Nothing moves, stops, speeds up or turns by itself — a force is always behind it. Newton’s three laws are the rulebook that the whole universe obeys, from a rolling ball to a rocket.
Force
A push or pull that can change an object’s speed, direction or shape.
Inertia
The natural laziness of matter — it resists any change in its state of rest or motion.
Momentum
p = m × v. The “quantity of motion” an object carries.
Newton’s Laws
Three laws that connect force, mass and motion completely.
What is a force?
A force is a push or a pull acting on an object. We cannot see a force directly, but we see what it does. A force can (1) make a stationary object move, (2) stop a moving object, (3) make a moving object go faster or slower, (4) change the direction of motion, and (5) change the shape or size of an object (like squashing dough). Forces are of two main kinds: balanced forces and unbalanced forces.
Balanced and unbalanced forces
When two equal forces act on an object in opposite directions, they cancel out — these are balanced forces. The net force is zero, so the object does not change its state of motion (it stays at rest or keeps moving steadily). In a tug-of-war, when both teams pull equally hard, the rope does not move — the forces are balanced. Unbalanced forces are when the forces do not cancel; there is a leftover net force. Only an unbalanced force can change the speed or direction of an object — that is, produce acceleration. To keep an object moving at constant speed against friction, we must keep applying a force only to balance the friction.
Newton’s First Law of Motion (Law of Inertia)
An object continues to be in its state of rest or of uniform motion in a straight line unless acted upon by an unbalanced (external) force. This means objects do not change their motion on their own. A book on a table stays put; a moving ball would roll forever if there were no friction. The tendency of an object to resist a change in its state of motion is called inertia. The first law is therefore also called the law of inertia. Galileo first arrived at this idea using inclined planes before Newton stated it formally.
Inertia and mass
All objects resist change, but some resist more than others. A loaded truck is far harder to start or stop than a bicycle. The mass of an object is the measure of its inertia — more mass means more inertia. This is why it takes a bigger force to move a heavy almirah than a light chair. Everyday examples of inertia: when a bus starts suddenly we fall backward (our body wants to stay at rest); when a bus stops suddenly we lurch forward (our body wants to keep moving); dust flies off when we beat a carpet (the carpet moves, the dust stays); a coin placed on a card on a glass drops into the glass when the card is flicked.
Newton’s Second Law of Motion
The rate of change of momentum of an object is directly proportional to the applied unbalanced force, and the change takes place in the direction of the force. Momentum (p) is the product of mass and velocity, p = m×v. The second law gives us the most useful equation in mechanics: F = m × a, where F is the net force (in newtons), m is the mass (in kg) and a is the acceleration (in m/s²). This law explains how much an object accelerates: the same force gives a small object a large acceleration and a big object a small one. The second law actually contains the first law — if F = 0, then a = 0, so velocity does not change.
The SI unit of force — the newton
From F = m×a, one newton (N) is the force that gives a mass of 1 kg an acceleration of 1 m/s². So 1 N = 1 kg·m/s². The second law also explains why a cricketer pulls his hands back while catching a fast ball — this increases the time of contact, which reduces the rate of change of momentum, which reduces the force on the hands. The same idea explains why high-jumpers land on a thick foam mattress and why a karate expert can break a slab with a sharp, fast blow (very short contact time means a very large force).
Newton’s Third Law of Motion
To every action there is an equal and opposite reaction. Forces always occur in pairs. If object A exerts a force on object B, then B exerts an equal force on A in the opposite direction. Crucially, action and reaction act on different objects, so they never cancel each other out. When we walk, we push the ground backward and the ground pushes us forward. A swimmer pushes water backward and is pushed forward. A gun recoils when a bullet is fired. A rocket pushes hot gases downward and is thrown upward — this is how rockets work even in empty space.
Conservation of momentum
When two objects collide, the total momentum before the collision equals the total momentum after, provided no external unbalanced force acts. This is the law of conservation of momentum, and it follows directly from Newton’s third law. For two objects of masses m1 and m2 with initial velocities u1 and u2 and final velocities v1 and v2: m1u1 + m2u2 = m1v1 + m2v2. This single rule explains gun recoil, rocket propulsion, and what happens when carrom coins or billiard balls strike each other.
