Exploring Forces – Important Questions and Answers for Class 8 Science
Complete premium revision post based on the CBSE Class 8 Science chapter Exploring Forces. Includes important keywords, very short answers, short answers, long answers, case studies, assertion-reason questions, competency-based questions, HOTS, diagram-based questions, revision box, exam tips, FAQs, and an interactive quiz.
Short Introduction
Forces are present in almost every activity around us. When we push, pull, lift, stop, stretch, bend, throw, or move an object, force is involved. This chapter explains the meaning of force, effects of force, contact and non-contact forces, muscular force, friction, magnetic force, electrostatic force, gravitational force, weight, spring balance, mass, buoyant force, floating, and sinking.
Chapter Overview
Meaning of Force
A force is a push or pull on an object resulting from interaction with another object.
Effects of Force
A force can make an object move, stop it, change its speed, change its direction, or change its shape.
Contact Forces
Muscular force and frictional force act only when objects are in physical contact.
Non-contact Forces
Magnetic, electrostatic, and gravitational forces can act from a distance without contact.
Weight and Mass
Weight is a force measured in newton, while mass is the amount of matter measured in gram or kilogram.
Floating and Sinking
Liquids apply an upward buoyant force. Objects float or sink depending on weight and buoyant force.
Important Keywords with Meanings
| Keyword | Meaning |
|---|---|
| Force | A push or pull on an object resulting from interaction with another object. |
| Push | A force applied to move an object away from the person or object applying it. |
| Pull | A force applied to bring an object closer to the person or object applying it. |
| Interaction | Action between two or more objects due to which force comes into play. |
| Newton | SI unit of force. Its symbol is N. |
| Contact force | A force that acts only when two objects are in physical contact. |
| Muscular force | Force produced by the action of muscles. |
| Friction | Force that opposes motion when an object moves or tries to move over another surface. |
| Frictional force | Another name for friction; it acts opposite to the direction of motion. |
| Non-contact force | A force that acts without physical contact between objects. |
| Magnetic force | Force exerted by a magnet on another magnet or magnetic material. |
| Electrostatic force | Force exerted by a charged body on another charged or uncharged body. |
| Static charges | Charges that build up on objects after rubbing and do not move by themselves. |
| Charged object | An object that has acquired static charges. |
| Positive charge | One kind of electric charge. |
| Negative charge | Another kind of electric charge. |
| Gravitational force | Force with which the Earth attracts objects towards itself. |
| Gravity | Another name for the gravitational force exerted by Earth. |
| Vertical motion | Motion of an object in a vertical direction under the influence of gravity. |
| Weight | Force with which the Earth pulls an object towards itself. |
| Spring balance | A device used to measure weight or force. |
| Mass | Amount of matter in an object, measured in gram or kilogram. |
| Upthrust | Upward force applied by a liquid on an object placed in it. |
| Buoyant force | Another name for upthrust. |
| Archimedes’ Principle | An immersed object experiences an upward force equal to the weight of liquid displaced by it. |
Very Short Answer Questions
1. What is force?
Answer: Force is a push or pull on an object resulting from interaction with another object.
2. What is the SI unit of force?
Answer: The SI unit of force is newton.
3. What is the symbol of newton?
Answer: The symbol of newton is N.
4. Name any two effects of force.
Answer: Force can change the speed of an object and change the shape of an object.
5. Can force change the direction of motion?
Answer: Yes, force can change the direction of motion of an object.
6. What is a contact force?
Answer: A force that acts only when objects are in physical contact is called a contact force.
7. Give two examples of contact forces.
Answer: Muscular force and frictional force.
8. What is muscular force?
Answer: The force produced by the action of muscles is called muscular force.
9. What is friction?
Answer: Friction is the force that opposes motion when an object moves or tries to move over another surface.
10. In which direction does friction act?
Answer: Friction acts opposite to the direction of motion or attempted motion.
