Newton’s laws

Newton's laws
Newton's laws

Physics is a science that studies phenomena and events in nature. Generally, observations converge to create theories and general laws. Newton’s laws cover different aspects of physics and can be applied in different situations.

To help you understand this subject and how it appears in entrance exams, Owl has created a summary with the main concepts and information about Newton’s laws. At the end, find a test question solved by our team. Let’s go?

What are Newton’s laws?

Newton’s laws are three physical rules proposed by the scholar Isaac Newton during the time he dedicated himself to the study of natural phenomena. They are the foundation for classical Mechanical Physics and concern the dynamics of bodies and the interaction between them. 

In addition to theoretical statements, the laws proposed by Isaac Newton have mathematical expressions to explain natural events. Through them, it was possible to understand the trajectory of objects that fall to the ground, the movement of the tides, among others.

The set of three laws describes the ability of bodies to move, stop movement or remain at rest. Therefore, it is present in several everyday situations, such as the creation of pulleys that facilitate the transport of heavy objects, the constant speed of cars, and other examples.


The main physical quantity present in Newton’s laws is force (F) , which is measured in kg.m/s 2 , or simply, in N (newtons, in honor of the scholar). The formula for calculating the vector magnitude of the force is:

F = ma 

m=mass of the object, in kg
a=acceleration of the object, in m/s 2

Newton’s first law

Newton’s first law is known as the law of inertia, which can be stated as: “Every body continues at rest or in uniform motion in a straight line unless some force alters the balance of the forces applied to it.”

This phrase brings the following idea: bodies tend to remain in the state they are in , unless some force “disturbs” them and takes away this inertia. For example, if a glass of wine is on the table during dinner, it can stay there indefinitely until something removes it from that state. 

When a child accidentally hits their hand on the glass, a force is created on the object and it falls onto the table, staining the white tablecloth. 

In this sense, Newton’s first law always refers to the sum of the forces acting on a body, which can also be called the resultant force. When resulting F =0, the object tends to remain in the state it is in, when resulting F ≠0, the object tends to change its state of inertia, as illustrated in the following image:

It is important to emphasize that, when the resultant of the forces is equal to zero, the object can be in motion. In this case, the movement takes place without acceleration (uniformly) and is not curved, that is, it is a straight-line movement that has a constant speed . 

Furthermore, it is important to note that inertia is greater in magnitude proportional to the mass of the body . In other words, large bodies have a great tendency to remain in the state they are in. That’s why it’s easier to push a grocery cart than to move a car, for example. 

Newton’s second law

This law is considered the fundamental principle of dynamics and concerns the movement of the body after the application of a force, its statement may be as follows: “The change in movement is proportional to the imparted motor force and is produced in the straight line direction to which that force is applied.”

It is from this law that the formula for the resultant force on a body can be inferred, which will be:

resulting = m object .a 

From this equation, it is possible to understand that for the same force, the greater the mass of the object, the lower its acceleration . For example, imagine that you push the door of your house with force and then, using the same force, you want to push a full gas cylinder. Obviously, the acceleration of the door was greater because it is lighter. 

In cases where a body is being acted upon by n different forces, the resulting force will be:

1 + F 2 + F 3 + F 4 + F n = resulting F = m object .a 

And the direction of movement of the object will be the same as that presented by the resulting vector, as well as the direction acquired by it.

Newton’s third law

Finally, Newton’s third law, also known as the law of action and reaction, deals with the creation of a reaction force in a body. When you apply a force to an object it automatically applies a force to you — these forces have equal intensity and direction , but their directions are opposite . 

To state it: “To every action there is always an opposite reaction of equal intensity: the mutual actions of two bodies on each other are always equal and directed in opposite directions.”

Action and reaction pairs always act in different bodies and are important in understanding everyday situations. For example, when we are standing on the ground, our weight (in the sense of physical magnitude measured in N) causes a reaction on the earth’s surface called Normal — in this case the vectors are vertical, of equal intensity and point in opposite directions. See a schematic:

Newton's laws - action and reaction

Other examples are traction force, which appears in cases where we use a cable or thread to pull objects, and elastic force, which appears in elastic or spring-loaded objects.

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