Exploring ForcesClass 8 Science NCERT Solutions
10 Solutions
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Solution 1 of 10
Q1Keep the curiosity alive
Match items in Column A with the items in Column B.
Column A (Type of force) Column B (Example) (i) Muscular force (a) A cricket ball stopping on its own just before touching the boundary line (ii) Magnetic force (b) A child lifting a school bag (iii) Frictional force (c) A fruit falling from a tree (iv) Gravitational force (d) Balloon rubbed on woollen cloth attracting hair strands (v) Electrostatic force (e) A compass needle pointing North
Solution
The correct matches are:
- (i) Muscular force (b) A child lifting a school bag
- Muscular force is a contact force resulting from the action of muscles, used for activities like lifting.
- (ii) Magnetic force (e) A compass needle pointing North
- A compass needle (a small magnet) aligns itself with Earth's magnetic field, demonstrating magnetic force.
- (iii) Frictional force (a) A cricket ball stopping on its own just before touching the boundary line
- Friction is a force that opposes motion, causing moving objects to slow down and stop.
- (iv) Gravitational force (c) A fruit falling from a tree
- Gravitational force is the attractive force exerted by Earth, pulling objects towards its centre.
- (v) Electrostatic force (d) Balloon rubbed on woollen cloth attracting hair strands
- Electrostatic force is exerted by a charged body on another charged or uncharged body, like a charged balloon attracting hair.
Q2Keep the curiosity alive
State whether the following statements are True or False.
(i)
A force is always required to change the speed of motion of an object.
(ii)
Due to friction, the speed of the ball rolling on a flat ground increases.
(iii)
There is no force between two charged objects placed at a small distance apart.
Solution
(i)
True. According to the chapter, a force is essential to change the speed, direction of motion, or shape of an object. Without the action of force, these changes do not occur.
(ii)
False. Friction is a force that always acts in a direction opposite to the direction of motion or attempted motion. Therefore, friction causes the speed of a rolling ball to decrease, eventually bringing it to a stop.
(iii)
False. There is an electrostatic force between two charged objects, even if they are not in physical contact. Like charges repel each other, while unlike charges attract each other. This force can also act between a charged object and an uncharged object.
Q3Keep the curiosity alive
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Solution
When two balloons are rubbed with the same woollen cloth, they both acquire similar electrical charges. According to the principles of electrostatic force, objects with similar (like) charges repel each other.
Therefore, when these two similarly charged balloons are brought near each other, they would move away from each other, exhibiting repulsion. This is because the electrostatic force between like charges is repulsive.
Q4Keep the curiosity alive
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.
Solution
This phenomenon can be explained by the interplay of two forces: the gravitational force (weight of the object) acting downwards and the buoyant force (upthrust) exerted by the water acting upwards.
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Coin: When a coin is dropped in water, its weight (gravitational force) is greater than the maximum buoyant force that the water can exert on it (which is the weight of the water displaced by the coin's full volume). Because the downward gravitational force is stronger than the upward buoyant force, the coin sinks.
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Wooden Block: When a wooden block is placed in water, it floats because its weight (gravitational force) is equal to the buoyant force exerted by the water on the submerged part of the block. This means the wooden block displaces a volume of water whose weight is exactly equal to the block's own weight. Since the block's weight is less than the maximum buoyant force it could experience if fully submerged, it floats.
Q5Keep the curiosity alive
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
(ii)
During its downward motion
(iii)
At its topmost position
Solution
(i)
During its upward motion:
* Gravitational force: Acts downwards, pulling the ball towards the Earth.
* Frictional force (Air resistance): Acts downwards, opposing the upward motion of the ball.
(ii)
During its downward motion:
* Gravitational force: Acts downwards, pulling the ball towards the Earth.
* Frictional force (Air resistance): Acts upwards, opposing the downward motion of the ball.
(iii)
At its topmost position:
* Gravitational force: Acts downwards, pulling the ball towards the Earth. At this momentary point, the ball's vertical velocity is zero, so there is no frictional force due to air resistance.
