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Class 8 Physics – Chapter 1: Force and Pressure
Class 8 Physics — Chapter 1: Force and Pressure
Introduction
In daily life we perform many actions like pushing a door, pulling a bucket, lifting a bag, squeezing a sponge or kicking a ball. All these actions involve a force. Force is responsible for changing the motion, direction, speed, shape and size of objects.
Force
Definition: A force is a push or a pull applied on an object.
Effects of Force
- Change in shape or size
- Change in speed
- Change in direction of motion
- Start or stop motion
Types of Forces
1. Contact Forces
- Muscular Force: Force exerted by muscles of humans or animals.
- Frictional Force: Force that opposes motion between two surfaces in contact.
2. Non-Contact Forces
- Gravitational Force: Force of attraction between Earth and objects.
- Magnetic Force: Force between magnets.
- Electrostatic Force: Force between electrically charged objects.
Net Force
When more than one force acts on an object:
- If forces act in the same direction, they add up.
- If forces act in opposite directions, the difference decides the motion.
- If net force is zero, the object remains at rest or moves with uniform speed.
Pressure
Definition: Pressure is the force acting on a unit area.
Formula: Pressure = Force / Area
- Smaller area → greater pressure
- Larger area → smaller pressure
This explains why sharp knives cut better, camels have broad feet, and school bags have broad straps.
Pressure in Liquids
Liquids exert pressure on the walls and bottom of the container. Pressure in liquids increases with depth. That is why dams are built thicker at the bottom and divers feel more pressure deep in water.
Atmospheric Pressure
Air exerts pressure on all objects. Atmospheric pressure helps in drinking with a straw, affects weather conditions, and decreases with altitude.
Class 8 Physics — Chapter 2: Friction
Introduction
Friction is a force that opposes the motion of an object when it moves over another surface. It plays an important role in our daily life by enabling us to walk, write, grip objects and apply brakes.
What is Friction?
Friction is the force that resists the relative motion between two surfaces in contact.
Factors Affecting Friction
- Nature of the surfaces in contact
- Roughness of the surface
- Weight of the object
Types of Friction
- Static Friction: Friction acting on an object at rest.
- Sliding Friction: Friction when an object slides over another surface.
- Rolling Friction: Friction when an object rolls over a surface.
- Fluid Friction: Friction offered by liquids and gases.
Advantages of Friction
- Helps us walk without slipping
- Makes writing possible
- Helps vehicles move on roads
- Allows us to hold objects
Disadvantages of Friction
- Causes wear and tear of machines
- Produces heat
- Reduces efficiency of machines
Methods of Reducing Friction
- Using lubricants like oil and grease
- Polishing surfaces
- Using ball bearings
- Streamlining of vehicles
Class 8 Physics — Chapter 3: Sound
Introduction
Sound is a form of energy that produces the sensation of hearing. It is produced by vibrating objects and travels through a medium such as air, water or solids.
Production of Sound
Sound is produced when an object vibrates. The vibrating object makes the surrounding medium vibrate, creating sound waves.
Propagation of Sound
Sound needs a medium to travel. It cannot travel in vacuum. Sound travels in the form of longitudinal waves consisting of compressions and rarefactions.
Characteristics of Sound
- Loudness: Depends on amplitude of vibration.
- Pitch: Depends on frequency of vibration.
- Quality: Helps distinguish different sounds of same pitch and loudness.
Audible and Inaudible Sounds
The human ear can hear sounds between 20 Hz and 20,000 Hz. Sounds below 20 Hz are infrasonic and above 20,000 Hz are ultrasonic.
Noise and Music
Noise is unpleasant sound while music is pleasant and harmonious sound. Noise pollution can cause headache, stress and hearing problems.
Speed of Sound
Speed of sound is fastest in solids, slower in liquids and slowest in gases.
Class 8 Physics — Chapter 4: Light
Introduction
Light is a form of energy that enables us to see objects. It travels in a straight line and can be reflected or refracted.
Reflection of Light
When light strikes a surface and returns back, it is called reflection.
Laws of Reflection
- Angle of incidence = Angle of reflection
- Incident ray, reflected ray and normal lie in the same plane
Regular and Irregular Reflection
Smooth surfaces produce regular reflection while rough surfaces produce irregular reflection.
Image Formation by Plane Mirror
- Image is virtual and erect
- Image is of same size as object
- Distance of image = distance of object from mirror
- Image is laterally inverted
Multiple Reflection
Multiple reflection is used in periscope and kaleidoscope.
Dispersion of Light
White light splits into seven colours when passed through a prism. This is called dispersion.
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