Chapter 5- Exploring Forces

1. Introduction: The Invisible Movers

Have you ever wondered why a bicycle slows down if you stop pedaling? Or why it is harder to pedal uphill than on a flat road? In our daily lives, we see things moving, stopping, or changing direction constantly. But these changes don’t happen by magic. There is always a cause behind every motion.

In science, we call this cause a Force. Whether you are kicking a football, opening a door, lifting your school bag, or even just sitting on a chair, forces are acting all around you. In this chapter, we will go on a journey to explore what forces are, how they behave, and why they are essential for our existence.

Think of the last time you went to a playground. When you sit on a swing and someone pushes you, you move forward. When you catch a ball, you stop it. In every such action, you are either applying a push or a pull. This simple concept is the foundation of physics in Class 8.

2. What Exactly is a Force?

If we try to define force in the simplest language, we say: A force is a push or a pull acting on an object.

However, as science students, we need to look deeper. A force is not just a push or pull; it is an interaction. A force cannot exist in isolation. You cannot “push” nothing. You need something to push against. Similarly, you cannot “pull” unless there is an object to be pulled.

The Concept of Interaction

This is a very important rule in science: Forces result from an interaction between at least two objects.

Let’s break this down with an example from your textbook. Imagine a book lying on a table. If you want to move it, your hand (Object 1) must interact with the book (Object 2). If you just wave your hand in the air without touching the book (and without using magnets!), the book won’t move. The moment your hand touches the book and applies a push, the force comes into play.

Teacher’s Tip: Always remember that force is a two-way street. When you push the wall, the wall pushes you back! This interaction is what makes forces exist.

• SI Unit of Force: In science, we measure everything. Just as we measure mass in kilograms (kg), we measure force in a unit called Newton. It is denoted by the capital letter N.

3. What Can a Force Do? (Effects of Force)

We cannot see “force” itself. You cannot look at the air and say, “Oh look, there goes a force!” However, we can clearly see the effects of force. We know a force is present because of what it does to objects. There are five main effects you need to remember:

A. Force Can Move a Stationary Object

Imagine a football placed at the center of the field. It is at rest (speed is zero). It will remain there forever unless a player kicks it. The kick is the force that changes its state from rest to motion. Similarly, pushing a stalled car makes it move.

B. Force Can Stop a Moving Object

Think of a goalkeeper in a soccer match. When the ball comes flying towards the goal, the goalkeeper applies force with his hands to stop the ball. Here, force is used to bring a moving object to rest.

C. Force Can Change the Speed of an Object

This is very common in cycling. If you are already moving and your friend pushes you from behind, your speed increases. However, if your friend pulls you from behind, your speed decreases. The brake on your bicycle is also a mechanism to apply a force (friction) that reduces speed.

D. Force Can Change the Direction of Motion

In a game of cricket, the bowler throws the ball towards the batsman. The batsman hits the ball with the bat. By doing so, he applies a force that changes the direction of the ball, sending it to the boundary. The ball was coming towards him, but after the force was applied, it went in a completely different direction.

E. Force Can Change the Shape of an Object

Not all forces result in motion. Sometimes, force changes the physical structure.

• Squeezing a toothpaste tube.

• Stretching a rubber band.

• Kneading dough (atta) in the kitchen.

In all these cases, the object may not move from one place to another, but its shape changes due to the applied force.

Figure-1: Forces in action: A batsman changes the direction of the ball, while a hand changes the shape of a stress ball.

4. Classification of Forces

Forces are broadly classified into two categories based on whether the interacting objects are touching each other or not.

Category 1: Contact Forces

These forces act only when there is physical contact between the two interacting objects.

(i) Muscular Force

This is the most biological force! It is the force exerted by the muscles of the body. When you lift a bucket of water, your biceps contract to pull it up. When you kick a ball, your leg muscles exert force.

Did you know? Muscular force is not just outside our body. Inside us, the heart muscles pump blood, and the food pipe pushes food down to the stomach using muscular force. Animals like bullocks, horses, and camels use muscular force to perform heavy tasks for us.

