Chapter 5: Exploring Forces

 Chapter 5: Exploring Forces

 Class 8  |  Science  |  Questions with Answers

Section A: Multiple Choice Questions (30 x 1 mark)

1. A push or a pull on an object is called a/an:

(a) Energy    (b) Force    (c) Work    (d) Speed

Answer: (b) Force

2. The SI unit of force is:

(a) Joule    (b) Watt    (c) Newton    (d) Pascal

Answer: (c) Newton

3. Which of the following is NOT an effect of force?

(a) Change in shape    (b) Change in speed    (c) Change in colour    (d) Change in direction

Answer: (c) Change in colour

4. A force is the result of interaction between:

(a) One object only    (b) At least two objects    (c) Only living objects    (d) Only magnetic objects

Answer: (b) At least two objects

5. Muscular force is an example of a:

(a) Non-contact force    (b) Contact force    (c) Magnetic force    (d) Gravitational force

Answer: (b) Contact force

6. The force that opposes relative motion between two surfaces in contact is called:

(a) Gravity    (b) Friction    (c) Magnetism    (d) Buoyancy

Answer: (b) Friction

7. Friction acts in a direction:

(a) Same as motion    (b) Opposite to motion    (c) Perpendicular to motion    (d) It has no fixed direction

Answer: (b) Opposite to motion

8. A ball rolling on a rough surface stops sooner than on a smooth surface because rough

surfaces have:

(a) Less friction    (b) More friction    (c) No friction    (d) More gravity

Answer: (b) More friction

9. Which of these is a non-contact force?

(a) Muscular force    (b) Frictional force    (c) Gravitational force    (d) Force applied while pushing a box

Answer: (c) Gravitational force

10. Like poles of two magnets:

(a) Attract each other    (b) Repel each other    (c) Have no effect    (d) Cancel each other's force

Answer: (b) Repel each other

11. The force exerted by a magnet on another magnet or magnetic material is called:

(a) Electrostatic force    (b) Magnetic force    (c) Muscular force    (d) Gravitational force

Answer: (b) Magnetic force

12. Static charges are called 'static' because they:

(a) Move very fast    (b) Do not move by themselves    (c) Only exist in liquids    (d) Are always positive

Answer: (b) Do not move by themselves

13. When a plastic scale is rubbed with polythene and brought near paper bits, the paper bits:

(a) Repel the scale    (b) Are attracted to the scale    (c) Remain unaffected    (d) Get magnetised

Answer: (b) Are attracted to the scale

14. Two balloons charged in the same way, when brought close, will:

(a) Attract each other    (b) Repel each other    (c) Stick together permanently    (d) Show no force

Answer: (b) Repel each other

15. Unlike charges:

(a) Repel each other    (b) Attract each other    (c) Have no interaction    (d) Cancel gravity

Answer: (b) Attract each other

16. The force exerted by a charged body on another body is called:

(a) Magnetic force    (b) Electrostatic force    (c) Muscular force    (d) Buoyant force

Answer: (b) Electrostatic force

17. The force with which the Earth attracts objects towards itself is called:

(a) Magnetic force    (b) Electrostatic force    (c) Gravitational force    (d) Frictional force

Answer: (c) Gravitational force

18. Gravitational force is always:

(a) Attractive    (b) Repulsive    (c) Either attractive or repulsive    (d) Zero on Earth

Answer: (a) Attractive

19. The weight of an object is the force with which:

(a) The object pulls the Earth    (b) The Earth pulls the object    (c) Air pushes the object    (d) A magnet pulls the object

Answer: (b) The Earth pulls the object

20. The SI unit of weight is the same as that of:

(a) Mass    (b) Force    (c) Volume    (d) Density

Answer: (b) Force

21. Which quantity remains the same everywhere in the universe: mass or weight?

(a) Weight    (b) Mass    (c) Both change equally    (d) Neither

Answer: (b) Mass

22. A device used to measure weight by the stretching of a spring is called a:

(a) Beam balance    (b) Spring balance    (c) Thermometer    (d) Barometer

Answer: (b) Spring balance

23. The weight of a 1 kg object on the Earth is approximately:

(a) 1 N    (b) 6 N    (c) 10 N    (d) 100 N

Answer: (c) 10 N

24. The upward force exerted by a liquid on an object placed in it is called:

(a) Gravitational force    (b) Electrostatic force    (c) Buoyant force (upthrust)    (d) Magnetic force

Answer: (c) Buoyant force (upthrust)

25. An object sinks in a liquid when:

(a) Buoyant force is greater than gravitational force    (b) Buoyant force equals gravitational force    (c) Gravitational force is greater than buoyant force    (d) There is no gravitational force