11. Is friction a contact force?
Answer: Yes, friction is a contact force.
12. What causes friction between surfaces?
Answer: Friction is caused by minute irregularities present on surfaces in contact.
13. On which surface is friction greater?
Answer: Friction is greater on rough surfaces.
14. What is a non-contact force?
Answer: A force that acts without physical contact is called a non-contact force.
15. Give two examples of non-contact forces.
Answer: Magnetic force and gravitational force.
16. What is magnetic force?
Answer: The force exerted by a magnet on another magnet or magnetic material is called magnetic force.
17. What happens between like poles of magnets?
Answer: Like poles repel each other.
18. What happens between unlike poles of magnets?
Answer: Unlike poles attract each other.
19. What is electrostatic force?
Answer: The force exerted by a charged body on another charged or uncharged body is called electrostatic force.
20. What are the two kinds of static charges?
Answer: Positive charge and negative charge.
21. What is gravitational force?
Answer: The force with which the Earth attracts objects towards itself is called gravitational force.
22. What is weight?
Answer: Weight is the force with which the Earth pulls an object towards itself.
23. What is the SI unit of weight?
Answer: The SI unit of weight is newton.
24. What is upthrust?
Answer: The upward force applied by a liquid on an object placed in it is called upthrust.
Short Answer Questions
1. Why is force called a push or pull?
Answer: Force is called a push or pull because most actions involving force either push an object away or pull it closer. For example, pushing a box, pulling a drawer, lifting a bag, and stretching a rubber band all involve force.
2. What can a force do to an object?
Answer: A force can make an object move from rest, stop a moving object, change its speed, change its direction of motion, change its shape, or cause some of these effects together.
3. Why are at least two objects required for force?
Answer: A force comes into play only when two objects interact. For example, when a hand pushes a table, the hand and the table are two interacting objects.
4. Give examples showing that force can change shape.
Answer: Pressing an inflated balloon, rolling a chapati, stretching a rubber band, and squeezing a sponge show that force can change the shape of an object.
5. What are contact forces?
Answer: Contact forces are forces that act only when there is physical contact between objects. Muscular force and frictional force are examples of contact forces.
6. Explain muscular force with examples.
Answer: Muscular force is the force produced by muscles. It is used while walking, running, lifting, pushing, jumping, stretching, chewing food, and pumping blood through heart muscles.
7. Why does a rolling ball stop after some time?
Answer: A rolling ball stops after some time because friction acts between the ball and the ground. Friction opposes the motion of the ball and gradually reduces its speed.
8. Why is friction greater on rough surfaces?
Answer: Rough surfaces have more irregularities. These irregularities lock into each other and oppose motion more strongly, so friction is greater.
9. Why do objects move more easily on smooth surfaces?
Answer: Smooth surfaces have fewer irregularities than rough surfaces. Therefore, friction is less and objects move more easily.
10. What are non-contact forces?
Answer: Non-contact forces are forces that act from a distance without physical contact. Magnetic force, electrostatic force, and gravitational force are non-contact forces.
11. How does a charged plastic scale attract paper pieces?
Answer: When a plastic scale is rubbed with polythene, static charges build up on it. The charged scale exerts electrostatic force and attracts small paper pieces without touching them.
12. What happens when two similarly charged balloons are brought near each other?
Answer: They repel each other because like charges repel each other.
13. Why do objects fall towards the Earth?
Answer: Objects fall towards the Earth because the Earth attracts them by gravitational force.
14. What is the difference between mass and weight?
Answer: Mass is the amount of matter in an object and remains the same everywhere. Weight is the gravitational force acting on the object and can change from place to place.
15. What is a spring balance used for?
Answer: A spring balance is used to measure weight or force. It works by measuring the stretching of a spring when an object is hung from it.
Long Answer Questions
1. Define force. Explain the effects of force with examples.
Answer: A force is a push or pull on an object resulting from interaction with another object. The effects of force are:
- Can move an object from rest: Pushing a box makes it move.
