Hello, and welcome to today's physics lesson. We are going to explore one of the most fundamental chapters in physics — Laws of Motion. This is where we understand why objects move, why they stop, and what governs everything from a falling apple to a rocket launching into space.
Let us begin with something simple — what exactly is a force? A force is that physical cause which changes, or tends to change, either the size or shape of a body, or its state of rest or motion. Think about pushing a book across a table — it moves. Squeeze a rubber ball — its shape changes. These are both effects of force.
Forces can be classified into two broad categories based on how they act. First, contact forces — these require physical touch between two bodies. Friction is a perfect example. When you slide a book across a table, friction acts opposite to the direction of motion, trying to slow it down. Then there is the normal reaction force — when you place a book on a table, the table pushes back up with a force equal to the chapter's weight, preventing it from falling through. Tension in strings and springs also fall under contact forces.
Second, non-contact forces — these act even without physical touch. Gravitational force is the most familiar. The Earth pulls you downward even though you are not touching its center. Similarly, electric and magnetic forces can attract or repel objects across empty space. A key property of non-contact forces is that their strength decreases with distance — specifically, they follow an inverse square law. Double the distance, and the force becomes one-fourth as strong.
Now we come to the heart of this chapter — Newton's three laws of motion. Let us start with the first law.
Newton's first law of motion states: if a body is in a state of rest, it will remain in the state of rest, and if it is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it. This law introduces us to the concept of inertia. Inertia is the property of an object by virtue of which it tends to retain its state of rest or of motion. The greater the mass of a body, the greater its inertia.
Imagine standing in a bus when it suddenly starts moving. Your feet move with the bus, but your upper body tends to stay behind — you fall backward. This is inertia of rest. Similarly, when a moving bus stops suddenly, you lurch forward — this is inertia of motion. Mass is the measure of inertia. A cricket ball has more inertia than a tennis ball because it has more mass.
Moving to Newton's second law — this is where we get quantitative. The law states: the rate of change of momentum of a body is directly proportional to the force applied on it, and the change in momentum takes place in the direction in which the force is applied.
Let us break this down. Linear momentum, denoted by p, is defined as the product of mass and velocity, so it is a vector quantity having the direction of motion of the body. So, momentum equals mass multiplied by velocity, or p = mv. The S.I. unit of momentum is kg m s⁻¹.
When a force acts on a body, it changes the body's momentum. The rate of this change equals the force applied. Mathematically, force equals the rate of change of momentum, which gives us F = ma when mass remains constant and velocity is much less than the speed of light. This is the special form of the second law. Here, F is force in newtons, m is mass in kilograms, and a is acceleration in m s⁻².
Let us work through an example. Imagine a cricket ball of mass 100 grams, or 0.1 kilograms, moving at 30 metres per second. A player stops it in 0.03 seconds. The change in momentum is 3 kilogram metres per second. Dividing by the time gives an average force of 100 newtons. This is why cricketers pull their hands back while catching — increasing the time reduces the force and prevents injury.
The S.I. unit of force is the newton, defined as the force which when acts on a body of mass 1 kilogram, produces an acceleration of 1 m s⁻². In the C.G.S. system, the unit is the dyne, defined as the force which when acts on a body of mass 1 gram, produces an acceleration of 1 cm s⁻².
One newton equals ten to the power five dyne, or 1 N = 10⁵ dyne.
Now, Newton's third law of motion. This law states: To every action, there is always an equal and opposite reaction. Action and reaction forces are equal in magnitude, opposite in direction, and act simultaneously on two different bodies.
When you walk, you push the ground backward with your feet — that is the action. The ground pushes you forward with an equal force — that is the reaction. When a gun fires a bullet, the bullet moves forward, and the gun recoils backward. A rocket works on this principle too — it expels gases backward, and the reaction force propels the rocket forward.
Finally, let us discuss gravitation and the distinction between mass and weight. Newton's universal law of gravitation states: Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Mathematically, F = Gm₁m₂/r², where G is the universal gravitational constant with a value of 6.67 × 10⁻¹¹ N m² kg⁻². This force is always attractive and acts along the line joining the two masses.
The acceleration due to gravity, denoted by g, is the acceleration produced in a freely falling body due to Earth's gravitational pull. Its average value on Earth's surface is 9.8 m s⁻², directed towards the center of the Earth. The relationship between g and G is g = GM/R², where M is Earth's mass and R is its radius.
Mass and weight are often confused, but they are distinct. Mass is the quantity of matter in a body — it is constant everywhere. Weight is the force of gravity on that body — it changes with location. Weight equals mass times acceleration due to gravity, or W = mg. Weight is measured in newtons, while mass is measured in kilograms. On the Moon, where g is about one-sixth of Earth's value, your mass stays the same, but your weight drops to one-sixth. This is why astronauts feel lighter on the Moon, though their inertia remains unchanged.
Let us recap the key takeaways from this lesson. First, forces can be contact forces like friction and tension, or non-contact forces like gravity and magnetism. Second, Newton's first law introduces inertia — the tendency of bodies to resist changes in motion. Third, Newton's second law quantifies force as the rate of change of momentum, giving us F = ma when mass remains constant and velocities are much less than the speed of light. Fourth, Newton's third law tells us that every action has an equal and opposite reaction. Fifth, gravitation is a universal attractive force governed by the inverse square law, and weight is the force of gravity on a body. And sixth, mass is constant, but weight depends on the gravitational field.
Understanding these laws opens the door to comprehending everything from planetary motion to everyday phenomena. Keep observing the world around you — physics is happening everywhere. Thank you for listening, and see you in the next lesson.