Do you want to study or do you need to review the **laws of dynamics**? In this article you will find a **lesson on the principles of dynamics**, also called *Newton’s laws* . The laws of dynamics are fundamental physical laws because they are the basis of **Dynamics**, a word that derives from the Greek *dynamic* or force, which is that branch of mechanical physics that studies forces and their effects on bodies.

In fact, **Newtonian dynamics** describes the behavior of a moving body and defines the concepts of force, balance, and movement. Also, remember that Newton formulated the three laws of motion in the 17th century in his work *Philosophiae Naturalis Principia Mathematica* . *Keep reading. *Below you will find the statements of the three laws of dynamics, the formulas designed specifically for high school students and for candidates for admission tests to restricted access faculties, with theory and a problem solved step by step.

## First law of dynamics

The first law of dynamics is also known as the **principle of inertia** or *Galileo’s principle* . The **statement of the first law of dynamics** is:

A body at rest remains stationary if the sum of the forces acting on it is zero or if no force acts. If the body is in motion, it will continue to move in a uniform rectilinear motion.

Hence, **what is inertia? **Inertia is the property of a body to maintain its state, which can be of motion or of rest, until a force intervenes on it.

In reality, you have probably observed that a body that is moving slowly eventually stops. This observation does not contradict Newton’s first law, because in a real reference system the friction force acts on that body. Instead, Newton’s first law must be applied in an empty space in which all forces are in equilibrium.

## The second law of dynamics

The second law of dynamics is also called **the principle of proportionality** or *the principle of conservation*.

The **statement of the second law of dynamics** is:

The force acting on a body is directly proportional to the mass of the body and to the acceleration, and has the same direction.

Thus, acceleration is proportional to force and inversely proportional to mass.

The **formula of the second law of dynamics** is:

*F= m·a* with *F* and *a* having the same direction and towards →.

The mass Newton talks about in this second law is called **inertial mass** because it is the measure of the resistance of an accelerated body. In fact, imagine if you exerted a force of equal intensity first on a body of small mass, such as an apple, and then on one of greater mass, such as a chair. The apple will fly away. The chair will move a little.

From Newton’s second law comes the unit of measurement **of force** which in the International System, *SI* is the **newton**, *N. **1N* is equivalent to the force needed to impart an acceleration of *1m/s ^{2}* to a body with a mass of

*1kg.*

## The third law of dynamics

The third principle of dynamics is also known, simplifying, as **the principle of action and reaction** .

The **statement of the third law of dynamics** is:

For every force that a body A exerts on another body B, there is another equal force, in magnitude and direction, and opposite in direction, that B exerts on A.

The **formula of the third law of dynamics** is:

The two forces, which act on different bodies, are two equal and opposite vector quantities.

*But if the force with which a thug pushes a kid is the same as the force that pushes the kid back onto the thug, why does the kid fly away and the thug doesn’t? *Because force, as we saw with the second law, also depends on mass. Objects that experience the same force but have different masses will experience different accelerations.

So, **what is strength? **Force is a vector quantity that originates from the interaction of two or more bodies and is always associated with a variation in speed.

Furthermore, from the third law of dynamics derives the principle according to which if a body were alone in space it would not be subjected to any force.

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