Learn Class 8 Science Chapter 5 Exploring Forces: contact and non-contact forces, friction, weight, mass, and floating. Simple notes with key points and FAQs.
This chapter introduces you to forces — the pushes and pulls that make things move, stop, change direction, or change shape. You will discover that a force arises when two objects interact, and that forces can act either by touching an object (contact forces) or from a distance (non-contact forces). The chapter explains muscular force, friction, magnetic force, electrostatic force and gravitational force with simple activities. It also teaches you how weight is measured using a spring balance, the difference between mass and weight, and why some objects float while others sink in water. These ideas explain everyday experiences such as cycling uphill, slipping on a wet floor, and a ball falling back to the ground. By the end, you will be able to identify forces around you and describe their effects using correct scientific terms and units.
What you'll learn
1Define force and state its SI unit
2Identify the effects of force on motion and shape
3Distinguish between contact and non-contact forces
4Explain friction and the factors affecting it
5Describe magnetic, electrostatic and gravitational forces
6Measure weight using a spring balance
7Differentiate between mass and weight
8Explain floating and sinking using buoyant force
Chapter at a glance
01Chapter Overview
02Effects of Forces on Motion
03Types of Forces
04Pressure and Its Applications
05Simple Machines and Work
Detailed chapter notes
01
What Is a Force?
A force is a push or a pull applied on an object. It comes into play only when two objects interact with each other. For example, when you push a table, your hand and the table interact. The SI unit of force is the newton, written as N. Force can make an object move from rest, change its speed, change its direction of motion, or change its shape. If an object is at rest, it does not mean no force acts on it; the forces acting on it may be balancing one another. When two objects interact, each object experiences a force from the other, and the force disappears as soon as the interaction stops.
Force = push or pull resulting from interaction between objects
Contact forces act only when there is physical contact between objects. Muscular force is caused by the action of muscles in our body, such as when we walk, lift, push or stretch. Animals also use muscular force for movement. Friction is another contact force that arises when an object moves or tries to move over another surface. It always acts opposite to the direction of motion. Friction is caused by irregularities in the surfaces in contact. Rough surfaces produce more friction than smooth ones. Friction also acts on objects moving through liquids and gases, which is why aeroplanes, ships and high-speed trains are designed with special shapes to reduce it.
Muscular forcedue to contraction and elongation of muscles
Frictionopposes motion, acts opposite to direction of motion
Friction depends on the nature of surfaces in contact
03
Non-Contact Forces: Magnetic, Electrostatic and Gravitational
Non-contact forces can be experienced even when objects are not touching. Magnetic force is exerted by a magnet on another magnet or a magnetic material. Like poles repel and unlike poles attract. Electrostatic force is exerted by a charged body on another charged or uncharged body. When certain materials are rubbed together, they acquire static charges. Like charges repel and unlike charges attract. Gravitational force is the force with which the Earth attracts objects towards itself. It is always attractive. All objects thrown up fall back to the ground because of gravity. Unlike magnetic and electrostatic forces, gravitational force is only attractive, never repulsive.
Magnetic forcelike poles repel, unlike poles attract
Gravitational forcealways attractive, acts from a distance
04
Weight and Its Measurement
The force with which the Earth pulls an object towards itself is called the weight of the object. Since weight is a force, its SI unit is the newton (N). A spring balance is used to measure weight. It consists of a spring fixed at one end with a hook at the other. When an object is hung from the hook, the spring stretches, and the amount of stretch indicates the weight. The scale is marked in newtons, and often also in grams for mass. The smallest value a spring balance can read depends on its divisions. Mass is the amount of matter in an object and remains the same everywhere, while weight can change from place to place.
Weight = gravitational force on an object; SI unit: newton (N)
Spring balance measures weight by stretching of a spring
Mass is constant; weight varies with gravitational force
05
Floating and Sinking
When an object is placed in a liquid, the liquid exerts an upward force called upthrust or buoyant force. The gravitational force acts downwards on the object. If the gravitational force is greater than the buoyant force, the object sinks. If the two forces are equal, the object floats. The buoyant force depends on the density of the liquid. Archimedes' Principle states that when an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid it displaces. An object floats if the weight of the displaced liquid equals its own weight, and sinks if the displaced liquid weighs less.
Buoyant forceupward force exerted by a liquid
Object floats if buoyant force equals gravitational force
Archimedes' Principleupthrust = weight of displaced liquid
Want the complete chapter resources?Topic notes, quizzes and flashcards for Exploring Forces.
