Class 8 Science · Chapter 5

5. Exploring Forces

Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).

Keep the Curiosity Alive

1Match items in Column A (type of force) with the items in Column B (example).

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

2State 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

Friction opposes motion, so the speed of the ball decreases.

(iii) There is no force between two charged objects placed at a small distance apart. — False

Charged objects exert an electrostatic force on each other, even without contact.

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

The two balloons will move away from each other (repel).

Both balloons get the same kind of charge on rubbing with the woollen cloth. Like charges repel each other.

4When you drop a coin in a glass of water, it sinks, but a bigger wooden block placed in water floats. Explain.

Two forces act on an object in water. Its weight acts downwards. The buoyant force (upthrust) of water acts upwards.

Coin: its weight is more than the buoyant force on it. So it sinks. The coin is made of metal, which is denser than water.
Wooden block: the buoyant force on it becomes equal to its weight. So it floats. Wood is less dense than water.

Floating or sinking does not depend on size. It depends on whether the object is denser or less dense than water.

5A ball is thrown upwards. It slows down, stops momentarily, and then falls back. Name the forces acting on the ball and specify their directions.

(i) During upward motion: gravitational force acts downwards. Air resistance (friction due to air) also acts downwards, opposite to the motion.

(ii) During downward motion: gravitational force acts downwards. Air resistance acts upwards, opposite to the motion.

(iii) At the topmost position: only the gravitational force acts, downwards. The ball is at rest for a moment, so there is no air resistance.

6A ball released from point P rolls down an inclined plane and stops at point A on the horizontal surface. Think of a way so that the ball released from P stops (i) before point A (ii) after crossing point A.

(i) To stop before A: increase friction. Spread a rough cloth, a towel or sand on the horizontal surface.

(ii) To stop after A: reduce friction. Make the horizontal surface smooth, for example by covering it with a glass sheet or a polished surface.

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

We can walk only because of friction between our feet and the ground. Friction gives our feet a grip.

Ice and polished floors are very smooth. They offer very little friction. Our feet do not get a proper grip, so we slip.

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

Yes. In non-uniform motion, the speed or the direction of motion (or both) keeps changing.

A force is needed to change the speed or direction of motion. So a force must be acting on the object.

9The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass also become one-sixth?

Weight is the force with which a planet or the Moon pulls an object. The gravitational force of the Moon is about one-sixth of that of the Earth. So the weight becomes one-sixth.

No, the mass does not change. Mass is the amount of matter in the object. It remains the same everywhere.

10Three objects 1, 2 and 3 of the same size and shape but made of different materials dip to different depths in water (Fig. 5.17). If their weights are w₁, w₂ and w₃, then

Answer: (ii) w₁ > w₂ > w₃

All three objects float. For a floating object, the buoyant force equals its weight. Object 1 dips the most, so it displaces the most water and its weight is the greatest. Object 3 dips the least, so its weight is the least.

Discover, Design, and Debate

The “Discover, design, and debate” projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

← 4. Electricity: Magnetic and Heating Effects 6. Pressure, Winds, Storms, and Cyclones →