Exploring Forces | CBSE Class 8 Science Notes

This note covers pushes and pulls, interactions between objects, effects of force, contact and non-contact forces, muscular force, friction, magnetic and electrostatic forces, gravity, weight and mass, spring-balance measurements, and floating and sinking.

What is a force, and why does it involve an interaction?

Definition: A force is a push or pull on an object resulting from its interaction with another object.

An interaction occurs when objects act on one another. When a hand pushes a table, the hand and the table are the two interacting objects. At least two objects must interact for a force to come into play.

Generally, a push or pull applied to an object is called force. Moving a large cardboard box makes this idea easy to recognise. Pushing the box, pulling it, or lifting it all involve applying force. Lifting is a pull upwards.

How can we identify the interacting objects?

Begin with the object whose motion or shape is being considered. Then identify what pushes or pulls it. In the table example, the table receives a push from the hand. The push is part of the interaction between them.

Whenever two objects interact, each experiences a force from the other. While pushing a table, a person also feels a force on the hand. When that pushing interaction ceases, the force associated with it is no longer experienced.

The SI unit, meaning the unit used in the International System of Units, of force is the newton. Its symbol is N. The unit name begins with a small letter, while its symbol is a capital letter.

Does rest mean that no force acts?

An object at rest need not be free of forces. Forces acting on it can balance one another. Balanced forces are forces that balance each other in their effect on the object. Rest alone therefore does not establish the absence of force.

Note: Identify the interaction as well as the effect. A force may act even when an object remains at rest, because other forces can balance it.

What changes can a force produce in an object?

An applied force may start an object moving from rest, change its speed, change its direction of motion, or change its shape. It may cause some or all of these effects. The word “may” matters: applying force does not guarantee one particular visible change.

Speed describes how fast an object moves, while its direction of motion describes where it is moving. A force can affect either. A change in shape is different from a change in motion, although force can cause both.

How do familiar actions show these effects?

These actions show why force cannot be identified with movement alone. Pressing the balloon is an example of changing shape. Holding the bicycle from behind is an example of reducing motion. Hitting the moving ball shows a change in direction.

Other familiar applications include opening a drawer, stretching a rubber band, a fielder stopping a ball, kicking a football, applying bicycle brakes, rolling a chapati, and turning an autorickshaw’s steering handle. Each involves a push or pull acting through an interaction.

How should an observation be explained?

1. Identify the object on which the force acts.
2. Identify the other object involved in the interaction.
3. Describe the action as a push or pull.
4. State whether the observed effect concerns speed, direction, shape, or movement from rest.

A change in speed, direction, or shape does not take place without the action of force. However, the presence of force and the observation of a particular change are different statements. Balanced forces explain why an object can experience forces and still remain at rest.

How does muscular force act as a contact force?

A contact force acts only when there is physical contact between the interacting objects. Contact can be direct, through a hand or another body part, or indirect, through an object such as a stick or rope.

Muscular force is force produced by the action of muscles. Muscles contract and elongate during physical activity. Contracting means becoming shorter, while elongating means becoming longer. These actions enable living beings to apply muscular force.

Where is muscular force used?

Walking, running, lifting, pushing, jumping, and stretching involve muscular action. Lifting a school bag is an example of muscular force. A hand pushing a table also applies muscular force, with physical contact connecting the hand and the table.

Animals, birds, fish, and insects use muscular forces for movement and survival. Humans have also used the muscular force of some animals to perform tasks for a long time. Muscular force is therefore not confined to human activity.

Does muscular force act inside the body?

The alimentary canal is the passage through which food travels during digestion. Muscular force helps move food through this passage. It also helps with chewing. These uses occur inside the body rather than in the movement of an external object.

Circulation is the movement of blood through the body. The expansion and contraction of heart muscles allow this movement. Thus, muscular force supports both visible activities, such as lifting, and internal functions essential for survival.

Why does friction oppose motion between surfaces?

Definition: Friction is the force that acts when an object moves, or tries to move, over another surface.

Friction always acts opposite to the direction in which the object is moving or trying to move. It is a contact force because it arises between surfaces in contact. An object need not already be moving for friction to act.

How can friction be investigated?

1. Place an object with a flat base, such as an empty lunch box, geometry box, or notebook, on a table or floor.
2. Push it gently and observe how it slides before stopping.
3. Push it in the opposite direction and observe its motion again.
4. Repeat with the same object on surfaces such as glass, cloth, wood, ceramic tile, and sand.

The object stops after sliding because friction acts between its surface and the table or floor. When it slides in the opposite direction, friction again opposes its motion. Its direction is linked to the direction of movement, not to a fixed side of the room.