Answer: (c) Gravitational force is greater than buoyant force

26. Which of the following is an example of a non-contact force?

(a) Kicking a football    (b) A compass needle aligning with the North    (c) Rolling a chapati    (d) A

fielder catching a ball

Answer: (b) A compass needle aligning with the North

27. According to Archimedes' Principle, the upward force on an immersed object equals the

weight of:

(a) The object itself    (b) The liquid displaced by the object    (c) The container    (d) The air above the liquid

Answer: (b) The liquid displaced by the object

28. Pumice stone floats on water because it is:

(a) Denser than water    (b) Less dense than water    (c) Magnetic    (d) Electrically charged

Answer: (b) Less dense than water

29. When we stop pedalling a bicycle on a flat road, it eventually stops due to:

(a) Gravitational force    (b) Magnetic force    (c) Frictional force    (d) Electrostatic force

Answer: (c) Frictional force

30. Mass is measured in:

(a) Newton    (b) Kilogram    (c) Pascal    (d) Joule

Answer: (b) Kilogram

Section B: Short Answer Questions (20 x 2 marks)

1. Define force. Give the SI unit of force.

Answer: Force is a push or pull on an object resulting from its interaction with another object. SI unit:  newton (N).

2. List any four effects that a force can produce on an object.

Answer: A force can (i) move an object from rest, (ii) change its speed, (iii) change its direction of motion, (iv) change its shape.

3. What do you understand by a contact force? Give two examples.

Answer: A contact force acts only when there is physical contact between objects, e.g. muscular force, frictional force.

4. What do you understand by a non-contact force? Give two examples.

Answer: A non-contact force acts even without physical contact, e.g. magnetic force, gravitational force, electrostatic force.

5. Why is muscular force called a contact force?

Answer: Because it is caused by the direct action of the body's muscles contracting/elongating while touching or moving an object.

6. Define the force of friction. In which direction does it act?

Answer: Friction is the force that opposes the motion of an object over another surface; it acts opposite to the direction of motion or attempted motion.

7. Why does a ball rolling on the ground eventually come to a stop?

Answer: Because the force of friction between the ball and the ground acts opposite to its motion and gradually slows it down until it stops.

8. How does the roughness of a surface affect the force of friction?

Answer: Friction is greater on rough surfaces (more irregularities) and smaller on smooth surfaces, so objects stop sooner on rough surfaces.

9. State one everyday situation where friction is useful and one where it is a hindrance.

Answer: Useful: walking/writing needs friction. Hindrance: friction wears out machine parts / makes pedalling harder.

10. What is a charged object? How does an object become charged?

Answer: An object that has acquired static charges after being rubbed with another object; charging happens due to rubbing (friction) transferring charge.

11. State the law of attraction and repulsion for electric charges.

Answer: Like charges repel each other; unlike charges attract each other.

12. Two balloons rubbed with wool are brought close to each other. What is observed, and why?

Answer: They move away from each other (repel) because both balloons acquire similar (like) charges from the woollen cloth.

13. Define gravitational force. Give one example that shows the Earth exerts a gravitational force on objects.

Answer: Gravitational force is the force with which the Earth attracts objects towards itself, e.g. a fruit falling from a tree.

14. Why is gravitational force considered a non-contact force?

Answer: Because it acts on objects without them being in physical contact with the Earth.

15. Define the weight of an object. State its SI unit.

Answer: Weight is the force with which the Earth pulls an object towards itself. SI unit: newton (N).

16. Differentiate between mass and weight (any two points).

Answer: Mass is the amount of matter in an object (kg/g) and stays constant everywhere; weight is the gravitational force on the object (N) and varies with location.

17. Why does the weight of an object vary from planet to planet while its mass does not?

Answer: Because gravitational force differs from planet to planet, and weight depends on gravitational force, while mass (amount of matter) stays constant.

18. What is a spring balance? What does it measure?

Answer: A device with a spring and hook that measures weight (force) by the amount the spring stretches when an object is hung from it.

19. Define buoyant force (upthrust). State the direction in which it acts.

Answer: Buoyant force (upthrust) is the upward force exerted by a liquid on an object placed in it; it acts in the upward direction.

20. State Archimedes' Principle in your own words.

Answer: When an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid displaced by it.

Section C: Three-Mark Questions (10 x 3 marks)

1. Explain, with two examples from Table 5.1 of your textbook, how a force can change the speed, direction, and shape of an object.

Answer: E.g. hitting a moving ball with a bat changes its direction; pressing an inflated balloon changes its shape. A force can act on a moving or stationary object to alter speed, direction, or shape depending on how it is applied.

2. Explain why at least two objects must be involved for a force to come into play. Illustrate with the example of pushing a table.

Answer: When you push a table, your hand (one object) applies force on the table (second object); both experience the interaction. Without a second object to interact with, there is nothing to push or pull, so force needs at least two interacting objects.