- Can stop a moving object: A fielder stops a moving ball.
- Can change speed: Applying brakes slows a bicycle.
- Can change direction: Hitting a ball with a bat changes its direction.
- Can change shape: Pressing a balloon changes its shape.
2. Explain why force is an interaction between two or more objects.
Answer: Force does not act by itself. It appears only when two or more objects interact. For example, when we push a table, our hand applies force on the table. At the same time, the table also applies force on our hand. When the interaction stops, the force also disappears. Therefore, at least two objects must interact for a force to come into play.
3. Differentiate between contact forces and non-contact forces.
| Contact Forces | Non-contact Forces |
|---|---|
| They act only when objects are in physical contact. | They act without physical contact. |
| They may act directly or through a rope, stick, or other object. | They act from a distance. |
| Examples: muscular force, frictional force. | Examples: magnetic force, electrostatic force, gravitational force. |
4. Explain friction and its cause.
Answer: Friction is the force that comes into play when an object moves or tries to move over another surface. It always acts opposite to the direction of motion or attempted motion. Friction arises because surfaces have minute irregularities. When two surfaces are in contact, these irregularities lock into each other and oppose motion. Rough surfaces produce greater friction than smooth surfaces.
5. Why is friction both useful and problematic?
Answer:
- Useful: Friction helps us walk, hold objects, write with a pencil, apply brakes, and prevent slipping.
- Problematic: Friction slows moving objects, causes wear and tear, produces heat, and makes motion difficult on rough surfaces.
- Thus, friction is necessary in some situations but a problem in others.
6. Explain magnetic force as a non-contact force.
Answer: Magnetic force is the force exerted by a magnet on another magnet or magnetic material. A magnet can attract iron objects without touching them. Like poles of magnets repel each other and unlike poles attract each other. Since magnetic force acts from a distance without contact, it is a non-contact force.
7. Explain electrostatic force with an activity.
Answer: Rub a plastic scale or straw with polythene and bring it near small pieces of paper. The paper pieces get attracted towards the scale or straw. This happens because rubbing produces static charges on the plastic object. A charged object can attract uncharged paper pieces without contact. This force is called electrostatic force and it is a non-contact force.
8. Explain gravitational force and vertical motion.
Answer: The force with which the Earth attracts objects towards itself is called gravitational force or gravity. It is always attractive and acts without contact. When an object is thrown vertically upwards, its speed decreases, it stops momentarily at the highest point, and then falls down. While falling, its speed increases due to gravity. This is called vertical motion under the influence of gravitational force.
9. Explain weight and how it is measured using a spring balance.
Answer: Weight is the force with which the Earth pulls an object towards itself. Since weight is a force, its SI unit is newton. A spring balance is used to measure weight. It has a spring fixed at one end and a hook at the other. When an object is hung from the hook, the spring stretches. The amount of stretching shows the weight of the object on the scale marked in newton.
10. Explain floating and sinking using buoyant force.
Answer: When an object is placed in a liquid, the liquid applies an upward force on it. This upward force is called upthrust or buoyant force. The Earth also pulls the object downward by gravitational force. If the gravitational force is greater than the buoyant force, the object sinks. If the gravitational force and buoyant force are equal, the object floats.
Case-Study Based Questions
Case Study 1: Cycling Uphill and Downhill
Sonali and Ragini were cycling near their village. They found it harder to pedal uphill. While coming down the slope, their bicycles moved fast even when they were not pedalling.
Q1. Why was it harder to pedal uphill?
Answer: It was harder because they had to apply more muscular force against gravity and friction.
Q2. Why did the bicycle move faster downhill?
Answer: Gravity pulled the bicycle downhill, increasing its speed.
Q3. Which forces acted on the bicycle?
Answer: Muscular force, frictional force, and gravitational force acted on the bicycle.