On different surfaces, the object travels different distances before stopping. This shows that friction depends on the nature of the surfaces in contact. Friction is greater on rough surfaces. A bicycle stops sooner on a rough road than on a smoother road after pedalling stops.

What causes friction?

Surfaces contain tiny irregularities, meaning small uneven parts. Even surfaces that appear smooth have many minute irregularities. When surfaces touch, these irregularities lock into one another and oppose attempts to move one surface over the other.

The drawing shows a smaller block on a larger surface. Their touching edges are uneven and fit into one another, illustrating the irregularities responsible for friction.

See Fig. 5.6 in your NCERT textbook

Friction also acts on objects moving through air, water, and other liquids. Aeroplanes, ships, boats, and high-speed trains have specific shapes to reduce friction from the surrounding air or water. Friction is therefore not restricted to movement across solid surfaces.

How can magnets exert force without touching?

A non-contact force can act even when the interacting objects are not touching. A magnet can exert force on another magnet or on a magnetic material from a distance. Physical contact is therefore not necessary for every force.

Magnetic force is the force exerted by a magnet on another magnet or on a magnetic material. A magnetic material is a material attracted by a magnet. Magnetic force belongs to the non-contact group of forces.

What happens between magnetic poles?

The poles of a magnet are its north and south pole regions. Like poles are poles of the same kind, such as north with north or south with south. Unlike poles are north and south together.

Attraction means pulling towards one another, while repulsion means pushing away from one another. Like magnetic poles repel, and unlike magnetic poles attract. Both attraction and repulsion are forms of force because both are pushes or pulls.

What do ring magnets demonstrate?

Place a wooden stick vertically on a wooden table and put a ring magnet over it. Add a second ring magnet above the first, arranging like poles to face one another. The upper magnet stays above the lower magnet without touching it.

Gently pushing the upper magnet down makes the opposing force noticeable. The separation between the magnets demonstrates that a magnet can act on another magnet without contact. Reversing both magnets still leaves like poles facing one another.

Figure: Force between two ring magnets (NCERT Class 8 Figure 5.7). The photograph shows two grey ring magnets around a vertical wooden stick. A visible gap separates the upper ring from the lower one.

How do static charges produce attraction and repulsion?

Rubbing together objects made of certain materials can build up electrical charges on their surfaces. Electrical charge is the property involved in electrical attraction and repulsion. These are called static charges because they do not move by themselves.

An object that acquires static charge is a charged object. An uncharged object has not acquired such a charge. A charged object can attract uncharged objects made of certain materials, including small pieces of paper, even before touching them.

How can attraction of paper be observed?

1. Collect a plastic scale or straw, a piece of polythene, and small pieces of paper.
2. Rub the scale or straw vigorously with the polythene.
3. Avoid touching the rubbed part with a hand or metal object.
4. Bring it close to the paper pieces without touching them and observe their movement towards it.

The paper pieces are pulled towards the rubbed plastic and stick to it. The attraction begins while they are separated, showing that the force does not require physical contact between the plastic and the paper.

What do rubbed balloons reveal?

Hang two inflated balloons so that they do not touch. Rub both with a woollen cloth, avoiding contact with the rubbed surfaces of the balloons. The balloons move away from one another as if repelling each other.

Bring the rubbing cloth near one rubbed balloon. They move towards each other as if attracting each other. Balloons charged in the same way acquire similar charges. The rubbing object and rubbed object both become charged, but acquire opposite kinds of charge.

The two kinds are called positive and negative charges. Like charges repel, while unlike charges attract. These names distinguish the kinds of charge; the balloon activity establishes their similarity or difference without requiring a particular sign to be assigned.

Electrostatic force is the force exerted by a charged body on another charged or uncharged body. It is a non-contact force. A charged object attracting paper and similarly charged balloons repelling are two different effects of this force.

How does gravity affect a falling or upward-thrown object?

Gravitational force is the force with which the Earth attracts objects towards itself. The gravitational force exerted by the Earth is also called the force of gravity. It acts without physical contact between the Earth and the object.

Gravity is always attractive. This distinguishes it from magnetic and electrostatic forces, which can be attractive or repulsive. An object falling towards the Earth is being pulled towards it, rather than pushed away from it.

What happens when a ball is thrown upwards?

A ball thrown vertically upwards rises, slows down, stops momentarily at its highest point, and then falls. Vertical motion here means motion in a vertical direction under the influence of gravity. Throwing the ball harder does not prevent it from returning to the ground.

