Chapter Notes: Exploring Forces
Science Class 8 (NCERT)
1. Introduction
Have you ever wondered why cycling uphill feels like a much tougher workout, but when you are coming downhill, your cycle seems to almost zoom forward on its own? These experiences are related to invisible but powerful forces acting on us and on the things around us.
Imagine Sonali and Ragini, two friends setting out for an adventure on their bicycles during the summer holidays. As they cycle against a strong wind, struggle up rough and hilly paths, and then speed down effortlessly without pedalling, they experience the effects of different forces. Nature also seems to join in: the wind pushes them, the ground offers resistance, and gravity finally pulls them down the hill.
What are these forces? What makes it hard or easy to move in different situations? Let us explore what happens when we push, pull, slip, slide, speed up or slow down.
2. What Is a Force?
Understanding Force through Activity
Take a large cardboard box.
Try to move the box in as many different ways as possible. You may push it, pull it, slide it, drag it or roll it.
Notice: In every method, you are either pushing or pulling the box.
Methods of moving include: a) Pushing, b) Pulling, and c) Lifting.
Definition: In science, a force is a push or a pull applied to an object. Whenever an object is moved, stopped, made to change its direction or speed, or made to change its shape, a force is involved.
Force can be a push: For example, when you push a swing away from yourself.
Force can be a pull: For example, when you draw water from a well using a rope.
No matter how you move the box in the activity, whether by dragging, sliding, rolling or lifting, some form of push or pull is always involved.
3. What Can a Force Do to the Bodies on Which It Is Applied?
We experience forces all the time, often without noticing them. The following examples show what a force can do:
Holding a moving bicycle from behind to stop it: A force opposing the motion slows down or stops the bicycle.
Hitting a moving ball with a bat: The force exerted by the bat can change the direction of the moving ball.
Pressing an inflated balloon: A force changes the shape of the balloon.
Kicking a football: A force moves the football from rest and starts its motion.
Applying brakes on a moving cycle: Frictional force slows down or stops the cycle.
Stretching a rubber band: A pulling force changes the shape of the rubber band.
Rolling a chapati: A pushing force changes the shape of the dough.
Turning the steering handle of an autorickshaw: A push or pull changes the direction of the autorickshaw.
Opening a drawer: A pulling force moves the drawer out from rest.
Stopping a cricket ball when a fielder catches it: The force exerted by the fielder stops or slows down the ball.
From the above examples, we see that the application of force can:
Start or move an object at rest: for example, by pushing a stationary object.
Stop or slow down a moving object: for example, by catching a moving ball or applying brakes.
Change the speed of a moving object: for example, by pushing a swing harder so that it moves faster.
Change the direction of a moving object: for example, by hitting a cricket ball towards another direction.
Change the shape of an object: for example, by squeezing, stretching, rolling or twisting it.
In summary: A force can make an object move, stop, speed up, slow down, change direction or change shape.
4. Are Forces an Interaction Between Two or More Objects?
Whenever you push or pull something, there are always two objects involved. A force is not produced by an object acting alone; it results from an interaction between objects.
Example 1: When you push a table, your hand and the table are involved.
Example 2: When a bat hits a moving ball, the bat and the ball interact.
Forces are the result of interactions between two or more objects. A force is a push or pull on an object resulting from its interaction with another object. The SI unit, or international scientific unit, for measuring force is the newton, with the symbol N.
Is Force Needed for Change?
Every change in the speed, direction or shape of an object means that a force has acted on it.
If an object remains at rest or continues moving without a change in speed or direction, the forces acting on it may be balanced.
An object being at rest does not always mean that no force is acting on it. It may mean that the forces acting on it are balanced.
Balanced forces may act on an object without changing its state of motion. Unbalanced forces can change its motion.
The SI unit for measuring force is the newton, written as N.
One newton is the force needed to give a mass of one kilogram an acceleration of one metre per second squared. In symbols, 1 N = 1 kg m s⁻².
When you apply a force on another object, that object also applies a force back on you.
Example: When you push a table, your hand feels a resistance from the table.
These two forces act on different objects. They arise because of the interaction between the objects.
When the interaction stops, the interaction forces between the two objects also stop.
Forces can be divided into two main categories: Contact forces and Non-contact forces.
Contact Forces
Forces that act only when there is physical contact between objects are called contact forces.