- Momentum: p = m × v (unit: kg·m/s)
- Second law: F = m × a (unit of force: newton, N)
- 1 N = 1 kg·m/s²
- Force from momentum: F = (mv − mu) ÷ t
- Conservation of momentum: m1u1 + m2u2 = m1v1 + m2v2
- Mass = measure of inertia; more mass → more inertia
- Balanced forces → no acceleration; unbalanced force → acceleration
A force is applied to a 5 kg box and it accelerates at 2 m/s². Find the force. Then find the acceleration if the same force acts on a 2 kg box.
- Use Newton’s second law: F = m × a.
- F = 5 kg × 2 m/s² = 10 N.
- For the 2 kg box with the same force: a = F ÷ m = 10 ÷ 2.
- a = 5 m/s² — the lighter box accelerates more for the same force.
A 0.5 kg ball moving at 10 m/s is stopped by a player in 0.1 s. Find (a) the change in momentum and (b) the force applied.
- Initial momentum p1 = m × u = 0.5 × 10 = 5 kg·m/s.
- Final momentum p2 = m × v = 0.5 × 0 = 0 kg·m/s.
- Change in momentum = p2 − p1 = 0 − 5 = −5 kg·m/s (negative means it opposes motion).
- Force = change in momentum ÷ time = (−5) ÷ 0.1 = −50 N.
Remember the three laws as “Lazy — Push harder — Push back”: 1st = objects are lazy (inertia), 2nd = the harder you push the faster it goes (F = ma), 3rd = whatever you push pushes back equally. For F = ma, think “Force = mass × acceleration” spells F-MA, like “Fast Moving Action”.
The biggest mistake is saying action and reaction cancel out. They are equal and opposite but they act on two different objects, so they can never cancel. Also, always convert units (mass in kg, velocity in m/s, time in s) before using F = ma, and never forget the unit “N” in your final answer.
Q1. State Newton’s first law of motion and define inertia.
Answer: Newton’s first law states that an object remains at rest or in uniform motion in a straight line unless an unbalanced external force acts on it. Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion. Because mass measures inertia, heavier objects have greater inertia. This is why the first law is also called the law of inertia.
Q2. Why does a cricketer pull his hands backward while catching a fast-moving ball?
Answer: A fast ball has large momentum. By pulling his hands backward, the cricketer increases the time over which the ball’s momentum is brought to zero. Since force = rate of change of momentum (F = change in momentum ÷ time), increasing the time reduces the force exerted on his hands. This prevents injury. It is a direct application of Newton’s second law.
Q3. A bullet of mass 20 g is fired from a gun of mass 2 kg with a velocity of 200 m/s. Find the recoil velocity of the gun.
Answer: Use conservation of momentum. Before firing, both are at rest, so total momentum = 0. After firing: momentum of bullet + momentum of gun = 0. Mass of bullet = 0.02 kg. So (0.02 × 200) + (2 × v) = 0, giving 4 + 2v = 0, so v = −2 m/s. The gun recoils at 2 m/s in the direction opposite to the bullet.
Q4. State Newton’s third law and explain how a rocket moves using it.
Answer: Newton’s third law states that to every action there is an equal and opposite reaction, and these act on two different bodies. In a rocket, burning fuel produces hot gases that are pushed out downward at high speed (the action). By the third law, the gases push the rocket upward with an equal and opposite force (the reaction). This reaction thrust propels the rocket, and it works even in the vacuum of space because no surrounding medium is needed.
- ✅ Force is a push or pull; unbalanced force causes acceleration.
- ✅ First law: no force, no change in motion (inertia); mass measures inertia.
- ✅ Second law: F = m × a; force equals rate of change of momentum.
- ✅ Third law: equal and opposite reaction on a different body.
- ✅ Momentum (p = mv) is conserved when no external force acts.
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