1. During the upward journey, the ball moves straight upwards while its speed decreases.
2. At the highest point, it stops for a moment.
3. Its direction of motion then changes from upwards to downwards.
4. During the downward journey, its speed increases as it falls towards the Earth.

An object simply dropped from a height follows a straight vertical path downwards before reaching the ground. The upward-thrown ball has an additional rising part of its journey, but it also returns towards the Earth.

How do the non-contact forces compare?

The common feature is action without contact. Their behaviour is not identical: attraction towards the Earth, repulsion between like magnetic poles, and attraction of paper by rubbed plastic are explained by different kinds of force.

What is weight, and how does a spring balance measure it?

Weight is the force with which the Earth pulls an object towards itself. It tells us how strongly the Earth attracts that object. Since weight is a force, the SI unit of weight is newton, represented by N.

Hang a spring from a nail and suspend different objects from its lower end, one at a time. The spring stretches because the Earth pulls the suspended object downwards. Different stretches indicate different weights in this investigation.

How does the instrument work?

A spring balance measures weight using the stretching of a spring. One end is fixed, and the other carries a hook. Suspending an object from the hook stretches the spring. A marked scale shows the weight in newtons.

Usually, another scale shows corresponding mass values in grams. Mass is the amount of matter in an object; a gram, with symbol g, is a unit of mass. This mass scale assumes that the balance is being used on the Earth.

How are range and the smallest division found?

The range is the interval of weights that the instrument can measure. The illustrated spring balance has a range of 0 to 10 N. Its larger marks are 1 N apart, with five divisions between neighbouring larger marks.

Worked example 1. A spring balance has larger marks 1 N apart and five equal divisions between them. Find the value of one small division.

Answer: One division represents $\frac{1\,\mathrm{N}}{5} = 0.2\,\mathrm{N}$. Its smallest readable weight interval is therefore 0.2 N.

Figure: A spring balance and close-up of its scale (NCERT Class 8 Figure 5.13). The photographs show a balance with a top suspension ring, a lower hook, and an enlarged scale. The scale is labelled GRAMS on the left and NEWTONS on the right.

1. Inspect the instrument’s range before choosing an object to weigh.
2. Determine the value of its smallest division from the marked intervals.
3. Suspend an object from the hook, keeping within the maximum measurable weight.
4. Read the weight scale carefully and record the measured value with its unit.

Overloading can damage the balance. Different balances may have different ranges and smallest divisions, so inspect the particular instrument being used. A pencil box or partially filled water bottle needs an actual measurement; its weight cannot be supplied merely from its name.

How do mass and weight differ from place to place?

Mass and weight describe different quantities. Mass is the amount of matter in an object and remains the same at every place. Weight is the gravitational force exerted on that object by the Earth or another planet.

Mass is measured in grams or kilograms, whose symbol is kg. Weight is measured in newtons. Gravitational force can vary very slightly from place to place on Earth and can be very different on different planets. Weight can change while mass does not.

What does the comparison of places show?

Here, $W$ denotes the weight of the 1 kg object, measured in newtons (N). The subscript names the place where the weight is measured.

The mass row stays unchanged across the five places. The weight row changes because the gravitational pull differs. For example, the same mass is associated with a smaller weight on the Moon than on Earth and a larger weight on Jupiter.

Why is weighing used to find mass?

An object’s mass can be measured indirectly through its weight using a spring balance. A beam balance compares the object’s weight with the weight of an object of known mass. Both methods use weight to find mass indirectly.

Weight remains almost the same everywhere on Earth. For all practical purposes, weighing an object to find its mass is therefore acceptable. This practical use does not make the scientific definitions of mass and weight identical.

Note: Everyday speech may call a wheat bag’s “weight” 10 kg. Scientifically, 10 kg states its mass. A statement of weight uses a force unit such as newton.

The assumption behind a spring balance’s mass scale matters here. The scale connects the measured pull to mass under Earth conditions. The amount of matter remains unchanged if the object is taken elsewhere, even though its weight can change.

Why do some objects float while others sink?

A mug can feel lighter while it is inside water. Similarly, an empty bottle with its lid tightly closed gives an upward push when pressed into water and bounces back to the surface when released. Water exerts an upward force on it.

Upthrust, also called buoyant force, is the upward force applied by a liquid to an object. All liquids apply a similar force. Gravity still pulls the object downwards while the liquid pushes upwards.

How do the two forces determine the result?

If gravitational force is greater than buoyant force, the object sinks. If the two forces are equal, the object floats. Floating does not mean that gravity has disappeared; the upward and downward forces balance.

