Newton's laws (summary): what they are, formulas and examples

What are Newton's Laws?

The laws of Newton are three principles that describe the motion of bodies, based on an inertial reference system (real forces at constant speed).

Newton's three laws are:

• First law or law of inertia. Second law or fundamental law of dynamics. Third law or principle of action and reaction.

These laws on the relationship between force, velocity, and the motion of bodies are the basis of classical mechanics and physics, and were postulated by the English physicist and mathematician Isaac Newton, in 1687.

Newton's first law: law of inertia

The law of inertia or first law postulates that a body will remain at rest or in straight motion with a constant speed, unless an external force is applied.

In other words, it is not possible for a body to change its initial state (either at rest or movement) unless one or more forces intervene.

Newton's first law formula is:

Σ F = 0 ↔ dv / dt = 0

If the net force (Σ F) applied to a body is equal to zero, the acceleration of the body, resulting from the division between speed and time (dv / dt), will also be equal to zero.

An example of Newton's first law is a ball in a state of rest. For it to move, it requires a person to kick it (external force); otherwise, it will remain at rest. On the other hand, once the ball is in motion, another force must also intervene so that it can stop and return to its resting state.

Although this is the first of Newton's proposed laws of motion, this principle had already been postulated by Galileo Galilei in the past, for which the latter is credited for its authorship, and Newton for its publication.

See also: Physics.

Newton's second law: fundamental law of dynamics

The fundamental law of dynamics, Newton's second law or fundamental law postulates that the net force that is applied to a body is proportional to the acceleration that it acquires in its trajectory.

Newton's second law formula is:

F = ma

The net force (F) is equal to the product resulting from the mass (m), expressed in kg, by the acceleration (a), expressed in m / s2 (meter per second squared).

This formula is only valid if the mass is constant. When the body mass is variable, it is necessary to calculate the amount of movement, which is the product of the object's mass times its velocity (mv).

In this case, the formula of the law of dynamics would be:

F = d (mv) / dt

Force (F) is equal to the derivative of momentum (d (mv) between the derivative of time (dt).

An example of Newton's second law can be seen by placing balls of different mass on a flat surface and applying the same force to them. The lighter ball will move faster than the one with a higher mass.

This is perhaps one of the most important laws of motion in classical physics, since it answers the question of what force is and how it should be calculated.

See also Dynamics.

Newton's third law: principle of action and reaction

Newton's third law postulate says that every action generates an equal reaction, but in the opposite direction.

The formula of law of action and reaction is:

F _1-2 = F _2-1

The force of body 1 on body 2 (F _1-2), or action force, is equal to the force of body 2 on body 1 (F _2-1), or reaction force. The reaction force will have the same direction and magnitude as the action force, but in the opposite direction.

An example of Newton's third law can be seen when we have to move a sofa, or any heavy object. The action force applied to the object causes it to move, but at the same time it generates a reaction force in the opposite direction that we perceive as an object resistance.

See also Types of movement.

Newton's fourth law: universal law of gravitation

The postulate of this law of physics states that the attractive force of two bodies is proportional to the product of their masses.

The intensity of that attraction will be stronger the closer and more massive the bodies are.

Newton's fourth law formula is:

F = G m1.m2 / d2

The force exerted between the two bodies with mass (F) is equal to the universal gravitational constant (G). This constant is obtained by dividing the product of the two masses involved (m1.m2) by the distance that separates them, squared (d2).

We have an example of Newton's fourth law in the gravitational attraction that two bowling balls exert. The closer they are to each other, the greater the attractive force.

See also:

• Gravity. Branches of physics.