Exploring ForcesClass 8 Science Notes

What Is a Force?

Imagine you're helping a friend move. Whether you're pushing a heavy box, pulling a stubborn drawer, or lifting a chair, you're applying a force. In simple terms, a force is a push or a pull.

Forces are essential for moving objects, changing their speed, or even changing their shape. Let's explore what forces can do.

What Can a Force Do to the Bodies on Which It Is Applied?

We use forces all the time, often without realizing it. Let's think about some common situations:

• Opening a drawer: You pull the drawer outwards.
• Stretching a rubber band: You pull on both ends of the rubber band.
• A fielder stopping a ball: The fielder applies a force to stop the ball's motion.
• Kicking a football: You apply a force to make the ball move.
• Applying brakes on a moving bicycle: The brakes apply a force to slow down or stop the bicycle.
• Rolling a chapati: You use a rolling pin to apply force and change the shape of the dough.
• Turning the steering handle of an autorickshaw: You apply a force to change the direction of the autorickshaw.

So, what can a force actually do? A force applied on an object may:

• Make an object move from rest: A football at rest will only move when you kick it.
• Change the speed of an object if it is moving: When you pedal harder on your bicycle, you increase its speed.
• Change the direction of motion of an object: Steering your bicycle changes its direction.
• Bring about a change in the shape of an object: Squeezing a clay ball changes its shape.
• Cause some or all of these effects.

What if an object is at rest? Does that mean no force is acting on it? Not necessarily! It could mean that the forces acting on the object are balancing each other out. You'll learn more about balanced forces later on.

Are Forces an Interaction Between Two or More Objects?

You can't apply a force to something without interacting with it. When you push a table, your hand is one object applying force on another object—the table. Your hand and the table are interacting with each other.

Forces always result from an interaction between at least two objects.

A force is a push or pull on an object resulting from the object's interaction with another object. The SI unit (the standard unit used by scientists) of force is the newton, written with a small 'n' and its symbol is N.

When you push a table, you also feel a force on your hand. As soon as you stop pushing, the force on your hand disappears. Whenever two objects interact, each object experiences a force from the other. As soon as the interaction stops, the forces disappear.

What Are the Different Types of Forces?

Forces can be broadly classified into two types:

• Contact forces
• Non-contact forces

Contact Forces

In many situations, you need to physically touch an object to apply a force on it. This is called a contact force. This contact can be direct, like using your hands, or indirect, like using a stick or rope.

Muscular Force

An example of a contact force is muscular force. When you walk, run, lift, push, jump, or stretch, the force is caused by the action of muscles in your body. This force is known as muscular force. Animals, birds, fish, and insects also use muscular forces for movement and survival.

Humans have also used the muscular force of animals for a long time to carry out tasks.

Muscular force also plays an important role in many functions inside our body, such as chewing food and circulating blood.

Friction

Have you ever noticed a ball rolling on the ground eventually stopping? Or a bicycle slowing down when you stop pedaling? This is due to friction.

Friction is the force that comes into play when an object moves or tries to move over another surface. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move. The force of friction is a contact force because it happens due to two surfaces being in contact.

Friction arises due to the irregularities in the two surfaces in contact. Even surfaces that appear smooth have a large number of tiny irregularities. When placed in contact, the irregularities of the two surfaces lock into each other and oppose any effort to move one surface over the other. Rougher surfaces have greater friction.

Friction doesn't only act on solid surfaces. Air, water, and other liquids also exert a force of friction on objects moving through them. That's why airplanes, ships, and boats are designed with specific shapes to reduce friction from the air or water around them.

Non-contact Forces

What if you can experience a force without touching the object? These forces are called non-contact forces.

Magnetic Force

Remember learning about magnets? A magnet attracts objects made of magnetic materials. When two magnets are brought close to each other, like poles (North-North, South-South) repel each other while unlike poles (North-South) attract each other. Magnets can exert force on another magnet or a magnetic material without being in contact with it.

The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. Since a magnet can exert a force from a distance without being in contact, it is called a non-contact force.

Electrostatic Force

Have you ever rubbed a plastic scale on your hair and then used it to pick up small pieces of paper? This is due to electrostatic force.

When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges because they don't move by themselves. The object that acquires static charges is said to be a charged object. A charged object attracts, that is, exerts a force on uncharged objects made of certain materials, such as small pieces of paper. This force comes into play even when the bodies are not in contact.

There are two kinds of static charges: positive and negative. Similar (like) charges repel each other, while opposite (unlike) charges attract each other.

The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force. It is a non-contact force.

When charges move, they create an electric current. This is the same current that makes a lamp glow or generates heat.

Gravitational Force

When you throw a ball up in the air, it eventually falls back down. This is because the Earth attracts (pulls) all objects towards itself. 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 force of gravity or simply gravity.

Since the gravitational force acts without contact with the object it attracts, it is a non-contact force. Gravitational force is always an attractive force.

When an object is thrown upwards, it slows down, stops momentarily at the top, and then falls back down. This happens because of the constant pull of gravity.

Weight and Its Measurement

The force with which the Earth pulls an object towards itself is called the weight of the object. Weight measures how strongly an object is pulled by the Earth. Since weight is a force, it is measured in the same unit as that of force: newton (N).

Does the Earth pull every object with equal force? No. The Earth pulls different objects with different forces, meaning the weight of different objects is different.

A spring balance is a simple device used to measure weight (force). When you hang an object from the hook, the spring stretches, and the amount of stretching indicates the weight of the object.

The mass of an object is the amount of matter in it and is measured in grams (g) or kilograms (kg). Mass remains the same everywhere, while weight can vary depending on the gravitational force.

The gravitational force of different planets is different. This means the weight of an object is different on different planets, even though its mass remains the same.