1. Muscular Force
Muscular force is the force produced by the action of muscles in humans and animals. Walking, lifting, pushing, pulling, chewing and the beating of the heart involve muscular force.
Lifting a bag, kicking a football, chewing food and pumping blood with the heart muscles involve muscular force.
Humans have used the muscular force of animals for tasks such as pulling carts and turning wheels.
Muscular force acts when a person or animal is in direct contact with an object, or when a tool provides indirect contact with it.
Friction is a force that opposes the motion of an object when it moves or tries to move over another surface. It acts in a direction opposite to the direction of motion or the attempted motion.
How friction works: Surfaces have tiny irregularities. These irregularities can interlock and resist the motion of one surface over another.
When you push a box on a table, it stops after some time because friction between the box and the table slows it down and eventually brings it to rest.
Nature of surfaces: Friction is different on different surfaces. Rough surfaces, such as sandpaper or cloth, generally produce more friction than smooth surfaces, such as glass or ceramic tile. Therefore, a box usually stops more quickly on a rough surface.
Friction in air and water: Air and water also oppose motion. This resistance is called drag. Air resistance and water resistance affect aeroplanes, cars and boats. Such vehicles are often given streamlined shapes to reduce resistance.
Friction is a contact force because it acts only when two surfaces are in contact.
Non-contact Forces
Some forces can act even when the objects are not in physical contact. These forces are called non-contact forces.
1. Magnetic Force
Magnetic force is the force exerted by a magnet on magnetic materials, such as iron, or on another magnet, even when they are at a distance.
Like poles of two magnets repel each other (e.g., north and north poles or south and south poles).
Unlike poles attract each other (e.g., a north pole and a south pole).
One ring magnet can appear to float above another ring magnet when like poles face each other.
Magnetic force is a non-contact force because magnets do not have to touch to attract or repel one another.
2. Electrostatic Force
When two objects made of certain materials are rubbed together, electrical charges can build up on their surfaces. These charges are called static charges because they remain at rest on the object for some time. An object that acquires static charges is called a charged object.
The force exerted by a charged body on another charged or uncharged body is called electrostatic force. It is a non-contact force.
Rubbing a plastic scale with polythene: The charged scale attracts small pieces of paper.
Rubbing two balloons with wool and hanging them: Similarly charged balloons repel each other. A charged balloon and the wool can attract each other.
Explanation: When certain materials are rubbed, they gain static charges. Like charges repel and unlike charges attract.
When charges move through a conducting path, they constitute an electric current in an electrical circuit. Electric current can make a lamp glow and can produce heating and magnetic effects.
6. Gravitational Force
The force with which the Earth attracts objects towards itself is called the gravitational force. The gravitational force exerted by the Earth is also called the force of gravity, or simply gravity.
If you throw a ball upwards, it comes back to the ground because gravity pulls it downwards.
Gravity is always attractive. Unlike magnetic and electrostatic forces, gravitational force does not repel.
Gravitational force acts between objects that have mass. The Earth attracts objects towards itself, and every object also attracts other objects, although this attraction may be very small.
While an object moves upwards, its speed decreases until it stops momentarily at the highest point. Its direction of motion then changes, and its speed increases while it comes down.
An object undergoes vertical motion when it moves in a vertical direction under the influence of gravitational force.
Gravitational force works at a distance and does not require contact.
7. Weight and Its Measurement
Weight is the force with which the Earth pulls an object towards itself due to gravity.
Weight tells us how strongly the Earth attracts an object.
The SI unit of weight is the newton, written as N, because weight is a force.
Measuring Weight with a Spring
When an object is suspended from a spring, the spring stretches because of the force with which the Earth pulls the object.
If objects of different masses are hung one after another, they produce different amounts of stretching.
This shows that heavier objects are generally pulled with a greater force. The weight of an object depends on its mass; heavier objects have greater weight at the same place.
Spring Balance: Measuring Force and Weight
Spring balance: A spring balance is an instrument with a spring fixed at one end and a hook at the other. The amount by which the spring stretches indicates the force or weight applied by the hanging object.
Scale markings: Spring balances are usually marked in newtons for measuring force and weight. Some balances also have gram or kilogram markings for mass under standard Earth gravity.
Observing the maximum weight measurable: The range of a spring balance is the maximum weight that it can measure. If a spring balance shows values from zero to ten newtons, its range is zero to ten newtons, and it can measure weights up to ten newtons.