Q6Keep the curiosity alive
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 .
Solution
The ball stops due to the force of friction acting between its surface and the horizontal surface. The distance it travels before stopping depends on the magnitude of this frictional force.
(i)
To stop before the point A:
* To make the ball stop earlier, the force of friction needs to be increased. This can be achieved by making the horizontal surface rougher. For example, one could spread sand, place a rough cloth, or use a surface with a higher coefficient of friction in the path of the ball.
(ii)
To stop after crossing the point A:
* To make the ball travel further, the force of friction needs to be decreased. This can be achieved by making the horizontal surface smoother. For example, one could polish the surface, apply a lubricant like oil or grease, or use a very smooth material like glass or a highly polished tile in the ball's path.
Q7Keep the curiosity alive
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Solution
We sometimes slip on smooth surfaces like ice or polished floors because the force of friction acting between our feet (or shoes) and the surface is very low. Friction arises due to the irregularities present on the surfaces in contact.
Smooth surfaces have very few minute irregularities compared to rough surfaces. When these surfaces come into contact, their irregularities do not interlock effectively, resulting in a significantly reduced frictional force. Since friction is the force that opposes relative motion and provides the grip necessary for walking, a lack of sufficient friction makes it difficult to push against the surface and maintain balance, leading to slipping.
Q8Keep the curiosity alive
Is any force being applied to an object in a non-uniform motion?
Solution
Yes, a force is indeed being applied to an object in non-uniform motion. Non-uniform motion implies that either the speed of the object, or its direction of motion, or both are changing. According to the chapter, a change in the speed or direction of an object's motion cannot occur without the action of a force. Therefore, for an object to be in non-uniform motion, there must be a net force acting upon it.
Q9Keep the curiosity alive
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?
Solution
The change in an object's weight on the Moon compared to the Earth is caused by the difference in their gravitational forces.
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Cause of change in weight: Weight is defined as the force with which a celestial body (like Earth or Moon) pulls an object towards itself. The Moon has a significantly smaller mass and size than the Earth. Consequently, the gravitational force exerted by the Moon is much weaker than that of the Earth. This weaker gravitational pull results in an object experiencing only about one-sixth of the gravitational force it would on Earth, hence its weight is one-sixth.
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Change in mass: No, the mass of the object does not become one-sixth of its mass on the Earth. Mass is the amount of matter contained in an object, and it is an intrinsic property of the object. Its value remains constant regardless of its location in the universe or the gravitational field it is in. The chapter states that 'The mass of an object remains unchanged whereas its weight may vary from place to place.'
Q10Keep the curiosity alive
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 , and , respectively, then
(i)
(ii)
(iii)
(iv)
Solution
To determine the relationship between the weights of the three objects, we apply the principles of floating and sinking, which depend on the balance between the object's weight (gravitational force) and the buoyant force from the water.
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Object 1 and Object 2 (Floating): Both objects 1 and 2 are floating, which means the buoyant force acting on each object is equal to its weight. The buoyant force is equal to the weight of the water displaced by the submerged part of the object. From Fig. 5.17, object 2 is submerged deeper than object 1. This implies that object 2 displaces a greater volume of water than object 1. Therefore, the buoyant force on object 2 is greater than on object 1, which means .
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Object 3 (Sinking): Object 3 has sunk to the bottom of the water. This indicates that its weight is greater than the maximum possible buoyant force the water can exert on it (which is the weight of the water displaced by the object's entire volume). Since objects 1 and 2 are floating, their weights are equal to or less than the weight of the water displaced by their full volume. Thus, object 3 must have the greatest weight among the three: is the largest.
Combining these observations, the correct order of weights is .
Upon reviewing the given options:
(i)
(Incorrect)
(ii)
(Incorrect)
(iii)
(Incorrect)
(iv)
(Incorrect, as it implies which contradicts the observation that object 2 is more submerged than object 1).
Therefore, none of the provided options accurately represent the relationship between the weights of the three objects as derived from the principles of buoyancy and the visual information.