What Are Newton’s Three Laws of Motion? Statements, Meanings & Practical Examples
Understanding the Newton’s Laws of Motion is essential for cracking JEE Main Physics. These laws explain how every force produces specific changes in the motion of objects and anchor the classical mechanics syllabus. Most types of Physical questions on movement or force analysis for JEE are based on these foundational principles.
These laws—often cited as the three laws of motion—cover everything from why a stationary ball remains at rest to how rockets are launched. Mastering them gives students an advantage when approaching mechanics, momentum, friction, and constrained motion problems. Vedantu’s curated explanations and practice examples help streamline your revision for all these related concepts.
Newton’s Laws of Motion: Definition, History, and Importance
The Newton’s Laws of Motion were proposed by Sir Isaac Newton in 1687. These three statements laid the groundwork for classical physics and modern engineering. They describe the link between force, mass, and acceleration, showing how and why objects move or resist movement when a force acts.
For syllabus topics like conservation of momentum, friction, kinematics, and equilibrium, these laws provide problem-solving structure. The influence of Newton’s work is foundational across modern science and JEE preparation alike.
Newton’s First Law of Motion (Law of Inertia)
Newton’s first law states: “An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force.” This principle is also called the law of inertia.
In simpler terms, motion does not change unless a force makes it. Here, inertia measures an object’s resistance to changes in its motion, based on mass.
- A book on a table remains at rest unless pushed.
- A moving ball rolls at constant speed until friction or another force stops it.
- Passengers lurch forward in a braking car due to their inertia.
- Spacecrafts continue moving in space when engines shut off.
There is no direct formula for first law, but it underpins all subsequent motion equations. For detailed problem types with inertia, see moment of inertia.
Newton’s Second Law of Motion (Law of Acceleration)
Newton’s second law of motion quantifies the effect of a force: “The rate of change of momentum of an object is directly proportional to the net external force applied, and occurs in the direction of the force.”
For JEE, this is usually expressed as F = ma where:
- F is net force (N, newton)
- m is mass (kg)
- a is acceleration (m/s2)
| Formula Name | Expression | Units |
|---|---|---|
| Force (Second Law) | F = ma | N = kg·m/s2 |
| Momentum | p = mv | kg·m/s |
| Change of Momentum | F = Δp/Δt | N |
Example: If a force of 10 N acts on a 2 kg block, acceleration = F/m = 10/2 = 5 m/s2.
Numerical questions on the law of acceleration may involve resolving forces, inclined planes, pulley systems, and friction. For more, see mass and weight differences or the acceleration formula.
Newton’s Third Law of Motion (Action-Reaction)
Newton’s third law of motion states: “For every action, there is an equal and opposite reaction.” When one body exerts a force on a second, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first.
- When you jump, your feet push down on the ground, and the ground pushes you up with equal force.
- A rocket expels gas backward (action), and moves forward (reaction).
- Recoil of a gun when fired is due to this law.
- Walking involves pushing the ground backward while the ground pushes you forward.
Both forces act on different bodies, so they do not cancel. Be careful not to confuse action-reaction with balanced forces! More situations using this law are found in collisions and impulse-momentum problems.
| Law | Statement | Formula | JEE Exam Examples |
|---|---|---|---|
| First Law | Object remains at rest/motion until net force acts. | – | Ball at rest, constant speed motion |
| Second Law | F = ma. Force causes acceleration. | F = ma | Pulley, elevator, incline force problems |
| Third Law | Action = reaction; equal and opposite. | F12 = –F21 | Gun recoil, rocket launch, collision |
Application of Newton’s Laws of Motion in JEE Main
All Newton’s Laws of Motion appear in JEE as core theory and multi-step numericals. Typical application areas include pulley and block systems, inclined planes, lifts, friction analysis, and motion with variable force.
- Calculate acceleration when multiple forces act using the acceleration formula.
- Solve problems with circular motion or collisions using conservation laws and third law pairs.
- Apply free-body diagrams to break forces into components.
- Remember, third law forces are always between two bodies, not within one body.
- Always use SI units (N, kg, m/s2).
- Carefully distinguish between net external force and internal action-reaction pairs.
For higher accuracy, combine these laws with kinematics, momentum principles, and work-energy analysis as needed.
Common Mistakes and Exam Pitfalls in Newton’s Laws of Motion
- Confusing first law (inertia) with second law (F = ma scenarios).
- Assuming action and reaction cancel each other—remember, they act on different objects.