Case Study 2: Ball Rolling on Different Surfaces
A ball rolls farther on a smooth floor but stops quickly on a rough surface.
Q1. Which force stops the ball?
Answer: Frictional force stops the ball.
Q2. Why does the ball stop quickly on a rough surface?
Answer: Rough surfaces have greater irregularities, so friction is more.
Q3. In which direction does friction act?
Answer: Friction acts opposite to the direction of motion.
Case Study 3: Charged Balloons
Two inflated balloons are rubbed with woollen cloth and then brought near each other. They move away from each other.
Q1. Why do the balloons repel each other?
Answer: Both balloons acquire similar charges, and like charges repel each other.
Q2. What force is acting between them?
Answer: Electrostatic force is acting between them.
Q3. Is this force contact or non-contact?
Answer: It is a non-contact force.
Case Study 4: Spring Balance Measurement
A student hangs a pencil box and a water bottle from a spring balance. The spring stretches more for the water bottle.
Q1. Why does the spring stretch?
Answer: The spring stretches because Earth pulls the object downward due to gravity.
Q2. Which object has greater weight?
Answer: The water bottle has greater weight because it stretches the spring more.
Q3. What is the unit of weight?
Answer: The unit of weight is newton.
Case Study 5: Floating Bottle
An empty closed bottle is pushed into water. It pushes back upward and bounces to the surface when released.
Q1. Which force pushes the bottle upward?
Answer: Buoyant force or upthrust pushes the bottle upward.
Q2. Why does the bottle float?
Answer: It floats because the upward buoyant force balances the downward gravitational force.
Q3. Name one factor on which buoyant force depends.
Answer: Buoyant force depends on the density of the liquid.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation.
B. Both Assertion and Reason are true but Reason is not the correct explanation.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
1. Assertion: A force is a push or pull. Reason: Force results from interaction between objects.
Answer: A
2. Assertion: Force can change the shape of an object. Reason: Pressing an inflated balloon can change its shape.
Answer: A
3. Assertion: Friction acts in the direction of motion. Reason: Friction helps objects move faster.
Answer: D
4. Assertion: Friction is greater on rough surfaces. Reason: Rough surfaces have more irregularities that lock into each other.
Answer: A
5. Assertion: Muscular force is a contact force. Reason: It usually involves physical contact while pushing, pulling, lifting, or moving objects.
Answer: A
6. Assertion: Magnetic force is a non-contact force. Reason: A magnet can attract iron objects without touching them.
Answer: A
7. Assertion: Like charges attract each other. Reason: Unlike charges repel each other.
Answer: Both Assertion and Reason are false.
8. Assertion: Gravitational force is always attractive. Reason: The Earth attracts objects towards itself.
Answer: A
9. Assertion: Weight is measured in newton. Reason: Weight is a force.
Answer: A
10. Assertion: Mass changes from place to place. Reason: Weight depends on gravitational force.
Answer: D
Competency-Based and Critical Thinking / HOTS Questions
1. Why does it feel harder to pedal a bicycle uphill than on flat ground?
Answer: While going uphill, the cyclist has to work against gravity and friction. More muscular force is needed to move the bicycle upward.
2. Why is it easier to slip on a wet surface?
Answer: Water reduces friction between the foot and the floor. With less friction, the grip becomes weaker and slipping becomes easier.
3. A ball is moving on a horizontal surface and comes to rest. Is any force acting on it?
Answer: Yes, frictional force acts on the ball opposite to its motion and slows it down until it stops.
4. If an object is at rest, does it mean no force is acting on it?
Answer: Not necessarily. Forces may be acting on it but balancing each other. When forces are balanced, the object may remain at rest.
5. A ball is thrown vertically upwards. Why does it slow down?
Answer: It slows down because gravitational force acts downward, opposite to its upward motion.