3. Explain, with the help of an activity, how friction can be demonstrated when a flat-based object is pushed across a table.

Answer: Pushing a lunch box or notebook across a table, it slides and then stops after some distance due to friction between its base and the table's surface, which acts opposite to its motion and gradually brings it to rest.

4. Explain why friction is greater on rough surfaces than on smooth surfaces, using the idea of surface irregularities.

Answer: Even seemingly smooth surfaces have microscopic irregularities; when two rough surfaces are in contact, more irregularities interlock, offering greater resistance to sliding, hence greater friction than smooth surfaces.

5. Describe an activity to show that a magnet can exert force on another magnet without being in contact with it.

Answer: When a second ring magnet with like poles facing the first is placed on a vertical stick above it, it floats without touching the first magnet, showing that magnetic force acts across a distance without contact.

6. Explain the difference between attraction and repulsion of charges with reference to the balloon-and-woollen-cloth activity.

Answer: Balloons rubbed with the same woollen cloth acquire similar (like) charges and repel each other when brought close; when the woollen cloth (with opposite charge) is brought near a balloon, they attract,showing unlike charges attract.

7. Explain, with an example, why an object thrown vertically upwards eventually falls back to the ground.

Answer: While going up, gravitational force acts downward, opposing motion and decreasing speed until it becomes zero momentarily at the top; then the same force pulls the object back down, increasing its speed as it falls.

8. Describe how a spring balance is used to measure the weight of an object, and explain why its readings would differ on the Moon.

Answer: An object is hung from the spring balance's hook; the spring stretches, and the scale shows the weight in newtons. On the Moon, gravitational force is about one-sixth of that on Earth, so the same object would show a much smaller reading.

9. Explain why a coin sinks in water while a large wooden block floats, using the ideas of gravitational force and buoyant force.

Answer: The coin's weight is greater than the buoyant force exerted by water on it, so it sinks; the wooden block, being larger, displaces more water and experiences a buoyant force equal to (or greater than) its weight, so it floats.

10. Explain why aeroplanes, ships, and high-speed trains are given specific streamlined shapes.

Answer: Streamlined shapes reduce the force of friction (drag) exerted by air or water as these vehicles move through them, allowing them to move faster and more efficiently.

Section D: Long Answer / Five-Mark Questions (10 x 5 marks)

1. What is force? Describe, with suitable examples, the four different effects a force can have on an object on which it is applied.

Answer: Force is a push or pull on an object due to its interaction with another object. Effects: (i) sets a stationary object in motion, e.g. pushing a stationary box; (ii) changes speed, e.g. pedalling harder speeds up a bicycle; (iii) changes direction, e.g. hitting a moving ball with a bat; (iv) changes shape, e.g. pressing an inflated balloon. A single force can also cause more than one of these effects at once.

2. What are contact forces and non-contact forces? Explain muscular force and frictional force as examples of contact forces, and magnetic, electrostatic, and gravitational force as examples of non-contact forces.

Answer: Contact forces act only with physical touch: muscular force (from muscle action, e.g. lifting a bag) and frictional force (opposing relative motion between surfaces in contact, e.g. a ball stopping on the ground). Non-contact forces act without touch: magnetic force (between magnets/magnetic materials), electrostatic force (between charged bodies), and gravitational force (Earth pulling objects towards itself, e.g. a fruit falling from a tree).

3. Explain the force of friction in detail: its cause, the direction in which it acts, the factors affecting it, and its role for objects moving through air and water.

Answer: Friction arises due to irregularities on surfaces in contact locking into each other, opposing relative motion; it always acts opposite to the direction of motion or attempted motion. It depends on the nature (roughness/smoothness) of the surfaces — greater on rough surfaces. Air and water also exert friction on moving objects, so aeroplanes, ships, and trains are streamlined to reduce this resistance.

4. Describe the phenomenon of electrostatic force. Explain, using two activities from the chapter, how objects become charged and how like and unlike charges behave.

Answer: When two objects are rubbed together, static electric charges build up on their surfaces (Activity 5.6: a rubbed plastic scale attracts paper bits). Charged bodies attract uncharged bodies and unlike-charged bodies, but repel bodies with like charges (Activity 5.7: two balloons rubbed with wool repel each other but are each attracted to the wool). This force is called electrostatic force and is a non-contact force; charges are of two kinds, positive and negative.

5. Explain gravitational force and its effect on objects thrown vertically upwards and dropped from a height. Describe the changes in speed and direction of motion during upward and downward vertical motion.

Answer: Gravitational force is the force with which the Earth attracts objects towards itself; it is always attractive and acts without contact. When a ball is thrown upward, gravity acts downward, decreasing its speed until it momentarily stops at the highest point (direction of motion changes here); then gravity pulls it back down, and its speed increases as it falls, following a straight vertical path — this is called vertical motion.

