1. Introduction to Force and Pressure

In our daily life, we perform many activities like pushing a door, pulling a bucket of water from a well, lifting a school bag, kicking a football, hammering a nail, squeezing a toothpaste tube, and so on. All these actions involve push or pull. In physics, these pushes and pulls are described using the term force.

Pressure is another important concept that you observe in daily life. Why do sharp knives cut better than blunt ones? Why do camels have broad feet? Why is it easy to cut vegetables with a sharp blade? All these are related to the idea of pressure, which tells us how much force is applied on a given area.

In this chapter, we will learn:

• What is force and what can it do?
• Different types of forces (contact and non-contact forces).
• What is pressure and how it depends on force and area.
• Pressure in solids, liquids, and gases (atmospheric pressure).

2. Understanding Force – Push or Pull

2.1 Definition of Force

A force is a push or pull acting on an object. It is an interaction that can change the state of rest or motion of an object.

Key Point: We cannot see force, but we can observe its effects on objects.

Whenever you:

• Push a trolley, you apply force on it.
• Pull a drawer, you apply force on it.
• Lift your school bag, you apply force against gravity.

2.2 When Does a Force Come into Play?

A force always arises due to the interaction between two objects. For example:

• When you kick a football, your foot and the ball interact.
• When you pull a cart, your hand and the handle of the cart interact.
• When a magnet attracts an iron nail, magnet and nail interact.

So, we need at least two objects for a force to act – the object applying the force and the object on which the force is applied.

3. Effects of Force on Objects

The same force can produce different effects on different objects. The main effects of force are:

• Force can change the state of motion of an object.
• Force can change the speed of an object.
• Force can change the direction of motion.
• Force can change the shape or size of an object.

3.1 Changing the State of Motion

An object which is at rest can be set into motion by applying a force. Similarly, a moving object can be brought to rest by applying a force in the opposite direction.

3.2 Changing Speed and Direction

If a force is applied in the direction of motion, it may increase the speed of the object. If applied opposite to the direction of motion, it may decrease its speed. A force applied at an angle can change the direction of motion.

3.3 Changing Shape and Size

Forces can deform objects – they can change the shape or size of objects.

• Stretching a rubber band makes it longer.
• Squeezing a sponge changes its shape.
• Hammering a piece of metal can change its shape.

4. Types of Forces (Contact and Non-contact)

Based on whether direct contact between objects is required or not, forces are broadly grouped into:

• Contact forces
• Non-contact forces

4.1 Contact Forces

A contact force is a force that comes into play only when two objects are in physical contact.

4.1.1 Muscular Force

The force exerted by the muscles of our body is called muscular force. It is used in walking, running, lifting, pushing, and pulling.

4.1.2 Frictional Force

When two surfaces are in contact and one surface moves or tends to move over the other, a force called friction acts between them. It always acts in a direction opposite to the motion.

Friction helps us in many ways (walking, holding objects, stopping vehicles), but it also opposes motion and causes wear and tear of machine parts.

4.2 Non-contact Forces

A non-contact force is a force that can act between two objects even when they are not in physical contact.

4.2.1 Magnetic Force

The force exerted by a magnet on another magnet or magnetic material (like iron, nickel, cobalt) is called magnetic force. It can act from a distance.

4.2.2 Electrostatic Force

When objects are rubbed, they may get electrically charged and then exert attractive or repulsive forces on other charged or neutral objects. This is called electrostatic force.

4.2.3 Gravitational Force

The force with which the Earth attracts all objects towards its centre is called gravitational force. It is also a non-contact force.

Example: When you throw a ball up, it comes back down due to Earth’s gravitational pull.

5. Introduction to Pressure

We know that a force can act on an object. But sometimes, the effect of a force does not depend only on how large the force is, but also on how spread out or concentrated it is.

Pressure is a physical quantity that tells us how much force is applied on one unit area.

So, pressure increases when:

• Force increases (for the same area), or
• Area decreases (for the same force).

6. Pressure Exerted by Solids – Force per Unit Area

6.1 Effect of Area on Pressure

If you stand on loose sand, your feet sink into it. But if you lie down on the sand, you do not sink as much. When you lie down, your weight (force) is spread over a larger area, thus pressure is less.

This explains why:

• Porters place a cloth pad on their heads to increase area and reduce pressure.
• Cutting tools (knives, nails, pins) have sharp tips with very small area to increase pressure.

7. Pressure in Fluids (Liquids and Gases)

Substances that can flow are called fluids. Both liquids and gases are fluids. Fluids also exert pressure on the walls and bottom of the container and on objects immersed in them.

7.1 Pressure in Liquids

Liquids exert pressure in all directions. The pressure in a liquid:

• Increases with depth (more depth → more liquid above → greater pressure).
• Acts in all directions on the walls and bottom of the container.

7.2 Pressure in Gases

Air is a mixture of gases. Gas molecules are constantly moving and colliding with the walls of their container. These collisions produce gas pressure.

Examples of gas pressure in daily life:

• Air pressure in bicycle and car tyres.
• Inflated footballs and balloons.
• Air pressure used in spray bottles.

8. Atmospheric Pressure

The layer of air surrounding the Earth is called the atmosphere. The air in the atmosphere has weight and exerts pressure on the surface of the Earth and on all objects. This is known as atmospheric pressure.

8.1 Applications of Atmospheric Pressure (Qualitative)

Atmospheric pressure is used in many practical devices:

• Drinking straw: When you suck air out of the straw, the air pressure inside the straw becomes less. The greater atmospheric pressure on the liquid surface pushes the liquid up into the straw.
• Syringe: Pulling the plunger creates low pressure inside the syringe. Atmospheric pressure forces the liquid (medicine) inside.
• Suction pads/toy darts: When pressed on a smooth surface, air is pushed out and low pressure is created inside the cup. Atmospheric pressure outside keeps it pressed against the surface.

9. Important NCERT Points and Exam Tips

9.1 Key Definitions to Remember

• Force: A push or pull acting on an object arising due to interaction of objects.
• Contact force: Force that acts only when objects are in physical contact.
• Non-contact force: Force that can act even without contact (magnetic, electrostatic, gravitational).
• Pressure: Force acting per unit area of a surface.
• Atmospheric pressure: The pressure exerted by the air column of the atmosphere on the Earth’s surface.

9.2 Concept Checklist for Exams

• Be able to give examples of force as push and pull.
• Explain at least 3–4 effects of force with examples.
• Differentiate clearly between contact and non-contact forces.
• Give daily life examples of friction being useful and also causing problems.
• Use the formula Pressure = Force / Area in simple numerical problems (conceptual understanding only).
• Explain why pressure is higher at greater depths in liquids.
• Describe atmospheric pressure and its role in working of straw, syringes, and suction cups.

9.3 Exam Writing Tips

• Always define the term in one clear sentence, then give one example.
• Draw simple labelled diagrams wherever possible (e.g., force acting on an object, pressure in liquids, straw working).
• Use proper scientific terms like “interaction”, “contact”, “non-contact”, “atmospheric pressure”.
• For 3 or 4-mark questions, write the answer in points instead of a single paragraph.

Remember: Regular revision of these notes and solving NCERT questions will help you master Chapter 11: Force and Pressure and score well in your CBSE Class 8 Science exams.