The weight of a body represents the force (gravitational) with which the body is
attracted to the surface (by the Earth). Mass is a property of the body, its quantity of
matter, quantifying its inertia. In the International System units, the mass is
expressed in kg and force is expressed in Newtons. The unit of force, Newton, is
defined as the force required for causing an acceleration of 1 ms
−1 to a body with a
mass of 1 kg.
If we consider in Eq. (A2.1) a body subjected to a zero balance of forces, one can
deduce that the acceleration is null, which translates Newton’s First Law of Motion.
Newton’s First Law may thus be considered as a particular case of the second.
Newton’s Second Law quantifies the effect of force systems in motion. The next
question concerns the provenance of a force. In fact, the existence of one force
implies its application by one body against another body. A charging animal pulls a
sleigh, a hammer nails a nail, a magnetic body attracts a metal clip, or in the
biosphere, a gust of wind causes a ripple of the vegetable coverings. Strictly
speaking, the force application of a body or system of fluid particles is not
unilateral. In the case of the hammer, it is evident that the hammer exerts a force on
the nail, but also the nail exerts a counterforce on the hammer, as the hammer speed
cancels out because of the contact with the nail. By Newton’s Second Law this
hammer braking effect can only be caused by an enough force emitted by the nail.
Newton thus considered that the two bodies should be treated equally,
formulating his Third Law of Motion: whenever one body exerts a force on
another body, the second body exerts a force, on the former body, which is equal
and opposite to the first force.
Newton’s Third Law can be enunciated according to the principle of
action-reaction: each force of action always corresponds to an equal and opposite
force of reaction. The fundamental assumption is that the forces of action and
reaction are acting on different bodies. Newton’s Third Law explains processes
such as that of the movement of a person on the ground in which their movement is
initiated by the force exerted by the foot on the ground and the ground exerts an
opposite reaction force with opposite sign (friction) on the person, which causes the
movement. In fact, a person could not move on ice without friction. Likewise, a
bird flies because of the force of reaction of the air on its wings, equal and of
opposite signal of the force exerted by the bird on the air. Another example is the
movement of an automobile. The automobile motor causes rotation of the wheels
and the car’s displacement is caused by the friction of the ground in the tires, being
the reaction force in opposition to the force of action of the tires in the ground.
The normal tendency is to associate forces with active bodies such as people,
animals, motors or a hammer-like moving object, and it is more difficult to imagine
how an inanimate object at rest like a wall, a desk, or the ground can exert the same
forces. The explanation lies in the occurrence of forces less apparent such as the
elasticity of solid materials, existing in a greater or lesser degree, the superficial
friction, or the impulsion of fluids.
The Third Law of Motion may lead to the following misconception: if the two
forces are equal although of opposite signs, then they cancel each other, and no
acceleration of the bodies occurs. In fact, two equal forces of opposite signs cancel
Annex A2: Basic Topics on Laws of Motion and Evaporation
335
attracted to the surface (by the Earth). Mass is a property of the body, its quantity of
matter, quantifying its inertia. In the International System units, the mass is
expressed in kg and force is expressed in Newtons. The unit of force, Newton, is
defined as the force required for causing an acceleration of 1 ms
−1 to a body with a
mass of 1 kg.
If we consider in Eq. (A2.1) a body subjected to a zero balance of forces, one can
deduce that the acceleration is null, which translates Newton’s First Law of Motion.
Newton’s First Law may thus be considered as a particular case of the second.
Newton’s Second Law quantifies the effect of force systems in motion. The next
question concerns the provenance of a force. In fact, the existence of one force
implies its application by one body against another body. A charging animal pulls a
sleigh, a hammer nails a nail, a magnetic body attracts a metal clip, or in the
biosphere, a gust of wind causes a ripple of the vegetable coverings. Strictly
speaking, the force application of a body or system of fluid particles is not
unilateral. In the case of the hammer, it is evident that the hammer exerts a force on
the nail, but also the nail exerts a counterforce on the hammer, as the hammer speed
cancels out because of the contact with the nail. By Newton’s Second Law this
hammer braking effect can only be caused by an enough force emitted by the nail.
Newton thus considered that the two bodies should be treated equally,
formulating his Third Law of Motion: whenever one body exerts a force on
another body, the second body exerts a force, on the former body, which is equal
and opposite to the first force.
Newton’s Third Law can be enunciated according to the principle of
action-reaction: each force of action always corresponds to an equal and opposite
force of reaction. The fundamental assumption is that the forces of action and
reaction are acting on different bodies. Newton’s Third Law explains processes
such as that of the movement of a person on the ground in which their movement is
initiated by the force exerted by the foot on the ground and the ground exerts an
opposite reaction force with opposite sign (friction) on the person, which causes the
movement. In fact, a person could not move on ice without friction. Likewise, a
bird flies because of the force of reaction of the air on its wings, equal and of
opposite signal of the force exerted by the bird on the air. Another example is the
movement of an automobile. The automobile motor causes rotation of the wheels
and the car’s displacement is caused by the friction of the ground in the tires, being
the reaction force in opposition to the force of action of the tires in the ground.
The normal tendency is to associate forces with active bodies such as people,
animals, motors or a hammer-like moving object, and it is more difficult to imagine
how an inanimate object at rest like a wall, a desk, or the ground can exert the same
forces. The explanation lies in the occurrence of forces less apparent such as the
elasticity of solid materials, existing in a greater or lesser degree, the superficial
friction, or the impulsion of fluids.
The Third Law of Motion may lead to the following misconception: if the two
forces are equal although of opposite signs, then they cancel each other, and no
acceleration of the bodies occurs. In fact, two equal forces of opposite signs cancel
Annex A2: Basic Topics on Laws of Motion and Evaporation
335