Class 8 Science – Force and Pressure

1. Introduction

• Force is a push or pull on an object that can change its state of motion or shape.

• Forces are everywhere – from pushing a door to pulling a rope.

• Pressure is the effect of force applied over an area.

• Both concepts explain how objects move, float, or sink, and how they experience stress.

Key Concept:

• Force changes motion, direction, or shape of objects; pressure explains how that force is distributed on surfaces.

2. Effects of Force

1. Change in Motion

   • A stationary object can start moving, and a moving object can stop or change speed.

2. Change in Direction

   • Force can alter the direction of a moving object.

3. Change in Shape

   • Stretching, compressing, bending, or twisting occurs when force is applied.

4. Rotation

   • Force applied off-center can cause rotation or torque.

Everyday Examples:

• Pushing a swing → motion

• Kicking a ball → motion and direction

• Hammering a nail → shape change

• Opening a door → rotation

3. Pressure

• Pressure is the force acting on a unit area of a surface.

• The same force produces more pressure if applied on a smaller area.

• Pressure depends on force and area of contact.

Everyday Examples

1. High Pressure (small area)

   • Knife edge → cuts easily

   • Needle → pierces cloth

2. Low Pressure (large area)

   • Snowshoes → walk on snow without sinking

   • Wide tires → less damage to roads

Key Concept:

• Reducing area increases pressure, increasing effect of the same force.

4. Atmospheric Pressure

• The weight of the air above the Earth exerts pressure on everything on Earth.

• Atmospheric pressure is felt in daily life, even though air is invisible.

Examples and Effects

1. Drinking through a straw → sucking creates lower pressure inside the straw.

2. Vacuum-packed food → removed air → external atmospheric pressure keeps food packed.

3. Suction cups → attach to smooth surfaces because of atmospheric pressure.

4. Weather phenomena → wind and storms result from differences in air pressure.

Key Concept:

• Atmospheric pressure acts in all directions, not just downwards.

5. Key Points to Remember

• Force is a push or pull → changes motion, shape, direction, or causes rotation.

• Pressure is force per unit area → smaller area → higher pressure; larger area → lower pressure.

• Atmospheric pressure is the pressure exerted by the weight of air.

• Everyday life and nature demonstrate force and pressure in action, from cutting with knives to weather patterns.

• Understanding these concepts helps explain floating, sinking, and movement of objects.