(ii) Frictional Force (Friction)

Have you ever slid across a polished floor in your socks? It’s fun because you slide easily. Now try doing that on a rough concrete road. You can’t. Why?

This is due to Friction. Friction is a force that arises when two surfaces slide (or try to slide) against each other.

• Direction: It always acts in the opposite direction to the motion. If you move right, friction pushes left.

• Cause: It happens because of the interlocking of irregularities (tiny bumps) on the two surfaces. Even smooth-looking surfaces have tiny bumps when seen under a microscope.

• Effect: It slows down moving objects and eventually stops them. Without friction, a rolling ball would never stop!

Figure-2: Friction is caused by the interlocking of tiny irregularities on surfaces. Rough surfaces have stronger interlocking.

5. Category 2: Non-Contact Forces

These are “magical” forces because they can act from a distance without any physical touching.

(i) Magnetic Force

You must have played with magnets. If you bring a magnet near iron nails, the nails jump towards the magnet. The magnet didn’t touch them, yet it pulled them. This is magnetic force.

Experiment at Home: Take two bar magnets. Bring the North Pole of one near the North Pole of the other. They will push each other away (Repulsion). Bring North near South, and they will snap together (Attraction). This pushing and pulling without touching is a classic example of non-contact force.

(ii) Electrostatic Force

This is the force related to electric charges.

Try this: Rub a plastic comb or a drinking straw on your dry hair or a woollen cloth. Now bring it near tiny pieces of paper. The paper pieces will “dance” and jump up to stick to the comb.

Why? Rubbing creates a static electric charge on the comb. This charged object exerts a force on the uncharged paper pieces. Like magnetic force, this acts from a distance.

• Like charges repel each other.

• Unlike charges attract each other.

(iii) Gravitational Force (Gravity)

This is the force that holds the universe together. Sir Isaac Newton famously realized this when he saw an apple fall. Why did it fall down? Why not up? Why not sideways?

The answer is that the Earth pulls everything towards its center. This pull is called the Force of Gravity.

• It is an attractive force only (it never repels).

• It acts on all objects: stones, water, air, you, and me.

• It is not just Earth; every object in the universe attracts every other object. But Earth is so massive that its pull is the most obvious one we feel.

Figure-3: Gravity is the invisible leash that pulls falling apples to the ground and keeps the Moon orbiting the Earth.

6. Mass and Weight: Are They the Same?

In daily language, we use “mass” and “weight” as if they are the same thing. But in science, they are very different.

• Mass: This is the quantity of matter inside an object. If you are made of bones, flesh, and blood, your mass is the total amount of that “stuff”. Mass is constant. Whether you are on Earth, on the Moon, or floating in deep space, your body composition doesn’t change, so your mass stays the same. It is measured in Kilograms (kg).
• Weight: This is actually a force. Weight is the measure of how strongly gravity is pulling you down.
  
  Formula: Weight = Mass × Gravity
  
  Since gravity is different on different planets, your weight changes!
  
  • On Earth, gravity is strong, so you weigh more.
  
  • On the Moon, gravity is 1/6th of Earth’s, so you would weigh 6 times less!
  
  • Since weight is a force, it is measured in Newtons (N), not kg.

Figure-4: A spring balance measures the force of gravity acting on an object. The stretching of the spring indicates the weight.

7. Floating and Sinking: The Battle of Forces

When you drop a stone in water, it sinks. But when you drop a plastic bottle, it floats. Why?

This happens because of a hidden force in fluids (liquids and gases). When an object enters water, two forces fight against each other:

1. Gravitational Force (Weight): This pulls the object downwards.
2. Buoyant Force (Upthrust): This is the force exerted by the water pushing the object upwards.

The Rule of Floating:

• If the Upward Force (Buoyancy) is greater than or equal to the Downward Force (Weight), the object floats.

• If the Downward Force (Weight) is stronger than the Upward Force, the object sinks.

