SCIENCE CLASS- 8

CHAPTER-5 (Exploring Forces)

CBSEChapter 5 Keep the curiosity alive

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

Answer:

(i) → (b)
(ii) → (e)
(iii) → (a)
(iv) → (c)
(v) → (d)

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: According to Newton's laws of motion, a force is needed to change the speed of a moving object. Force can increase or decrease its speed.

(ii) Due to friction, the speed of the ball rolling on a flat ground increases.

Answer: False

Explanation: Friction opposes motion. Therefore, it decreases the speed of the rolling ball and eventually stops it.

(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. They may either attract or repel depending on the nature of charges.

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, both acquire the same type of electric charge.

Since like charges repel each other, the balloons will move away from each other when brought close together.

This is due to the electrostatic force of repulsion acting between them.

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:

Whether an object sinks or floats depends on its density and the upthrust exerted by water.

The coin is denser than water. Its weight is greater than the upward force exerted by water, so it sinks.

The wooden block is less dense than water. The upward force exerted by water balances its weight, so it floats.

Therefore, even though the wooden block is larger in size, it floats because its density is less than that of water.

Question 5. If a ball is thrown upward, 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:

The force acting on the ball is the gravitational force of the Earth.

Direction: Vertically downward towards the Earth.

This force opposes the upward motion of the ball and gradually reduces its speed.

(ii) During its downward motion

Answer:

The gravitational force continues to act on the ball.

Direction: Vertically downward towards the Earth.

This force causes the ball to accelerate downward.

(iii) At its topmost position

Answer:

Even at the highest point, gravitational force acts on the ball.

Direction: Vertically downward.

Although the velocity becomes zero momentarily, gravity continues to act and pulls the ball back towards the Earth.

Question 6. A ball is released from point P and moves along an inclined plane and then along a horizontal surface. It comes to rest at point A.

(i) Think of a way so that when the ball is released from the same point P, it stops before reaching point A.

Answer:

The ball can be made to stop before point A by increasing the friction between the ball and the surface.

This can be done by:

Making the surface rough.
Spreading sand on the path.
Using a rough cloth on the surface.

Greater friction will reduce the speed more quickly and stop the ball earlier.

(ii) Think of a way so that when the ball is released from the same point P, it stops after crossing point A.

Answer:

The ball can be made to cross point A by reducing friction.

This can be done by:

Polishing the surface.
Using a smoother surface.
Applying lubricants where possible.

Less friction allows the ball to travel a greater distance.

Question 7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.

Answer:

Friction provides grip between our feet and the ground.

Ice and polished floors offer very little friction.

Due to insufficient friction, our feet cannot get a proper grip and may slide unexpectedly.

As a result, we may lose balance and slip.

Question 8. Is any force being applied to an object in a non-uniform motion?

Answer: Yes.

Explanation:

Non-uniform motion means that the speed or direction of motion is changing.

A change in speed or direction can occur only when a force acts on the object.

Therefore, a force must be acting on an object moving with non-uniform 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 weight of an object depends on the gravitational pull acting on it.

The Moon's gravitational force is about one-sixth that of the Earth.

Therefore, an object weighs only one-sixth as much on the Moon.

Weight on Moon = Weight on Earth ÷ 6

However, the mass of an object remains unchanged because mass is the amount of matter contained in the object.

Mass on Earth = Mass on Moon

Question 10. Three objects 1, 2 and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects are w₁, w₂ and w₃ respectively, then which relation is correct?

Observation:

Object 1 is immersed the most, object 3 is immersed less, and object 2 is immersed the least.

The deeper an object sinks in water, the greater is its weight for the same size and shape.

Therefore:

w₁ > w₃ > w₂

Correct Option: (iv) w₁ > w₃ > w₂

Key Concepts Learned

Force can change the speed, direction and shape of an object.
Friction opposes motion.
Gravitational force always acts towards the Earth.
Electrostatic force acts between charged objects.
Magnetic force acts between magnets and magnetic materials.
Objects float or sink depending on density and upthrust.
Weight depends on gravity, but mass remains constant everywhere.
Non-uniform motion always involves the action of a force.

CHAPTER-5 (Exploring Forces)

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

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.

(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.

Question 3. Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?

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.

Question 5. If a ball is thrown upward, 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

Question 6. A ball is released from point P and moves along an inclined plane and then along a horizontal surface. It comes to rest at point A.

(i) Think of a way so that when the ball is released from the same point P, it stops before reaching point A.

(ii) Think of a way so that when the ball is released from the same point P, it stops after crossing point A.

Question 7. Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.

Question 8. Is any force being applied to an object in a non-uniform 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?

Question 10. Three objects 1, 2 and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects are w1, w2 and w3 respectively, then which relation is correct?

Key Concepts Learned

Question 10. Three objects 1, 2 and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects are w₁, w₂ and w₃ respectively, then which relation is correct?