6. Why does the same object have less weight on the Moon than on Earth?
Answer: The Moon’s gravitational force is weaker than Earth’s gravitational force. Therefore, the object’s weight is less on the Moon, but its mass remains the same.
7. Why do aeroplanes and high-speed trains have special shapes?
Answer: They are designed with special shapes to reduce friction due to air, allowing smoother and faster movement.
8. Why does a mug feel lighter inside water?
Answer: Water applies an upward buoyant force on the mug. This upward force reduces the effective downward pull felt by the hand.
9. Why can a big wooden block float while a small coin sinks?
Answer: Floating or sinking depends on the balance between weight and buoyant force, not only on size. The wooden block receives enough buoyant force to float, while the coin’s weight is greater than the buoyant force on it.
10. How can a ball released from the same point on an inclined plane be made to stop before or after a point A?
Answer: To stop before point A, increase friction by using a rougher horizontal surface. To stop after point A, reduce friction by using a smoother surface.
Diagram-Based Questions
Diagram 1: Effects of Force
Question: Name any three effects of force shown above.
Answer: Force can move an object, stop a moving object, and change the shape of an object.
Diagram 2: Friction on a Moving Object
Question: In which direction does friction act?
Answer: Friction acts opposite to the direction of motion or attempted motion.
Diagram 3: Non-contact Forces
Question: Why are these called non-contact forces?
Answer: These forces can act from a distance without physical contact between objects.
Diagram 4: Floating and Sinking
Question: When does an object float?
Answer: An object floats when the upward buoyant force balances the downward gravitational force.
Diagram 5: Spring Balance
Question: What does a spring balance measure?
Answer: A spring balance measures weight or force.
Quick Revision Box
- Force is a push or pull resulting from interaction between objects.
- The SI unit of force is newton, symbol N.
- Force can change speed, direction, shape, or state of motion.
- Muscular force and frictional force are contact forces.
- Magnetic force, electrostatic force, and gravitational force are non-contact forces.
- Friction opposes motion and acts opposite to motion.
- Friction is greater on rough surfaces.
- Like charges repel and unlike charges attract.
- Gravity is the force with which Earth attracts objects.
- Weight is a force and is measured in newton.
- Mass remains the same everywhere, but weight can vary.
- Liquids apply upward buoyant force on objects placed in them.
- An object sinks if its weight is greater than buoyant force.
- An object floats if buoyant force balances its weight.
Important Exam Tips
Interactive Quiz
Choose the correct answer and click submit.
Frequently Asked Questions
1. What is force in Class 8 Science?
Force is a push or pull on an object resulting from its interaction with another object.
2. What are the main effects of force?
Force can move an object, stop a moving object, change its speed, change its direction, change its shape, or cause multiple effects together.
3. What are contact forces?
Contact forces are forces that act only when objects are in physical contact. Muscular force and frictional force are examples.
4. What are non-contact forces?
Non-contact forces act without physical contact. Magnetic force, electrostatic force, and gravitational force are examples.
5. What is friction?
Friction is the force that opposes motion when an object moves or tries to move over another surface.
6. Why is friction greater on rough surfaces?
Rough surfaces have more irregularities that lock into each other and oppose motion more strongly.
7. What is gravitational force?
Gravitational force is the force with which the Earth attracts objects towards itself.
8. What is the difference between mass and weight?
Mass is the amount of matter and remains the same everywhere. Weight is the gravitational force on an object and can vary from place to place.
9. What is buoyant force?
Buoyant force is the upward force applied by a liquid on an object placed in it.
10. Why do some objects float and others sink?
An object floats if buoyant force balances its weight. It sinks if its weight is greater than the buoyant force.
Final Conclusion
The chapter Exploring Forces helps students understand that force is present in everyday actions like pushing, pulling, cycling, walking, stopping, floating, and falling. Forces may act through contact or without contact. By understanding friction, gravity, weight, mass, and buoyant force, students can explain many real-life events scientifically.
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