Determining the smallest readable value: The least count of a spring balance is the smallest difference in weight that it can measure. It is found by dividing the difference between two consecutive major marks by the number of equal divisions between them. Example: If one newton is divided into five small divisions, each small division represents 0.2 newton. Thus, the least count is 0.2 newton.
Measuring weight with a spring balance: Hang the object from the hook of the spring balance without exceeding its maximum range. The pointer or scale reading gives the weight of the object in newtons. The measurement can be repeated for different objects and recorded in a table.
Using a spring balance to measure mass: A mass scale in grams or kilograms on a spring balance assumes the standard gravitational pull of the Earth. Therefore, a mass reading based on that scale is correct only under the gravitational conditions for which the balance was calibrated. Mass can also be compared with a known mass using a beam balance.
Mass and Weight
Mass is the amount of matter present in an object. Mass is measured in grams or kilograms and remains the same at every place, whether the object is on Earth, the Moon or another planet.
Weight is the force with which the Earth, or another planet, pulls an object. Weight is measured in newtons. Weight depends on mass and the acceleration due to gravity, expressed as W = mg, where W is weight, m is mass and g is acceleration due to gravity. Near the surface of the Earth, the value of g is approximately 9.8 m s⁻² (often taken as 10 m s⁻² for simple calculations). Weight can change when an object is taken to another planet or when gravitational force changes.
Variation of Weight on Different Planets (For an object of mass 1 kg)
Everyday and Scientific Language
In daily life, units of mass, such as kilograms, are often used while talking about weight. For example, people may say, 'The weight of the wheat bag is 10 kg.'
Scientifically, this statement is not correct. The proper unit of mass is the kilogram, while the proper unit of weight is the newton.
It is important to use the correct terms and units in scientific work.
8. Buoyant Force: Upthrust, Floating and Sinking
Upthrust, or buoyant force, is the upward force exerted by a liquid on an object immersed in it. All liquids exert this force on objects placed in them. For example, when you push a tightly closed empty plastic bottle into a bucket full of water, you feel an upward push on your hand. The bottle moves upwards when it is released.
What Happens Physically?
When an object is placed in a liquid:
Gravity: pulls the object downwards.
The buoyant force: or upthrust, pushes the object upwards.
What Decides Whether an Object Floats or Sinks?
If the gravitational force, or weight of the object, is greater than the buoyant force, the object sinks.
If the buoyant force becomes equal to the weight of the object, the object floats.
The density of the liquid affects the buoyant force. This factor is studied in greater detail in later classes.
Interesting fact: Some rocks, such as pumice, which is formed during volcanic eruptions, can float on water. Pumice contains many holes and air spaces, making its average density less than that of water. Therefore, it can float.
9. Archimedes' Principle
Archimedes was an ancient Greek scientist.
Archimedes' Principle: When an object is fully or partially immersed in a liquid, it experiences an upward force called the buoyant force. This force is equal to the weight of the liquid displaced by the object.
Consequences
If the weight of the displaced liquid is less than the weight of the object, the object sinks.
If the weight of the displaced liquid becomes equal to the weight of the object, the object floats.
10. Key Points to Remember
Force: A force is a push or pull on an object resulting from its interaction with another object.
SI unit of force: The SI unit of force is the newton, written as N.
Types of force: Forces can act with or without contact between objects.
Contact forces: Muscular force and frictional force are examples of contact forces.
Non-contact forces: Magnetic force, gravitational force and electrostatic force are examples of non-contact forces.
Effects of force: A force can change an object's speed, direction of motion or shape. It can also start or stop motion.
Friction: The force that acts when an object moves or tries to move over another surface is called frictional force. Friction acts opposite to the direction of motion or attempted motion.
Magnetic force: The force exerted by a magnet on another magnet or on a magnetic material is called magnetic force.
Electrostatic force: The force exerted by a charged body on another charged or uncharged body is called electrostatic force.
Gravitational force: The force with which the Earth attracts objects towards itself is called gravitational force. This force is always attractive.
Weight: The force with which the Earth pulls an object towards itself is called the weight of the object. The SI unit of weight is the newton.
Mass and weight: The mass of an object remains unchanged wherever it is, while its weight may change from place to place.
Upthrust or buoyant force: The upward force exerted by a liquid on an object placed in it is called upthrust or buoyant force.
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