Newton’s Laws of Motion

Newton is one of the most influential scientists in physics. He worked on gravitational theories at an early age. He was only 23 when he developed the theory of gravity.

Newton’s Laws of Motion
Newton’s Laws of Motion


Newton’s Laws of Motion


Newton is one of the most influential scientists in physics. He worked on gravitational theories at an early age. He was only 23 when he developed the theory of gravity. He also made development in calculus along with Leibniz although their opinions varied from each other. But he will remain as an ever-fresh figure for his discovering the ‘Three Laws of Motion.’ His laws of motion are the fundamental foundations of classical mechanics. The laws were proved to be the root cause of a scientific revolutionary at that time. Scientists further researched the laws of Newton for many years. 


Why an inert object does not change its position or how an object moves when force is given, has been always a question for the scientists of different age. Aristotle, the Greek scholar believed that every object in the universe has a natural place of its own. According to him, heavy objects want to stay where they are. Light objects like smoke want to keep moving to the sky. But he was wrong about the moving objects. His view on a dynamic object is it continues to go on a straight line and an external agent propels it. His idea was partially true.     


Nicolaus Copernicus explained his theory that the earth encircles around the sun in his book ‘De revolutionibus orbium coelestium’ published in 1543. 


Galileo Galilei, an Italian scientist disagreed with Aristotle’s view. His researches prove that a necessary force can change the velocity or acceleration of a moving body. But a dynamic object does not need any extra effort to carry on the movement. In other words, Galileo says that without any external force, the moving body will continue its journey. 


Johannes Kepler came with his idea about the tendency of objects. He observed that an object does not want to change the condition of its motion. He called it ‘Inertia’. His book ‘New Astronomy,’ was published in 1609. This book contains his idea of three laws of planetary motion.


Newton’s first law is also known as the “Law of inertia”. As the law was previously found by Galileo, Newton gave him due respect. 


Leonardo da Vinci observed in his experiments that an object and air show equal resistance to each other. It was an inaugural base for the third law by Newton. 


Many scientists like Thomas Hobbes and René Descartes also experienced the law of inertia in their researches. Thomas Hobbes mentioned the law in his ‘Leviathan’. Descartes formulated the law but no proof was found about his experiments on this law. 


Newton’s laws of motion created a scientific revolution at that time. Although his laws were not applicable to very small bodies like a photon, they are still worthy to prove many scientific problems. His laws were later superseded by the ‘theory of relativity’ by Albert Einstein. 


The First Law


“Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.”






For better understanding, Newton has used the term ‘net force’ which is the vector sum of all forces acting on an object. This law describes that if the net force is zero or the object does not get external force, the object’s velocity becomes constant. It means if the body is at rest, its velocity is zero and it tends to stay inert without outer force. While a body with velocity keeps going on a straight line at a constant speed. 


We know this condition of the objects as inertia. Galileo was the first to experiment with this law on a horizontal basis. But René Descartes generalized the law later. 


An inert object wants to stay wherever it is for a long time. For example, say you are in front of a table. There is a tablecloth on the table and many plates on the tablecloth. If you quickly remove the tablecloth skillfully, the plates will stay on the table. For another example, you need a glass, a piece of paper and a coin. Place the paper on the top of the glass and keep the coin on it. Pull the paper quickly. What happened? The coin falls in the glass. In both experiments, the plates or the coin did not want to leave their previous place. 


To exemplify the condition of the moving objects we can simply kick a football. The football roles running straight for some time and then stops. According to Newton’s law of inertia, it should not have stopped. But the friction of the land resisted the force of the football. If you kick the ball in the space, it will go on and on. Because there is no friction or outer force in the space. When we are in a running vehicle, our bodies also become dynamic. So, the upper portion of our body abruptly comes forward when the driver pulls the break. This is why we use seat belts and airbags to protect ourselves. 


The Second Law  


“In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object. F = ma


An inertial frame of reference is purely an imaginary term by Newton as we cannot measure it on measurable physics. This particular frame is unable to measure our velocity relative to a far star. This law is the most effective among the three laws as it directly measures the calculations of dynamics. When we apply forces on an object, this law shows what type of change occurs in its motion and how.  This law also proves Aristotle wrong. We need force to change the velocity of an object, not to maintain it.





Newton states in this law that an object’s momentum-changing rate is directly proportional to the force applied to it. The direction of the force effects the change in momentum. It means the change works in the same direction as the applied force. 


For example, if you apply equal force to two objects with different mass, the lighter one will reach further than the heavier one. When you throw a basketball and a tennis ball with the same force, the tennis ball reaches greater destination. Pushing an empty shopping cart is easier. But when it is full, we need extra forces to push it. It happens because of Newton’s second law of motion. 


The Third Law


“For every action, there is an equal and opposite reaction.”


Mathematically we can state this law as below-


Fp = -Fq 


When a body p gives force to another body q, q alters the same amount of force to p. The force works in the opposite direction. As all the forces are accompanied by an opposite and equal force, a single force cannot exist. If p applies force on q, we can call it ‘action’ and the alternative force of q on p can be called ‘reaction’. Both ‘action’ and ‘reaction’ happen together every time. These two occur simultaneously as a part of a single interaction.  


When we walk on the streets, our feet apply force on the ground. If the reaction force does not work in this case, we would not be able to walk harmoniously. That is why we can walk easily on smooth ground and face difficulty while walking on fragile grounds. We cannot understand this force because it is unapparent. Let’s try another example. When you throw a basketball from the ground, you do not feel the reaction of your action. You will definitely see the reaction force if you stand on a skateboard. When you throw the basketball, you will go back as the throwing force works front. 



You may also notice that when you get down from a boat, the boat slightly floats back. If you try to jump from there, the boat may float further than you have estimated. You may not make it to the bank of the river and get drenched or drowned. 


Similarly, when car tires’ action works on the ground, the grounds reaction helps the tires to move functionally. Aircraft building also takes this law very seriously in making planes. 



So far, we have seen the applications of the laws by Newton. His laws have some limitations too. They do not work in the non-inertial frame of reference. Another boundary is that Newton stated ‘a body at rest’. In reality, it is not possible. Because any object on the earth is not at rest as our planet moves around the sun. The theory of general and specific relativity by Albert Einstein proves the limits of the laws of motion. The laws are also inapplicable in subatomic levels. 


If we ignore the limitations of these laws, Newton has been successful in establishing the laws with all his painstaking efforts. He spent a considerable portion of his life in developing physics. As a recognition of his endeavor for science, we should always remember this great scientist.