In everyday life, we often use the term "weight" when we actually mean "mass." For example, we might say a bag of wheat "weighs" 10 kg. However, in scientific use, it's important to use the correct terms with their correct units.

Floating and Sinking

Have you ever noticed that some objects float on water, while others sink? The gravitational force of the Earth acts on all objects, so why don't they all sink?

When you try to push an empty bottle into water, you feel an upward push. This is because liquids apply a force on objects in the upward direction, known as upthrust or buoyant force.

When an object is placed in a liquid, the gravitational force pulls it downwards, while the buoyant force pushes it upwards.

• If the gravitational force is more than the buoyant force, the object sinks.
• If the gravitational force is equal to the buoyant force, the object floats.

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.

Some rocks, like Pumice, can float on water. Pumice is formed during volcanic eruptions and contains tiny bubbles of gas, making it less dense than water.

What Is a Force?

Imagine you're trying to move a heavy cardboard box. To get it moving, you need to either push it or pull it. This push or pull is what we call force in science. Force is essentially what causes objects to move, change direction, or even change shape.

What Can a Force Do to the Bodies on Which It Is Applied?

We use forces every day, often without even realizing it. Let's explore what a force can actually do:

• Make an object move from rest: A parked car stays still until the engine provides the force to move it.
• Change the speed of an object if it is moving: When you pedal harder on your bicycle, you increase the force, and your speed increases.
• Change the direction of motion of an object: Turning the steering wheel of a car applies a force that changes the car's direction.
• Bring about a change in the shape of an object: Stretching a rubber band applies a force that changes its shape.
• Cause some or all of these effects: A combination of forces can affect an object's motion in multiple ways at once.

Even if an object is at rest, it doesn't necessarily mean that no force is acting on it. It simply means that the forces acting on the object are balanced.

Are Forces an Interaction Between Two or More Objects?

Forces don't just appear out of nowhere. They always involve an interaction between at least two objects. When you push a table, your hand is one object, and the table is the other. Your hand is applying a force on the table. As soon as you stop pushing, the force disappears.

Force is defined as a push or pull on an object resulting from the object's interaction with another object. The SI unit (the standard unit used by scientists) of force is the newton, written with a small 'n' and its symbol is N.

What Are the Different Types of Forces?

Forces can be broadly classified into two types: contact forces and non-contact forces.

Contact forces

Contact forces are forces that act only when there is physical contact between objects. This contact can be direct, like using your hands, or indirect, like using a stick or rope.

Muscular force

One common type of contact force is muscular force. This is the force generated by the muscles in our body. We use muscular force for all sorts of physical activities like walking, running, lifting, pushing, jumping, and stretching. Muscular force happens when our muscles contract and elongate.

Friction

Imagine a ball rolling on the ground. Eventually, it will slow down and stop. Why? Because of friction. Friction is the force that opposes motion when two surfaces are in contact. It acts in the opposite direction to the movement of the object.

Friction arises because even surfaces that look smooth have tiny irregularities. When two surfaces are in contact, these irregularities lock together, making it harder for one surface to move over the other. Rougher surfaces have more irregularities, which means greater friction.

Friction also acts on objects moving through liquids and gases. This is why airplanes, ships, and boats are designed with specific shapes to reduce friction with the air or water.

Non-contact forces

Non-contact forces are forces that can act even when objects are not physically touching each other.

Magnetic force

Remember learning about magnets? Magnets attract objects made of magnetic materials. When two magnets are brought close, like poles (North-North or South-South) repel, while unlike poles (North-South) attract. This attraction and repulsion is a force, and it can happen even without the magnets touching.

The magnetic force is the force exerted by a magnet on another magnet or a magnetic material. Because it can act from a distance, it's a non-contact force.

Electrostatic force

Have you ever rubbed a plastic scale on your hair and then used it to pick up small pieces of paper? This works because of electrostatic force.

When you rub certain materials together, electrical charges build up on their surfaces. These charges are called static charges. An object that acquires static charges is said to be a charged object. A charged object can attract uncharged objects, like paper, even without touching them.

There are two kinds of static charges: positive and negative. Objects with similar charges repel each other, while objects with opposite charges attract each other.

The electrostatic force is the force exerted by a charged body on another charged body or an uncharged body. It is a non-contact force.

Gravitational force

If you throw a ball up in the air, it will eventually come back down. This is because of gravity. The Earth attracts all objects towards itself.

The gravitational force is the force with which the Earth attracts objects towards itself. It's also called the force of gravity or simply gravity.

Weight and Its Measurement

The weight of an object is the force with which the Earth pulls it towards itself. Weight measures how strongly an object is pulled by the Earth. Since weight is a force, it is measured in newtons (N).

The Earth pulls different objects with different forces, meaning the weight of different objects is different. We can measure weight using a spring balance.

A spring balance is a device that uses a spring to measure force. When you hang an object from the spring balance, the spring stretches. The amount of stretching tells you the weight of the object.

Mass is the amount of matter in an object and is measured in grams (g) or kilograms (kg). Mass remains the same everywhere, while weight can change depending on the gravitational force.

The gravitational force of different planets is different. This means that the weight of an object will be different on different planets, even though its mass stays the same.

Floating and Sinking

Why do some objects float in water while others sink? It's all about forces.

When you try to push an empty plastic bottle underwater, you feel an upward push. This is because liquids exert an upward force on objects placed in them. This upward force is called upthrust or buoyant force.

When an object is placed in a liquid, two forces act on it:

• Gravitational force: pulls the object downwards
• Buoyant force: pushes the object upwards

If the gravitational force is greater than the buoyant force, the object sinks. If the buoyant force is equal to the gravitational force, the object floats.