Have you ever tried pushing an empty plastic mug upside down into a bucket of water? It feels like there is a spring pushing it back up. That “push back” is the Buoyant Force. This is the same reason why heavy ships made of steel can float—their shape is designed to maximize this upward buoyant force (based on Archimedes’ Principle).

8. Summary: Key Takeaways

• Force is a push or pull arising from the interaction of objects.
• Forces can change speed, direction, shape, or state of rest.
• Contact forces (Muscular, Friction) need touching; Non-contact forces (Magnetic, Electrostatic, Gravity) act from a distance.
• Friction always opposes motion.
• Mass is the amount of matter (constant); Weight is the force of gravity (variable).
• Objects float if the Buoyant Force balances their weight.

Practice Set (CBSE Pattern)

A. Very Short Answer Questions (1 Mark)

Q1: What is the SI unit of Force?

Answer: The SI unit of force is the Newton (N).

Q2: Name the force responsible for wearing out bicycle tires.

Answer: Frictional force (Friction).

Q3: Does a magnet need to touch an iron nail to attract it?

Answer: No, magnetic force is a non-contact force.

B. Short Answer Questions (2-3 Marks)

Q1: Why do we say force is an “interaction”? Explain with an example.

Answer: Force cannot exist with a single object alone. For a force to act, one object must apply it and another must receive it. For example, to kick a ball, the foot must interact with the ball. Without the ball, the foot cannot apply a “kicking force” on nothing.

Q2: Explain why a plastic comb rubbed on dry hair attracts small pieces of paper.

Answer: Rubbing the comb on dry hair generates a static electric charge on the comb. This charged comb exerts an electrostatic force on the uncharged paper pieces, attracting them even from a distance.

Q3: Differentiate between Mass and Weight.

Answer:

• Mass: It is the amount of matter in an object. It is measured in kg and remains constant everywhere.

• Weight: It is the gravitational force pulling the object. It is measured in Newtons and changes depending on the gravity of the place (e.g., Earth vs Moon).

Q4: What is Buoyant Force? When does an object sink?

Answer: Buoyant force is the upward force exerted by a liquid on an object immersed in it. An object sinks when its weight (downward force) is greater than the buoyant force (upward force).

C. Long Answer Questions (5 Marks)

Q1: Describe the various effects of force with one daily life example for each.

Answer:

1. Start Motion: Pushing a stationary trolley makes it move.

2. Stop Motion: A fielder catching a cricket ball stops its motion.

3. Change Speed: Pressing the accelerator in a car increases its speed.

4. Change Direction: A batsman hitting a ball changes its path towards the boundary.

5. Change Shape: Squeezing a lemon changes its shape to extract juice.

Q2: Friction is often called a “necessary evil”. Explain why it is important but also why it can be a problem.

Answer:

Why it is necessary (Good): Friction allows us to walk without slipping. It helps cars stop when brakes are applied. It allows us to write on paper (the pen tip needs friction to leave ink).

Why it is a problem (Bad): Friction causes wear and tear in machine parts, shoe soles, and tires. It also produces unwanted heat (like rubbing hands) and reduces the efficiency of machines by wasting energy.

D. Case-Based Question (4 Marks)

Case: Ravi is conducting an experiment. He hangs a stone on a Spring Balance, and it reads 5N. Then, he slowly lowers the stone into a beaker of water without touching the bottom. He notices the reading on the Spring Balance changes.

Q1: Will the reading increase or decrease when the stone is in water?

Answer: The reading will decrease.

Q2: Which force is responsible for this change in reading?

Answer: The Buoyant Force (or Upthrust) exerted by the water pushes the stone up, reducing its apparent weight.

E. Assertion–Reason Question

Assertion (A): An apple falls towards the Earth, but the Earth does not move towards the apple.

Reason (R): Gravitational force acts only on small objects.

Answer: (c) Assertion is True but Reason is False.

Explanation: Gravity acts on BOTH. The Earth pulls the apple, and the apple pulls the Earth with equal force. However, because the Earth’s mass is huge, the force doesn’t move it noticeably, while the small apple moves easily.