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Newton's laws of motion

Newton’s laws of motion, relations between the forces acting on a body and the motion of the body, first formulated by English physicist and mathematician Sir Isaac Newton.

Newton’s first law states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. This postulate is known as the law of inertia. The law of inertia was first formulated by Galileo Galilei for horizontal motion on Earth and was later generalized by René Descartes. Before Galileo it had been thought that all horizontal motion required a direct cause, but Galileo deduced from his experiments that a body in motion would remain in motion unless a force (such as friction) caused it to come to rest.

Newton`s laws of motion - Class 11

Newton’s second law is a quantitative description of the changes that a force can produce on the motion of a body. It states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. The momentum of a body is equal to the product of its mass and its velocity. Momentum, like velocity, is a vector quantity, having both magnitude and direction. A force applied to a body can change the magnitude of the momentum, or its direction, or both. Newton’s second law is one of the most important in all of physics. For a body whose mass m is constant, it can be written in the form F = ma, where F (force) and a (acceleration) are both vector quantities. If a body has a net force acting on it, it is accelerated in accordance with the equation. Conversely, if a body is not accelerated, there is no net force acting on it.

Newton’s third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The third law is also known as the law of action and reaction. This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion. The forces it describes are real ones, not mere bookkeeping devices. For example, a book resting on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a coiled spring.


Newton’s laws first appeared in his masterpiece, Philosophiae Naturalis Principia Mathematica (1687), commonly known as the Principia. In 1543 Nicolaus Copernicus suggested that the Sun, rather than Earth, might be at the centre of the universe. In the intervening years Galileo, Johannes Kepler, and Descartes laid the foundations of a new science that would both replace the Aristotelian worldview, inherited from the ancient Greeks, and explain the workings of a heliocentric universe. In the Principia Newton created that new science. He developed his three laws in order to explain why the orbits of the planets are ellipses rather than circles, at which he succeeded, but it turned out that he explained much more. The series of events from Copernicus to Newton is known collectively as the scientific revolution.


In the 20th century Newton’s laws were replaced by quantum mechanics and relativity as the most fundamental laws of physics. Nevertheless, Newton’s laws continue to give an accurate account of nature, except for very small bodies such as electrons or for bodies moving close to the speed of light. Quantum mechanics and relativity reduce to Newton’s laws for larger bodies or for bodies moving more slowly.

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1. What are Newton's laws of motion?
Ans. Newton's laws of motion are three fundamental principles in physics that describe the motion of objects. The laws are: 1) First law (Law of Inertia): An object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction, unless acted upon by an external force. 2) Second law (Law of Acceleration): The acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. The equation F = ma represents this law. 3) Third law (Law of Action-Reaction): For every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.
2. How do Newton's laws of motion apply to everyday life?
Ans. Newton's laws of motion apply to various aspects of everyday life. Here are a few examples: 1) First law: When you suddenly stop a car, your body tends to move forward due to inertia. Wearing a seatbelt helps to prevent this. 2) Second law: Pushing a heavy object requires more force than pushing a lighter object. This law also explains why it is harder to accelerate a larger vehicle compared to a smaller one. 3) Third law: When you swim, you push the water backward, and the water pushes you forward, propelling your movement. Similarly, when you walk, your feet push against the ground, and the ground pushes you forward.
3. Can you provide an example of Newton's first law of motion?
Ans. An example of Newton's first law of motion, also known as the law of inertia, is the movement of a book placed on a table. If the table is smooth and there are no external forces acting upon the book, it will remain at rest. However, if the table is suddenly pulled, the book will tend to stay at its original position due to its inertia and will slide along the table until a frictional force stops it.
4. How does Newton's second law of motion relate force, mass, and acceleration?
Ans. Newton's second law of motion states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. Mathematically, it can be expressed as F = ma, where F represents the net force acting on the object, m is the mass of the object, and a is the resulting acceleration. This equation shows that a larger force will cause a greater acceleration, and a larger mass will result in a smaller acceleration for the same force.
5. Can you explain Newton's third law of motion with an example?
Ans. Newton's third law of motion states that for every action, there is an equal and opposite reaction. An example of this law is the propulsion of a rocket. As the rocket expels gas out of its engines with a force, the gas exerts an equal and opposite force on the rocket, propelling it forward. This law also applies to everyday actions, such as pushing a wall. When you push the wall, you exert a force on it, and the wall exerts an equal and opposite force back on you, preventing you from moving through it.
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