Newton’s Laws of Motion

Newton’s laws of motion show the relationship between the forces acting on the body and the changes that takes place as a result of the forces. In the year 1686, Sir Issac Newton propagated the laws of motion in his book “Principia Mathematica Philosophiae Naturals”.

What are the three Laws of Motion?

The three laws of motion are:

  • Newton’s first law- According to Newton’s first law of motion, a body in rest or in motion, will be in that same state. It will change its state only when it is acted upon by an external source.
  • Newton’s second law- According to Newton’s second law of motion, the change which takes place in the body is directly proportional to the force acting on the body, and the momentum takes place along the direction of the force applied on it.
  • Newton’s third law- According to Newton’s third law of motion, whenever there is an action, then there has to be an equal and opposite reaction to it.

First Law of Motion

Galileo Galilei first discovered the law of inertia for explaining the horizontal motion of planet earth. Inertia is defined as the property of a body which is not able to change its state. Before Galileo discovered the law of inertia, there was a belief that a force is needed to keep a body moving. Galileo formulated that an external force is needed to change the state of a body whether the body is in a state of rest or in motion. Suppose a body is moving in a particular direction, then that body will keep on moving in that same direction unless and until it is acted upon by an external source.

Second Law of Motion

As per the second law of motion, the momentum of a body is equal to the product of the mass and velocity of the body. It talks about the quantitative description of force. Momentum is a vector quantity which have both velocity and magnitude. When an external force is applied on a body, then it can either change its momentum or the velocity or it can change both. Newton’s second law of motion is considered as one of the important laws of classical physics.

According to Newton’s formula, a body having a constant mass, is given as below.

F = ma,

Where F stands for the applied force, and ‘a’ stands for the acceleration occurred, and ‘m’ stands for the mass of the object.

When the net force acting on the body is positive, then the body accelerates. And if the net force is 0, then the body does not accelerate.

According to the second law of motion, when force is applied to two different objects having different masses, then different accelerations take place. The body which has less mass accelerates more than the body having bigger mass.

The result which will take place on applying a force of around 15 Newton on a football will be more profound than the effect of the same force applied to move a car. The difference occurs due to the difference of the masses of the body.

Third Law of Motion

According to Newton’s third law of motion, for every action there is an equal and opposite reaction, and the action and reaction happens to two bodies. When two bodies come together, there is an exchange of force which is equal in magnitude. But the force acts in opposite directions. This law is applied in static equilibrium in which the forces are balanced, and also for objects in which the accelerated motion is equal.

Let us take an example where a laptop is kept on a table and it exerts a downward force. The force is equal to its weight on the table. Hence, there is an equal and opposite force exerted by the table on the laptop. This force occurs because the force exerted by the laptop causes a minute change in the table. And as a result, the table exerts an opposing force to the laptop.

Few examples of Newton’s laws are mentioned below:

  • Force is applied when we want to shift some object, say a study table from one corner of the room to another.
  • A train running at 60 kilometers per hour.
  • When we swim in the water which has almost constant velocity.

Sir Issac Newton was a scientist who belonged to England. He had shabby looks but was a brilliant scientist. He founded the differential calculus and he devoted his entire life in the study of physics. The most significant work of his are the laws that rules classical physics which are called the Laws of motion.

Solve this question: If a body of mass m is carried by a lift moving with an upward acceleration a, then the forces acting on the body are (i) the reaction R on the floor of the lift upwards (ii) the weight mg of the body acting vertically downwards. The equation of motion will be given by?