For a floating object, $F_{\mathrm{b}} = W_{\mathrm{object}}$. Here, $F_{\mathrm{b}}$ is the upward buoyant force and $W_{\mathrm{object}}$ is the downward weight of the object. Both forces are measured in newtons (N).

The bottle investigation can be explained by identifying both interacting surroundings: the Earth provides the downward gravitational pull, and water provides the upward buoyant force. Considering only the Earth’s pull would leave the upward push unexplained.

What does Archimedes’ Principle state?

An object immersed in a liquid is partly or completely within it. Displaced liquid is the liquid moved aside by the object. These terms describe what happens when an object occupies space in a liquid.

Archimedes’ Principle states that an object fully or partially immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces. The condition includes both complete and partial immersion.

For full or partial immersion, $F_{\mathrm{b}} = W_{\mathrm{displaced\ liquid}}$. Here, $W_{\mathrm{displaced\ liquid}}$ is the weight of the displaced liquid, measured in newtons (N), and $F_{\mathrm{b}}$ is the buoyant force in newtons.

If the displaced liquid weighs less than the object, the object sinks. If the displaced liquid’s weight equals the object’s weight, it floats. This relates buoyant force to the displaced liquid while retaining the comparison between upward and downward forces.

A coin sinking in water and a bigger wooden block floating show why size alone does not settle the result. The sinking coin has a greater downward weight than upward buoyant force; the floating block has these forces balanced.

Glossary

• Force — A push or pull on an object resulting from its interaction with another object.
• Contact force — A force that acts only when interacting objects have physical contact with one another.
• Muscular force — Force produced by the action of muscles during physical activities and internal bodily functions.
• Friction — Force opposing the movement or attempted movement of an object over another surface.
• Non-contact force — A force whose effect can occur even when the interacting objects are not touching.
• Magnetic force — Force exerted by a magnet on another magnet or an object made of magnetic material.
• Static charges — Electrical charges that build up on surfaces and do not move by themselves.
• Electrostatic force — Force exerted by a charged object on another charged object or an uncharged object.
• Gravity — The attractive gravitational force exerted by the Earth on objects towards itself.
• Weight — The force with which the Earth pulls an object towards itself, measured in newtons.
• Mass — The amount of matter in an object, remaining the same at every place.
• Spring balance — An instrument that measures weight from the stretching of a spring carrying a suspended object.
• Buoyant force — The upward force applied by a liquid to an object placed in it.
• Archimedes’ Principle — An immersed object experiences an upward force equal to the weight of the liquid it displaces.

Common errors and misconceptions

• Misconception: An object at rest has no forces acting on it. Correct: Forces can act on an object at rest and balance one another.
• Misconception: Force must produce movement. Correct: Force may change speed, direction, or shape, and forces can also balance without starting motion.
• Misconception: Every force requires touching. Correct: Magnetic, electrostatic, and gravitational forces can act without contact between the interacting objects.
• Misconception: Friction occurs only after movement begins. Correct: Friction also acts when an object tries to move over another surface.
• Misconception: Smooth-looking surfaces have no irregularities. Correct: Even apparently smooth surfaces have many minute irregularities that contribute to friction.
• Misconception: Both balloons rubbed with the same woollen cloth should attract. Correct: They acquire similar charges and repel; the balloon and rubbing cloth acquire opposite charges.
• Misconception: Mass and weight are identical and are both measured in kilograms. Correct: Mass measures matter; weight is a force measured in newtons and can change with location.
• Misconception: Gravity does not act on a floating object. Correct: Its downward gravitational force is balanced by the liquid’s upward buoyant force.

Exam-style questions with model answers

Q1. Define force and state its SI unit and symbol. [2 marks]

1. A force is a push or pull on an object resulting from its interaction with another object.
2. The SI unit of force is newton, and its symbol is N.

Q2. A friend holds a moving bicycle from behind, a bat hits a moving ball, and a hand presses an inflated balloon. For each action, identify the push or pull and its effect. [3 marks]

1. Holding the moving bicycle from behind applies a pull. The effect is to decrease the bicycle’s speed or bring it to a stop.
2. Hitting the moving ball with a bat applies a push. This can change the direction in which the ball moves.
3. Pressing the inflated balloon applies a push. The effect is a change in the shape of the balloon.

Q3. An empty lunch box is pushed across a table and stops after sliding. Explain the force involved, its direction, and its cause. [3 marks]

1. Friction acts between the lunch box and the table and brings the sliding box to rest. It is a contact force.
2. The frictional force acts in the direction opposite to the direction in which the lunch box is sliding across the table.
3. Minute irregularities on the touching surfaces lock into one another. They oppose movement of one surface over the other.

