something speed up or slow down, a push or a pull is needed.

(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
See AnswerFalse. This is a classic trick question! Friction is a force that always opposes motion. It’s like a tiny brake. So, friction will always cause the speed of a rolling ball to decrease, not increase.

(iii) There is no force between two charged objects placed at a small distance apart.
See AnswerFalse. Charged objects create an invisible force field around them. When two charged objects are brought near each other, they will either attract or repel each other due to the electrostatic force between them.

3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
See AnswerWhen you rub two balloons with the same woollen cloth, they both pick up the same kind of static electrical charge. Think of it like putting the same team jersey on both balloons. Just like magnets with similar poles (North-North or South-South) push each other away, objects with similar electrical charges also push each other away. So, the two balloons will repel each other and move apart due to the electrostatic force.

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.
See AnswerThis is a fascinating battle between two opposing forces! First, you have the Earth’s gravitational force pulling every object downwards (this is the object’s weight). Second, you have the water pushing upwards on any object placed in it. This upward push is called the buoyant force.
For the coin, the downward pull of gravity is stronger than the upward push from the water, so it sinks.
For the wooden block, the upward buoyant force from the water is strong enough to balance the downward pull of gravity on the block. When the forces are balanced, the object floats!

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
See AnswerDuring its upward motion: Two forces are acting on the ball. The gravitational force is pulling it straight down. At the same time, air resistance (a type of friction) is also acting downwards, opposing its upward movement.

(ii) During its downward motion
See AnswerDuring its downward motion: The gravitational force is still pulling it straight down. However, now the air resistance is acting upwards, opposing its downward movement.

(iii) At its topmost position
See AnswerAt its topmost position: For just an instant, the ball stops moving. At this exact moment, the only significant force acting on it is the gravitational force, pulling it straight down. Air resistance is zero at this instant because the ball is not moving.

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.
See AnswerThis is all about controlling friction! The ball stops at point A because of the friction between the ball and the horizontal surface.
(i) To make the ball stop before point A, we need to increase the friction. We could make the horizontal surface rougher, for example, by laying a piece of carpet or sandpaper on it.
(ii) To make the ball stop after crossing point A, we need to decrease the friction. We could make the horizontal surface smoother. Polishing the surface or applying a lubricant like a little bit of oil or talcum powder would reduce friction and allow the ball to travel farther.

7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
See AnswerWe can walk because of the friction between the soles of our shoes and the ground. This friction provides the grip we need to push off the ground. Smooth surfaces like ice or a freshly polished floor have very few bumps and irregularities. This means there is very little friction. Without that essential grip, our feet slide easily, causing us to slip.

8. Is any force being applied to an object in a non-uniform motion?
See AnswerYes. Non-uniform motion means the object’s velocity is changing—it’s either speeding up, slowing down, or changing direction. Any change in velocity is called acceleration, and to make an object accelerate, you must apply a net (unbalanced) force. So, if you see an object in non-uniform motion, you can be sure a force is at work.

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?
See AnswerThis is a key difference between mass and weight!
The change in weight is caused by the difference in gravity. The Moon is much smaller and has less mass than the Earth, so its gravitational pull is much weaker—about one-sixth as strong. Since weight is just the measure of this gravitational pull, the object’s weight is less on the Moon.
However, the mass of the object does NOT change. Mass is the amount of ‘stuff’ or matter an object is made of. Whether you take a rock to the Moon, to Jupiter, or keep it on Earth, it’s still made of the same amount of stuff. So, its mass remains constant everywhere.

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
See AnswerThis is a brilliant puzzle about floating and sinking. Let’s analyze the clues from the picture.
For an object to float, the upward buoyant force must exactly balance its downward weight. The buoyant force is equal to the weight of the water the object pushes aside (displaces).
– Object 1 is floating very high, which means it displaces only a small amount of water. Therefore, it must be the lightest.
– Object 2 is floating lower in the water than object 1. It has to displace more water to stay afloat, which means it must be heavier than object 1. So, W₂ > W₁.
– Object 3 has sunk to the bottom. This means its weight (W₃) is greater than the maximum possible buoyant force the water can provide, even when the object is completely underwater.
From this, we know for certain that the sunken object (3) is the heaviest, and between the two floating objects, object 2 is heavier than object 1.
So the correct order of weights is W₃ > W₂ > W₁.
It appears that none of the multiple-choice options provided in the book match this correct conclusion. This can sometimes happen in textbooks! The most important thing is to understand the scientific principle, which clearly tells us the correct order is W₃ > W₂ > W₁.