Newton's Laws of Motion

Newton's Laws of Motion

Newton's laws of motion give a scientific relationship between the forces that act on a body and the changes that occur due to this force. Sir Isaac Newton formulated the laws of motion in the year 1686 in his book ‘Principia Mathematica Philosophiae Naturalis’

What are the three Laws of Motion?

The three laws of motion are:

  1. Newton’s first law - Newton's first law of motion states that, if a body is in the state of rest or is moving with a constant speed in a straight line, then the body will remain in the state of rest or keep moving in the straight line, unless and until it is acted upon by an external force. 

  2. Newton’s second law - Newton's 2nd law of motion states that the rate of change of momentum of a body is directly proportional to the force applied on it, and the momentum occurs in the direction of the net applied force.

  3. Newton’s third law - According to Newton's third law of motion, to every action, there is always an equal and opposite reaction

First Law of Motion

This property of a body unable to change its state is called Inertia. Galileo Galilei first formulated the law of inertia for the horizontal motion of planet Earth. Later on, it was generalized to René Descartes. Before Galileo, it was believed that a force is required to keep a body moving. Galileo deduced that a body can't change its state unless acted by force (like friction).

The state of motion or rest cannot be changed without applying force. If a body is moving in a particular direction, it will keep moving in that direction, until an external force is applied to stop it.

Second Law of Motion

Newton's second law gives a quantitative description of force. The momentum of a body is equivalent to the product of its mass and velocity. To speak, momentum is a vector quantity having both velocity and magnitude. When force is applied to a body, it can either change its momentum or its velocity or both. Newton's second law of motion is one of the most important laws of classical physics.

For a body of constant mass m, Newton's law formula is given as,

F = ma,

Where ‘F’ is the applied force, and ‘a’ is the acceleration produced, and m is mass of the object

If the net force acting on a body is positive, the body gets accelerated. Conversely, if the net force is 0, the body doesn't accelerate.

According to the second law of motion, if force is applied to two different objects of different masses, different accelerations (change in motion) are produced. The body with less mass accelerates more.

The effect of a force of around 15 Newton on football will be much more significant as compared to the impact of the same force applied to move a car. This difference, in fact, is due to the difference in masses of the two objects.

Third Law of Motion

According to Newton's third law of motion, to every action, there is always an equal and opposite reaction. Also, the action and reaction occur in two different bodies. When two bodies interact with each other, they exchange force, which is equal in magnitude but act in opposite directions. This law has a huge application in static equilibrium where the forces are balanced, and also for objects which undergo uniform accelerated motion.

For example, a laptop kept on a table exerts a downward force, which is equal to its weight on the table, and consequently, the table exerts an equal and opposite force on the laptop. This force comes into play because the weight of the laptop slightly deforms the table, and in return, the table pushes back the laptop.

Some of Newton’s laws examples are mentioned below.

1. You apply force when you want to move your study table from one side of the room to another side.

2. A cargo train is running on the track at 60 kilometers per hour.

3. Swimming in the water with an almost constant velocity.

Isaac Newton was a scientist from England. He was a bit stuffy with unpleasant hairs, but he was a brilliant person. Newton is the founder of differential calculus and has dedicated his life to the world of physics. One of his significant works is the laws that govern classical physics. His ideas and concepts have been experimentally verified throughout the years. These are called Newton's Laws of Motion.

FAQ (Frequently Asked Questions)

1. Why do Objects Slow Down?

Before Newton and Galileo, it was believed that the slowing down of objects was a natural tendency of the objects. Forces like friction and gravity were not known at that time. The frictional force opposes the motion of an object, causing it to lose energy resulting in slowing it down. 

For instance, when we see a toy car moving on a concrete surface, the car’s speed is determined by the force of friction between the road and the car wheels. When the toy car is driven, over a smooth surface, the wheel will meet with little resistance. This will set up a frictional obstacle and will let the car drive faster over all the plain tile surface. 

This is very much unlike when a car toy is moving in an unsound surface containing gravels. Newton's first law of motion gives this concept of change in the state of rest or state of motion.

2. What does Net Force Mean?

The net force ΣF is the vector sum of all the forces that are acting on a body at the same time. Adding the vector quantities is different from simple addition, as it involves direction in an account. 

Let us consider that an object is subjected to two different forces of 30 N in the right direction and 20 N in the left direction, respectively, as per the diagram. Since the force acting on the right side is more, so the net force will be given by

Σ F = 30N- 20N

Σ N = 10 N in the right

If there are multiple numbers of forces acting on an object, then Newton's second law of motion is given by

ā =    Σ f̅/𝑚

This is a universal formula for force, and it shows that the direction of acceleration is in the same direction as the net force.