6. Distinguish between mass and weight. Explain, with the help of the table of weights of a 1 kg object on Earth, Moon, Mars, Venus, and Jupiter, why weight differs while mass stays the same.

Answer: Mass is the amount of matter in an object (measured in kg/g) and stays the same everywhere. Weight is the gravitational force pulling the object towards a planet (measured in N) and varies with the planet's gravity. A 1 kg object weighs about 10 N on Earth, 1.6 N on the Moon, 3.8 N on Mars, 9 N on Venus, and 25.4 N on Jupiter — same mass, different weights, because gravitational pull differs.

7. Describe, in detail, how a spring balance works and how it is used to measure the weight of an object, including how its smallest readable division is calculated.

Answer: A spring balance has a spring fixed at one end with a hook at the other; hanging an object stretches the spring in proportion to its weight, and a marked scale shows the weight in newtons (with a corresponding mass scale in grams). The smallest readable division is found by measuring the gap between two marked values (e.g. 1 N) and dividing by the number of small divisions between them (e.g. 5), giving a smallest reading of 0.2 N.

8. Explain the phenomena of floating and sinking with reference to gravitational force and buoyant force. State Archimedes' Principle and explain it with an example.

Answer: When an object is placed in a liquid, gravitational force pulls it down while the liquid's buoyant force (upthrust) pushes it up. If gravity exceeds buoyant force, the object sinks; if the two are equal, it floats. Archimedes' Principle states that an immersed object experiences an upward force equal to the weight of the liquid it displaces; if this displaced weight is less than the object's weight, it sinks, and if equal, it floats.

9. With reference to Activity 5.1 (moving a cardboard box), explain the different ways a force can be applied to an object and identify whether each is a push or a pull.

Answer: A box can be pushed (Fig. 5.1a), pulled with a rope (Fig. 5.1b), or lifted/carried (Fig. 5.1c, a pulling-up force). All of these apply a push or pull to move the box, showing several different ways of applying the same underlying concept of force, and each can be classified clearly as either a push or a pull.

10. A ball released from point P rolls down an inclined plane and along a horizontal surface, stopping at point A due to friction. Explain, with reasons, what changes you could make so that the ball stops (i) before point A and (ii) after crossing point A.

Answer: To make the ball stop before point A, increase friction on the horizontal surface (e.g. use a rougher surface/cloth) or release the ball from a lower point on the incline so it has less speed. To make it stop after crossing point A, decrease friction (e.g. use a smoother/oiled surface) or release the ball from a higher point on the incline so it gains more speed before reaching the horizontal surface.

Section E: Assertion-Reason Questions (10 x 1 mark)

For questions 1-10, two statements are given: Assertion (A) and Reason (R). Choose the correct option:

(a) Both A and R are true, and R is the correct explanation of A.

(b) Both A and R are true, but R is NOT the correct explanation of A.

(c) A is true, but R is false.

(d) A is false, but R is true.

1. Assertion (A): A force can change the speed, direction, and shape of an object.

   Reason (R): A force is a push or pull that results from the interaction between two objects.

Answer: (a)

2. Assertion (A): Muscular force is classified as a contact force.

   Reason (R): Muscular force is produced by the contraction and elongation of muscles during physical activity.

Answer: (a)

3. Assertion (A): A ball rolling on a flat ground eventually stops on its own.

   Reason (R): The force of friction acts in the same direction as the motion of the ball, slowing it down.

Answer: (c)

4. Assertion (A): A rolling object stops sooner on a rough surface than on a smooth surface.

   Reason (R): Rough surfaces have more irregularities that lock into the irregularities of the object's surface, increasing friction.

Answer: (a)

5. Assertion (A): A magnet can exert a force on another magnet even without touching it.

   Reason (R): Magnetic force is a non-contact force.

Answer: (a)

6. Assertion (A): A charged plastic scale attracts small pieces of paper.

   Reason (R): Rubbing certain materials together causes electrical charges to build up on their surfaces.

Answer: (a)

7. Assertion (A): Two balloons rubbed with the same woollen cloth repel each other when

brought close.

   Reason (R): Both balloons acquire similar (like) charges, and like charges repel each other.

Answer: (a)

8. Assertion (A): An object thrown vertically upward slows down as it rises.

   Reason (R): Gravitational force acts downward on the object, opposing its upward motion.

Answer: (a)

9. Assertion (A): The weight of an object is different on the Earth and on the Moon.

   Reason (R): Weight depends on the gravitational force of the planet, which varies from planet to planet.

Answer: (a)

10. Assertion (A): A large wooden block floats on water while a small coin sinks.

   Reason (R): The buoyant force on the wooden block is greater than or equal to its weight,

while the buoyant force on the coin is less than its weight.

Answer: (a)