Notes Chapter 5 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

Exploring Forces

Introduction – Daily Life and Force

We experience forces everywhere in life:

• Pedalling uphill is harder because a force opposes motion.
• It is easier to slip on smooth/wet surfaces due to less friction.
• At the highest point of a swing, we feel light because forces are acting on us.

Force helps in activities like cycling, playing, lifting, pushing, and even natural events like falling objects.

5.1 What is a Force?

• Force = a push or a pull applied on an object.
• Example: pushing, pulling, lifting a box.
• Unit of force: newton (N).

5.2 What Can Force Do?

Force can:

1. Move a stationary object.
2. Change the speed of a moving object.
3. Change the direction of motion.
4. Change the shape of an object.
5. Cause one or more of these effects.

If speed, direction, or shape changes → force is acting. If an object is at rest, it doesn’t mean no force acts; sometimes forces are balanced.

5.3 Forces as Interactions

• A force occurs only when two objects interact.
• Example: pushing a table (hand + table).
• Both objects experience force during interaction.
• When interaction stops → force disappears.

5.4 Types of Forces

5.4.1 Contact Forces

Forces that require physical contact.

1. Muscular Force

• Force produced by muscles.
• Used in walking, running, pushing, lifting, chewing, heartbeat, digestion, etc.
• Animals also use muscular force.

2.Frictional Force

• Definition: Force that opposes motion between two surfaces in contact.
• Always acts in opposite direction of motion.
• Caused due to irregularities of surfaces.
• Greater on rough surfaces, smaller on smooth ones.
• Exists in solids, liquids, and gases (air resistance, drag in water).
• Importance: Helps us walk, write, and stop moving objects, but also causes wear and tear.

5.4.2 Non-contact Forces

Forces that act without contact.

1. Magnetic Force

• Magnet attracts iron and other magnetic materials.
• Like poles repel, unlike poles attract.
• Acts from a distance.

2.Electrostatic Force

• Produced when certain objects are rubbed and get charged.
• Charged objects attract or repel other objects.
• Like charges repel, unlike charges attract.
• Example: rubbed balloon attracting hair/paper bits.
• Charges are of two kinds: positive (+) and negative (−).

3.Gravitational Force

• Earth pulls all objects towards itself.
• This pull is called gravity.
• Always an attractive force.
• Responsible for falling objects and vertical motion (upwards and downwards).

5.5 Weight and Its Measurement

• Weight: The force with which Earth pulls an object.
• Unit: newton (N).
• Mass: Amount of matter in an object, measured in g or kg.

Difference between Mass and Weight

• Mass remains same everywhere.
• Weight changes from place to place (less on Moon, more on Jupiter).

Measuring Weight

• Spring Balance: A device with a spring and scale.
• Measures weight in newton (N).
• Smallest reading depends on divisions of scale.

Example of Weight on Different Planets (for 1 kg mass):

• Earth: 10 N
• Moon: 1.6 N
• Mars: 3.8 N
• Venus: 9 N
• Jupiter: 25.4 N

In daily life, we often say “10 kg weight” (wrong scientifically). Correct is: “Mass = 10 kg, Weight = 100 N on Earth.”

5.6 Floating and Sinking

• Liquids exert an upward force on objects = upthrust or buoyant force.
• If gravitational force > buoyant force → object sinks.
• If gravitational force = buoyant force → object floats.

Archimedes’ Principle

• An object immersed in liquid experiences upward force equal to the weight of displaced liquid.
• If displaced liquid weight < object weight → sinks.
• If displaced liquid weight = object weight → floats.

Example: Empty bottle floats, stone sinks. Some rocks like Pumice float on water because they have air pockets (low density).

Snapshots (Key Points)

• Force = push or pull due to interaction.
• SI unit = newton (N).
• Types: Contact (muscular, friction) and Non-contact (magnetic, electrostatic, gravitational).
• Force can change speed, direction, shape.
• Weight = gravitational pull by Earth.
• Mass remains constant, weight varies.
• Buoyant force explains floating and sinking