Q4. Two balloons are rubbed with the same woollen cloth and brought near each other. The cloth is then brought near one rubbed balloon. Explain the charging and the two observations, and classify the force. [4 marks]

1. Rubbing produces static charges. Both the balloons and the rubbing cloth become charged, with the rubbed and rubbing objects acquiring opposite kinds of charge.
2. The two balloons acquire similar charges because they are charged in the same way. They repel and move away from one another.
3. The balloon and the woollen cloth have unlike charges. They attract and move towards one another when brought close.
4. The interaction involves electrostatic force. It is a non-contact force because its effect occurs even before the objects touch.

Q5. A spring balance has a range of 0 to 10 N. Adjacent larger marks differ by 1 N, divided into five equal intervals. Explain its working, maximum measurable weight, smallest division, measurement procedure, and overload precaution. [5 marks]

1. The balance works through a spring fixed at one end with a hook at the other. A suspended object stretches the spring, indicating its weight.
2. The range is 0 to 10 N, so the maximum weight that this particular instrument can measure is 10 N.
3. The five equal intervals together represent 1 N. Therefore, one small division represents 1 N ÷ 5 = 0.2 N.
4. Suspend the object from the hook, read the weight scale carefully, and record the reading in newtons. Check the scale before measuring.
5. Do not suspend an object heavier than the maximum measurable weight. Exceeding the instrument’s range may damage the spring balance.

Q6. An object has mass 1 kg on both Earth and the Moon. Its listed weights are 10 N on Earth and 1.6 N on the Moon. Explain mass, weight, their units, and why these readings differ. [5 marks]

1. Mass is the amount of matter in an object. In the given comparison, this amount remains unchanged when the object moves from Earth to the Moon.
2. Weight describes the gravitational pull acting on the object. It measures force, rather than the amount of matter contained in the object.
3. The stated mass is 1 kg at both places. The symbol kg stands for kilogram, a unit used for measuring mass.
4. The weights are 10 N on Earth and 1.6 N on the Moon. The symbol N stands for newton, the unit of force.
5. The difference in weight is caused by the different gravitational pull at the two places. It does not show a reduction in the object’s mass.

Q7. A coin sinks in water while a bigger wooden block floats. Identify the two opposing forces and explain the result for each object. [3 marks]

1. The Earth pulls each object downwards through gravitational force, which is its weight. Water applies an upward buoyant force to each object.
2. The coin sinks because its downward gravitational force is greater than the upward buoyant force acting on it in the water.
3. The wooden block floats because its upward buoyant force equals its downward weight. Its larger size alone does not decide whether it sinks.

Q8. State Archimedes’ Principle and use it to describe the force balance on an object floating in a liquid. [2 marks]

1. A fully or partially immersed object experiences an upward force equal to the weight of the liquid it displaces.
2. For a floating object, this upward buoyant force equals the object’s downward weight, so the forces balance.

Key takeaways

• A force is a push or pull arising from interaction between objects, and its SI unit is newton.
• Force may start motion, change speed or direction, alter shape, or produce several of these effects together.
• Muscular force and friction are contact forces; magnetic, electrostatic, and gravitational forces can act without contact.
• Friction opposes movement or attempted movement between surfaces and is greater on rough surfaces than smoother ones.
• Like magnetic poles and like electrical charges repel, whereas unlike poles and unlike electrical charges attract.
• A spring balance measures weight through spring stretching; check its range and smallest division before making a measurement.
• Mass remains the same at every place, while weight can change because gravitational pull can differ between places.
• An object floats when upward buoyant force equals downward weight and sinks when its weight exceeds buoyant force.

Test yourself

Why does a force require more than one object?

A force results from an interaction, so at least two objects must act on one another for it to arise.

Can an object at rest experience force?

Yes. Forces may act on an object at rest and balance one another.

Why is friction classed as a contact force?

It arises between surfaces in contact when one moves or tries to move over the other.

What happens when like poles of two ring magnets face each other?

They repel one another, allowing the upper ring magnet on a vertical wooden stick to remain separated from the lower one.

What are the two kinds of static charge?

They are positive and negative charges. Like charges repel, while unlike charges attract.

What is one division worth if five equal divisions cover 1 N?

One division represents 1 N divided by five, which is 0.2 N.

What does the phrase “a wheat bag of 10 kg” specify scientifically?

It specifies the mass of the bag. Weight is a force and is measured in newtons.

Does water stop exerting force on an object that sinks?

No. Water still applies an upward buoyant force, but the object’s downward weight is greater.