- Mixing mass and weight—see mass versus weight.
- Forgetting to resolve forces along correct axes in inclined plane problems.
- Ignoring friction—always check for presence/absence as per question.
- Forgetting SI units or incorrect conversion to Newtons.
Practicing with varied examples on laws of motion practice papers and mock tests can help avoid these common traps.
For compact reference, download summary sheets or revise the topic with the laws of motion revision notes made by Vedantu’s expert faculty.
The Newton’s Laws of Motion remain the heart of mechanics for JEE, connecting all fundamental problems in force, equilibrium, and motion. Use regular practice of all three laws in both theory and numericals to solidify your understanding and boost problem-solving speed.
FAQs on Newton’s Laws of Motion Explained: Concepts, Formulas & Uses
1. What are Newton's 1st, 2nd, and 3rd laws of motion?
Newton's Laws of Motion are three fundamental principles describing how forces affect the motion of objects in classical mechanics.
1st Law (Law of Inertia): An object remains at rest or in uniform motion unless acted upon by an external force.
2nd Law (Law of Acceleration): The force acting on an object is equal to the mass of that object times its acceleration (F = ma).
3rd Law (Action-Reaction): For every action, there is an equal and opposite reaction.
These laws are essential for JEE, NEET, and board exams, and are widely applied in physics problem solving.
2. What is Newton's 2nd law also called?
Newton's Second Law is also called the Law of Acceleration.
- It states that the rate of change of momentum of an object is directly proportional to the net force applied, and occurs in the direction of the applied force.
- Its most common formula is F = ma (force equals mass times acceleration).
3. Is there a Newton's 4th law?
There is no officially recognized Newton's 4th law in classical physics.
- Isaac Newton formulated only three laws of motion.
- Sometimes, unofficial or misattributed statements are called a "fourth law,” but these are not part of the standard physics syllabus.
4. What are formulas for Newton’s laws of motion?
The key formulas for Newton's Laws of Motion are:
- First Law: No specific formula, but relates to inertia: Net force = 0 implies constant velocity.
- Second Law: F = ma (Force = mass × acceleration)
- Third Law: Faction = –Freaction (For every action, there is an equal and opposite reaction)
5. What is Newton's first law?
Newton's First Law states that an object will remain at rest or move in a straight line at constant speed unless acted upon by an external force.
- This law is also called the Law of Inertia.
- It explains why objects remain in their state of motion or rest if no net external force is applied.
6. How is Newton's first law applied in daily life?
Newton's First Law is seen in many daily life situations where inertia is observed:
- A book on a table stays at rest until someone pushes it.
- You lurch forward in a bus when it suddenly stops, as your body wants to keep moving.
- A rolling ball slows down and stops if friction or another force acts on it.
7. What is Newton's second law of motion?
Newton's Second Law states that the force on an object equals its mass multiplied by its acceleration.
- Mathematically: F = ma.
- This means greater force leads to greater acceleration, or more mass needs more force for the same acceleration.
8. What is Newton's third law of motion?
Newton's Third Law says that for every action, there is an equal and opposite reaction.
- If you push against a wall, the wall also pushes back with equal force in the opposite direction.
- This principle explains phenomena like walking, swimming, and rocket propulsion.
9. If action and reaction are equal, why don't they cancel each other out?
Though action and reaction forces are equal and opposite, they act on different objects, so they do not cancel each other out.
- For example, when you walk, your foot pushes backward on the ground, and the ground pushes you forward.
- Since the forces act on different systems, their effects cause movement rather than cancellation.
10. What are common mistakes when applying F=ma in problems?
Common mistakes when using F = ma in Newton's second law include:
- Not using the net (total) force; students sometimes use a single force instead of the sum of all forces acting on the object.
- Forgetting to convert mass into the correct units (SI: kilograms).
- Ignoring direction: force and acceleration are vector quantities, so their directions must match.
11. Why is Newton's first law also called the law of inertia?
Newton's First Law is called the law of inertia because it describes an object's tendency to resist changes in its motion.
- Inertia is the property of matter by which an object remains at rest or keeps moving at constant velocity unless a force acts on it.
12. How do Newton’s laws apply to non-contact forces like gravity?
Newton's Laws of Motion apply to both contact and non-contact forces, such as gravity.
- For example, an apple falls to the ground because gravity (a non-contact force) provides the external force required for acceleration, as per the second law.
- Similarly, gravitational action-reaction pairs exist: Earth pulls the apple down, and the apple pulls Earth up with equal force.