According to the chapter, which of the following is NOT an effect of applying force on an object?
AMaking an object move from rest
BChanging the speed of a moving object
CChanging the mass of an object
DChanging the shape of an object
Show answer
Answer: (C) Changing the mass of an object
The chapter lists that force can make an object move from rest, change its speed, change its direction of motion, or change its shape. It does not state that force changes the mass of an object.
Question 02
What is a force?
AA push or pull that changes the motion or shape of an object
BThe weight of an object
CThe speed of an object
DThe color of an object
Show answer
Answer: (A) A push or pull that changes the motion or shape of an object
A force is any push or pull that can change how an object moves or its shape.
Question 03
What is friction?
AA force that stops or slows down movement between two surfaces
BA force that makes things move faster
CA force that only works in water
DA force that pulls things upward
Show answer
Answer: (A) A force that stops or slows down movement between two surfaces
Friction is the force that opposes motion between two surfaces in contact. It always acts to slow down or stop movement.
Question 04
What is pressure?
AForce acting on a unit area
BThe weight of an object
CThe speed of movement
DThe direction of force
Show answer
Answer: (A) Force acting on a unit area
Pressure is defined as the force applied per unit area of surface.
Question 05
What is the SI unit of force?
Anewton (N)
Bjoule (J)
Cwatt (W)
Dpascal (Pa)
Show answer
Answer: (A) newton (N)
The SI unit of force is newton, denoted by N.
Ready for more practice?Unlock the full quiz for this chapter.
Q1. What is a force? Give two examples of contact forces and two examples of non-contact forces.
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Model answer
A force is a push or pull on an object resulting from the object's interaction with another object. Its SI unit is newton (N). Contact forces act only when there is physical contact between objects, e.g., muscular force and friction. Non-contact forces act without contact, e.g., magnetic force, electrostatic force, and gravitational force.
Sample question3 marks
Q2. A football is rolling on the ground. A player kicks it in the opposite direction. What effects of force are shown in this action?
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Model answer
The force applied by the player's kick changes the direction of the moving football. It may also change the speed of the ball. Thus, the force changes both the direction and speed of the ball.
Sample question3 marks
Q3. Define contact forces and give two examples.
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Model answer
Contact forces are forces that act only when there is physical contact between the objects. Examples include muscular force and frictional force.
Sample question3 marks
Q4. What is the force of friction? State the direction in which it acts and explain why it is called a contact force.
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Model answer
The force that comes into play when an object moves or tries to move over another surface is called the force of friction, or simply friction. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move. It is called a contact force because it arises due to two surfaces being in contact with each other.
Sample question3 marks
Q5. A ball rolling on a flat ground stops on its own after some time even though no one is pushing or pulling it. Name the force responsible and explain why it acts on the ball. State the direction in which this force acts.
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Model answer
The force responsible is the force of friction. It comes into play when an object moves or tries to move over another surface, and it arises due to the minute irregularities in the two surfaces in contact, which lock into each other and oppose motion. Friction always acts in a direction opposite to the direction in which the object is moving, so it gradually reduces the speed of the ball and brings it to a stop.
Want more questions with answers?Get the full practice set for this chapter.
Force is a push or pull on an object resulting from its interaction with another object. It can change an object's speed, direction of motion, or shape. The SI unit of force is the newton, written as N.
What is the difference between contact and non-contact forces?
Contact forces act only when objects are in physical contact, such as muscular force and friction. Non-contact forces act from a distance without contact, such as magnetic force, electrostatic force and gravitational force.
Why does a ball stop on its own after rolling on the ground?
A rolling ball stops because of friction between the ball and the ground. Friction acts opposite to the direction of motion and gradually reduces the ball's speed until it stops.
What is the difference between mass and weight?
Mass is the amount of matter in an object and remains the same everywhere. Weight is the gravitational force with which the Earth pulls an object; it can change from place to place. Mass is measured in kilograms, weight in newtons.
Why do some objects float and others sink in water?
An object floats if the buoyant force exerted by the liquid equals its weight. It sinks if its weight is greater than the buoyant force. The buoyant force depends on the density of the liquid and the volume of the object.
How is weight measured using a spring balance?
A spring balance has a spring with a hook. When an object is hung from the hook, the spring stretches. The amount of stretch indicates the weight of the object, which is read on a scale marked in newtons.