Newton's Laws, Description of the Laws of Motion

Newton's laws, description of the laws of motion

Newton's laws (or the fundamental principles of mechanics) are three physical physical laws that relate the forces that act on a body and the movement of that body. These laws were stated by Sir Isaac Newton (also based on Galileo's studies).

Newton's laws are sufficient to explain all movements in classical mechanics, that is, movements that take place at speeds much slower than the speed of light in vacuum. All of them can be demonstrated by applying the scientific method.

These three laws are based on an inertial reference system, that is, with real forces with constant velocity.

Newton's First Law: the Law of Inertia

Newton's first law is the law of inertia. The law of inertia says:

"Any body maintains a state of rest or uniform rectilinear motion as long as no other forces act on it or the sum of the forces acting on it is zero."

If a moving object does not receive any extra force, it moves in a straight line with a constant speed. If, by the contrary, we exert a lateral force on this same body, it will change direction. This is what happens with the Moon. The moon constantly experiences a centripetal force in the direction of the earth due to the force of gravity with which the Earth and the Moon are attracted (universal law of gravitation).

Newton's Second Law: Law of Dynamics

Newton's second law is the fundamental law of dynamics: it establishes the relationship between force and acceleration.

"A force acting on a body gives it an acceleration, proportional to the force and inversely proportional to the mass of the body."

Mass is a measure of the amount of matter in the body. Newton introduces the notion of momentum. This is a vector quantity equal to the product of the mass and the velocity vector.

p = mv


  • m is the mass

  • v is the velocity vector

Starting from the momentum we can deduce the most complete form of definition of force for a body of constant mass.

F = m a


  • F is the force vector (N)

  • m is mass (kg)

  • a is the acceleration vector (m / s)

If an object that is at rest does not receive any external force, it will remain at rest. If, on the other hand, we apply a net force to it, the body experiences an acceleration and acquires a uniformly accelerated motion.

Newton's Third Law: Law of Action and Reaction

The third law or law of action and reaction indicates the following:

“When a body acts on another body with a force (called an action force), the second body also acts on the first with a force (called a reaction force) of the same size and direction, but in the opposite direction. This principle is also known as the action-reaction principle. "


Published: October 30, 2021
Last review: October 30, 2021