Laws of Motion

Hundreds of years ago, Sir Isaac Newton revolutionized Physics study by discovering Newton’s laws of motion. Laws of motion describe the behavior of a physical object when it stands still, moving or when forces act upon it. 

Newton’s laws consist of three statements that describe the relationship between the forces applied to a body and the motion of that body. These are the basis of classical mechanics.

In this article, we will study the definition of Newton’s laws of motion and its three important laws, in detail.

Define Newton’s Laws of Motion

Newton’s Laws of Motion is defined by including three crucial laws, which are:

1. Unless an unbalanced force acts on an object, an object will continue remaining at rest if it is already at rest, and it will continue moving in uniform motion if it is already in uniform motion. The speed will remain constant and in a straight line in both scenarios. 

1. The acceleration of an object is directly equivalent to its mass and the amount of applied force. The acceleration is dependent on these two criteria.

1. For every amount of force applied on the first object, there is an equal and opposite force applied on the second one.

Science was revolutionized with the discovery of these three laws. Sir Issac Newton developed the gravitation theories at the age of 23 years in 1666. In his masterpiece, “Principia Mathematica Philosophiae Naturalis”, he presented these three laws in 1686.

Newton’s laws of motion, when combined with Kepler’s laws, explain to us the reason for planets moving in elliptical orbits and not in circles. Later, quantum mechanics replaced Newton’s laws in the 20th century. 

Newton’s First Law of Motion: The law of inertia

We define Newton’s first law of motion:

When an object is at rest or in motion, it will continue to be at rest or in motion until and unless an external unbalanced force acts on it, provided the object is in a straight line and the speed remains constant.

Anything that is in motion, cannot automatically start, change its direction, or stop. It requires some force externally to experience a change in the motion of the object. This law includes a concept called inertia. Inertia is the property in which large bodies that are moving resist changes. This law implements this property. Hence, it is also known as the law of inertia. 

The velocity of the object remains constant if, on an object, there exists no net force resulting from external forces. When the velocity changes, it is because of the external forces which are applied. When the object remains at rest, it means that the velocity is zero. 

Some examples include:

• When a ball in motion is falling 
• The flying of a kite according to the change in the speed and direction of the wind.
• Launching a model rocket into the atmosphere.
• A moving airplane when the throttle setting of an engine is changed by a pilot. 

Newton’s Second Law of Motion: Force (F = ma)

We define Newton’s second law of motion:

The net force produces the acceleration of an object, which is dependent on the mass of an object and the applied unbalanced external force. This acceleration is directly proportional to the magnitude, in the same direction, of the external force and inversely proportional to the mass of an object.

One of the examples includes the motion of an aircraft that results from aerodynamic forces, the weight of an aircraft and its related thrust.

It is defined by the expression, F=ma

We can derive this expression, mathematically, as:

Force = Difference in momentum with the time difference

F= m1V1 – m0V0t1 – t0

F= Force 

v= final velocity 

t= time 

When the mass is constant,  

F=mV1 – V0t1 – t0

We can express it as,

                                                       F is directly proportional to dPdt

F is directly proportional to mv – mut

F is directly proportional to m(v – u)t

                                                  F is directly proportional to ma

                                                    F = k ma

                                                    F  = ma

In the above expression, k is the proportionality constant that is equal to 1.  

Some examples of this law include:

• Pushing a Vehicle
• Launching a rocket
• Crashing a car
• Car driving

Newton’s Third Law of Motion: Action and Reaction 

We define Newton’s third law of motion:

For every action or external force that object 1 exerts on object 2, there is an equal and opposite reaction or force that object 2 exerts on object 1.

In simple words, for every action, there is always an equal and opposite reaction. The action is commonly referred to as force. Whenever there is an interaction between the objects, it results in a force that they apply to each other. This force has equal magnitude and is directed opposite to each other. Out of all the laws, this law implies the conservation of momentum. 

Some examples of this law include:

• Swimming in a pool or any water body.
• Sitting on a chair.
• The carpet moves backwards when someone is running on it.
• Walking.
• A spinning ball, when moving, deflects the air on one side and experiences the reaction of the moving ball on the opposite side.

Conclusion 

Newton’s laws of motion include three important laws developed by Sir Issac Newton. These laws are based on inertia, force, and action & reaction. In our article, we learned the definitions and explanations of the laws. Also, the examples are stated for each law. Some significant questions are also explained in detail in the FAQ section.