# About Newton’s laws

There are at least 5 things about Newton’s laws that you need to know to better understand the topic.

**Newton**‘s laws are the basis for understanding Mechanics , a branch of Physics that is dedicated to the study of motion. These laws deal with extremely everyday situations and help to understand numerous phenomena.

There are important observations regarding Newton’s laws that can go unnoticed and lead us to make mistakes in interpretation or not to understand exactly the occurrence of phenomena. Here are the five things you need to know about Newton’s Laws:

**1. Normal weight and strength ratio**

The weight force is the result of the product of the mass of a body by the acceleration of gravity and represents the attraction that the planet makes on a certain body. The normal force arises when an object is placed on a surface and is the force exerted by the surface on the object in order to support it.

These two forces are usually treated as if they were an action and reaction pair, but **according to Newton’s third law , weight and normal are not action and reaction** . Forces called action and reaction act on different bodies. When slapping a table, for example, there is the force made by the hand on the table (action) and the force made by the table on the hand (reaction). Note that the forces act on different bodies (table and hand). **The weight and the normal act on the same object and, therefore, cannot be considered an action and reaction pair.**

**2. Mass and inertia**

**Inertia is the tendency of a body to maintain its initial state of rest or motion. Such an initial state is only changed through the application of an external force** . Everyday experience leads us to realize that the greater the mass of a given object, the more complicated it will be to set it in motion or stop it. The opposite case occurs when the mass is small. Therefore, we can say that **mass is the quantitative measure of inertia and indicates the degree of difficulty imposed by an object on movement or rest.**

**3. Newton’s laws are not valid in all frames of reference**

The frame of reference is the object or place from which observations about motion and rest are made. **It is entirely possible for the same object to be stationary for one frame of reference and in motion for another** . **Newton’s laws are only valid when the frame of reference is stationary or in uniform motion . **Therefore, if any observation is made in an aircraft at full throttle pre-takeoff, for example, Newton’s laws will not be valid.

**Attention! **The Earth does not perform a uniform motion, but has a movement sometimes accelerated ( aphelion to perihelion ), sometimes delayed ( perihelion to aphelion ), but, for all intents and purposes, **our planet is considered an inertial reference.**

**4. Newton’s laws have limits**

If the speed of the analyzed object is very close to or equal to the speed of light , Newton’s laws will lose their validity and must be replaced by the application of Einstein’s relativistic proposals . For atomic-sized objects, analyzes based on Quantum Mechanics should be used. Thus, we can see that **Newton’s laws have limits of action and can be considered particular cases of major theories.**

**5. Newton’s second law was not originally written in the form F _{R} = ma**

The way most textbooks show Newton’s second law , the Fundamental Principle of Dynamics, is by putting **force as the result of the product of the body’s mass and its acceleration** , but originally Isaac Newton didn’t write this equation that way. When proposing this law, Newton said that force was the result of the variation of the momentum of the object over time. Therefore, we can write:

**F _{R} = ΔQ/Δt**

Momentum: **Q = mv** , where *m* is the mass and *v* is the speed of the object.

**F _{R} = m.Δv/Δt**

As the acceleration is the result of the ratio of the change in velocity by time, we can write:

**F **_{R}** = m. The**