PHYSICS

MASS

1. Definition of Mass:

Mass is the quantity of matter contained in a body. It is a scalar quantity and remains constant everywhere. Its SI unit is kilogram (kg).

2. Inertia:

Inertia is the property of a body due to which it resists any change in its state of rest or uniform motion in a straight line. Mass is a measure of inertia. A body having greater mass has greater inertia, while a body having smaller mass has less inertia.

Example: It is easier to push an empty trolley than a loaded trolley because the loaded trolley has greater mass and greater inertia.

3. Measurement of Mass:

Mass is measured by using a beam balance or an electronic balance.

• Beam Balance: It consists of a horizontal beam with two pans at its ends. The object is placed in one pan, while standard masses are placed in the other pan. When both pans balance, the objectis mass equals the total standard mass.

• Electronic Balance: It is a modern instrument used to measure mass. The object is placed on its platform, and its mass is displayed on the digital screen.

The SI unit of mass is kilogram (kg), while gram (g) is also commonly used.

1 kg = 1000 g

FORCE AND TYPES OF FORCES

1. Definition of Force:

Force is a push or pull that can change the state of rest or motion, direction, or shape of an object. Its SI unit is Newton (N).

2. Types of Forces:

Forces are mainly divided into two types:

A. Contact Forces:

Contact forces are those forces that act when two objects are in physical contact with each other.

Examples:

1. Frictional Force: It opposes the motion of an object when two surfaces are in contact. Example: A box sliding on the floor slows down due to friction.

2. Muscular Force: It is the force produced by the muscles of our body. Example: Lifting a school bag.

3. Tension Force: It is the pulling force produced in a stretched rope or string. Example: Pulling a bucket with a rope.

4. Normal Force: It is the supporting force exerted by a surface on an object in contact with it. Example: A table supporting a book.

B. Non-Contact Forces:

Non-contact forces are those forces that act between objects without physical contact.

Examples:

1. Gravitational Force: It is the force of attraction between objects having mass. Example: An object falling towards the Earth.

2. Magnetic Force: It is the force exerted by a magnet on magnetic materials. Example: A magnet attracting an iron nail.

3. Electrostatic Force: It is the force of attraction or repulsion between electrically charged objects. Example: A charged comb attracting small pieces of paper.

WEIGHT

1. Definition of Weight:

Weight is the gravitational force with which the Earth attracts a body towards its centre. It is represented by W and its SI unit is Newton (N).

The formula of weight is:

\[ W = mg \]

Where:

• W = Weight

• m = Mass

• g = Acceleration due to gravity (9.8 m/s²)

2. Comparison of Mass and Weight:

Mass and weight are different from each other in the following ways:

3. Measurement of Weight:

Weight is measured by using an instrument called a spring balance.

• A spring balance consists of a spring attached to a graduated scale.

• The object is hung from the hook of the spring balance.

• The weight of the object stretches the spring.

• The pointer moves along the scale and shows the reading.

• The reading gives the weight of the object in Newtons (N).

Thus, a spring balance is used to measure the weight of objects.

Weight completed

Important formulas to remember:

\[ W = mg \]

\[ g = 9.8 \, m/s^2 \]

GRAVITY OR GRAVITATIONAL FIELD

1. Gravity:

Gravity is the natural force of attraction between any two objects having mass. The Earth attracts all objects towards its centre due to gravity. It keeps us on the ground and causes objects to fall when dropped.

2. Gravitational Field:

The region around a mass in which another mass experiences gravitational force is called a gravitational field.

For example, the Earth has a gravitational field around it. Any object placed in this field experiences a gravitational force towards the Earth.

3. Gravitational Field Strength:

Gravitational field strength is defined as the gravitational force acting on a unit mass at a particular point.

Its formula is:

\[ g = \frac{F}{m} \]

Where:

• g = Gravitational field strength

• F = Gravitational force

• m = Mass

4. SI Unit:

The SI unit of gravitational field strength is Newton per kilogram (N/kg).

Its approximate value near the Earth's surface is:

\[ g = 9.8 \, N/kg \]

5. Examples of Gravity:

• Objects fall towards the Earth due to gravity.

• Gravity keeps us on the Earth's surface.

• The Earth's gravity helps keep the Moon in its orbit.

FUNDAMENTAL FORCES OF NATURE

There are four fundamental forces in nature. These forces are responsible for all basic interactions between objects and particles in the universe.

1. Gravitational Force:

It is the force of attraction between two objects having mass. It is the weakest fundamental force but has an unlimited range.

Example: The Earth attracts objects towards its centre due to gravity.

2. Electromagnetic Force:

It is the force that acts between electrically charged particles. It can be attractive or repulsive.

Example: A magnet attracts iron objects, and electric charges attract or repel each other.

3. Strong Nuclear Force:

It is the strongest fundamental force. It holds quarks together inside protons and neutrons. Its residual effect also holds protons and neutrons together inside the atomic nucleus.

Example: It keeps the atomic nucleus stable.

4. Weak Nuclear Force:

It is a fundamental force that acts between subatomic particles and is responsible for certain radioactive processes.

Example: It is responsible for beta decay in radioactive substances.

Conclusion:

These four fundamental forces play an important role in the structure and working of the universe.

Topic completed

Quick memory trick

Newton's Laws of Motion

Newton gave three laws of motion that explain the relationship between force and the motion of objects.

1. Newton's First Law of Motion (Law of Inertia)

Newton's first law states that an object remains at rest or continues moving with uniform velocity in a straight line unless acted upon by an external unbalanced force.

This law is also called the Law of Inertia because it describes the tendency of objects to resist a change in their state of rest or motion.

Example: When a moving bus suddenly stops, passengers move forward due to inertia of motion.

2. Newton's Second Law of Motion (Law of Acceleration)

Newton's second law states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass.

Its mathematical formula is:

\[ F = ma \]

Where:

• F = Force

• m = Mass

• a = Acceleration

The SI unit of force is Newton (N).

Example: A football kicked with greater force gains greater acceleration. Similarly, an empty trolley accelerates more than a loaded trolley when the same force is applied.

3. Newton's Third Law of Motion (Law of Action and Reaction)

Newton's third law states that for every action, there is an equal and opposite reaction.

It means that when one object applies a force on another object, the second object applies an equal force in the opposite direction on the first object.

Example: When we walk, our feet push the ground backwards, and the ground pushes us forward. This reaction helps us move forward.

Another example is a rocket, which moves upward by pushing gases downward.

TOPIC COMPLETED ✓

Remember the three laws:

1. First law – Inertia (resists change).

2. Second law – Acceleration (\(F=ma\)).

3. Third law – Action and Reaction (equal and opposite).