Module 110. Motion, Friction, Work, Energy and Power, Gravitation, Satellites

The study of motion, forces, and energy forms the foundation of classical physics, describing how objects move, interact, and exchange energy. These concepts explain a vast range of natural and technological phenomena, from a falling apple to orbiting satellites. Together, they constitute essential principles in mechanics — a branch of physics that governs the behaviour of matter under the influence of forces.

Motion

Motion is defined as the change in the position of an object with respect to time and a reference point. Every moving body follows a specific path, and its movement can be described in terms of distance, displacement, speed, velocity, and acceleration.

• Distance is the total path covered, whereas displacement is the shortest straight-line distance between the initial and final positions.
• Speed measures how fast an object moves (distance per unit time), while velocity includes both speed and direction.
• Acceleration refers to the rate of change of velocity with time.

The motion of objects is classified as:

1. Uniform Motion – When an object travels equal distances in equal intervals of time (constant speed).
2. Non-uniform Motion – When an object’s speed or direction changes over time.
3. Rectilinear Motion – Movement along a straight line.
4. Circular Motion – Movement around a fixed centre or axis.
5. Periodic Motion – Repeated motion at regular time intervals, such as a pendulum.

Newton’s Laws of Motion, formulated by Sir Isaac Newton, describe the relationship between motion and force:

• First Law (Law of Inertia): A body remains at rest or in uniform motion unless acted upon by an external force.
• Second Law: The rate of change of momentum is proportional to the applied force (F = ma).
• Third Law: For every action, there is an equal and opposite reaction.

These laws explain everyday movements, such as the motion of vehicles, sports actions, and celestial orbits.

Friction

Friction is the resisting force that opposes the relative motion of two surfaces in contact. It arises due to surface irregularities and intermolecular forces. Friction acts in the opposite direction of motion and can be both beneficial and undesirable.
Types of Friction:

• Static Friction: Acts when a body is at rest and resists the start of motion.
• Kinetic (Sliding) Friction: Acts when two surfaces slide over each other.
• Rolling Friction: Occurs when an object rolls over a surface, as with wheels and ball bearings.
• Fluid Friction (Drag): Resistance offered by fluids (liquids or gases) to moving objects.

Advantages of Friction:

• Enables walking, writing, and gripping objects.
• Allows brakes and tyres to function effectively.

Disadvantages:

• Causes wear and tear of machinery.
• Reduces efficiency by generating heat.

Lubrication, polishing, and the use of wheels help reduce undesirable friction, improving mechanical performance.

Work, Energy, and Power

These three quantities describe how forces cause motion and how energy is transferred or transformed.
Work: Work is said to be done when a force causes displacement in the direction of the force.
W=F×d×cos⁡θW = F \times d \times \cos \thetaW=F×d×cosθ
where F is force, d is displacement, and θ is the angle between them.If there is no displacement, no work is done.
Energy: Energy is the capacity to do work. It exists in various forms — mechanical, thermal, electrical, chemical, and nuclear energy. The two main mechanical forms are:

• Kinetic Energy (KE): Energy possessed by a body due to its motion.

  KE=12mv2KE = \frac{1}{2}mv^2KE=21​mv2

• Potential Energy (PE): Energy stored due to position or configuration, such as a raised object or stretched spring.

  PE=mghPE = mghPE=mgh

According to the Law of Conservation of Energy, energy cannot be created or destroyed; it only changes from one form to another. For example, in a pendulum, potential energy converts into kinetic energy and vice versa.
Power: Power measures the rate at which work is done or energy is transferred.
P=WtP = \frac{W}{t}P=tW​
The SI unit of power is the watt (W), where 1 watt = 1 joule per second.
Power indicates efficiency — a higher power output means more work done in less time.

Gravitation

Gravitation is the universal force of attraction between all masses in the universe. It was first quantified by Sir Isaac Newton through the Law of Universal Gravitation, which states:
F=Gm1m2r2F = G \frac{m_1 m_2}{r^2}F=Gr2m1​m2​​
where F is the gravitational force between two masses m₁ and m₂, r is the distance between them, and G is the gravitational constant (6.674 × 10⁻¹¹ N·m²/kg²).
This law explains why objects fall to Earth and why planets orbit the Sun. The acceleration due to gravity (g) on Earth’s surface is approximately 9.8 m/s².
Gravitation governs many natural phenomena:

• The motion of planets, stars, and galaxies.
• The tides caused by the Moon’s gravitational pull.
• The structure and stability of the solar system.

Newton’s theory of gravitation was later refined by Albert Einstein’s General Theory of Relativity, which described gravity as the curvature of space-time caused by mass.

Satellites

A satellite is any object that orbits around a planet due to gravitational attraction. Satellites may be natural, such as the Moon, or artificial, launched by humans for scientific, communication, or defence purposes.
1. Natural Satellites: These are celestial bodies that revolve around planets. For instance, the Moon orbits the Earth, controlling ocean tides and stabilising Earth’s axial tilt.
2. Artificial Satellites: Human-made satellites are placed into orbit for specific functions, including:

• Communication Satellites: Relay television, telephone, and internet signals (e.g., INSAT, Intelsat).
• Weather Satellites: Monitor atmospheric conditions and predict weather (e.g., METEOSAT).
• Navigation Satellites: Enable GPS and global positioning systems (e.g., NAVIC, GPS).
• Scientific Satellites: Study space, climate, and environmental data (e.g., Hubble Space Telescope).
• Military and Reconnaissance Satellites: Used for surveillance and strategic purposes.

Orbits of Satellites: Satellites follow curved paths called orbits, maintained by a balance between gravitational pull and their tangential velocity.

• Geostationary Orbit: Satellite appears stationary relative to Earth, completing one orbit every 24 hours at an altitude of about 36,000 km. Used for communication and broadcasting.
• Polar Orbit: Passes over Earth’s poles, covering the entire surface in successive revolutions, ideal for mapping and weather observation.

Escape Velocity: To launch a satellite, it must reach a minimum velocity to overcome Earth’s gravitational pull. This is known as escape velocity, given by:
ve=2grv_e = \sqrt{2gr}ve​=2gr​
For Earth, it is approximately 11.2 km/s.

Interconnection and Applications

The concepts of motion, friction, work, energy, and gravitation interrelate in both natural and engineered systems:

• Vehicles depend on balanced forces, friction, and energy conversion for movement.
• Hydroelectric power plants transform potential energy of water into electrical energy.
• Space exploration applies gravitational laws and energy principles to launch and manoeuvre satellites.
• Sports and biomechanics rely on understanding motion, friction, and energy efficiency.

These principles also underpin modern innovations such as renewable energy systems, robotics, transportation, and aerospace technology.

Significance in Modern Science

From Newton’s falling apple to advanced satellite missions, the study of mechanics has deepened our comprehension of the universe. Understanding motion, friction, energy, and gravitation not only explains physical phenomena but also drives technological progress.