each other out when applied to the same body. For example, if two equal forces of
opposite directions were applied to the same rock, there would never be any
movement of the rock, regardless of the force applied: the forces could cause
rupture or pulverization of the rock, without however displacing it, (e.g., Asimov
1993).
The Law of Interaction implies, however, the existence of two equal forces of
opposite signs, applied to two separate bodies. If a person exerts a force on a stone,
the stone exerts an equal force in opposite direction on the person. In this case,
neither force is compensated for: the rock is accelerated in the direction of the force
applied by the person, and the person is accelerated in the direction of the force of
equal magnitude applied by the rock on the person. If the launching of the stone
happens on rough ground, the friction developed between the shoes and the ground
introduces new forces to the system that prevent the person’s movement. The
acceleration is then cancelled, so the true effect of the Law of Interaction is no
longer visible. However, if the throw occurs on an icy surface without friction, we
will see the person sliding in the opposite direction to that of the throw.
By the same token, the gases formed by combustion in a rocket motor expand,
exerting a force against the respective inner walls as they exert an opposite and
equal force against the gases, expelling them. The gases are forced to a downward
acceleration and by the principle of action-reaction they exert an opposite force in
the structure of the rocket, impelling it to an ascending acceleration.
In these examples, the two bodies involved are physically separated or separable,
insofar that one of them can accelerate in one direction and the other can accelerate
in the opposite direction. If the two bodies are coupled, e.g. a horse pulling a
wagonette, all this analysis is apparently more difficult to accomplish.
According to the Law of Interaction, the sleigh will also pull the horse in the
opposite direction with equal speed, although it is visible that these two bodies do
not accelerate in opposite directions but are together safely evolving in the same
direction. If the forces connecting the horse and the wagonette were the only ones
existing, there would be no movement. The frictional force exerted by the ground
surface (exerted by the Earth) induces the progressive movement of the
horse-sleight system. On an ice surface, without any frictional force, neither the
horse nor the sleigh would progress.
In real conditions, and in the presence of frictional forces, the sleigh system
exerts a frictional force on Earth and this, in turn, causes a frictional force on the
sleight system. As a result of this interaction, the sleight system is displaced and
progresses in a certain direction, while Earth is displaced in the opposite direction.
As the planet has much more mass than the system, we only notice the movement in
progression of the system.
These concepts of various forces in interaction in a system are exemplified in
Fig. A2.1 exemplified by the effort made by a person to move a sleigh. In fact, it
can be verified by the figure that the two forces mainly responsible for the
progressive movement of the person-sleigh system are the friction force F ps exerted
by the ground on the person and the force F pt exerted by the sleigh on the person.
When the ground exerts a force F ps on the person oriented in direction of movement
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Annex A2: Basic Topics on Laws of Motion and Evaporation
opposite directions were applied to the same rock, there would never be any
movement of the rock, regardless of the force applied: the forces could cause
rupture or pulverization of the rock, without however displacing it, (e.g., Asimov
1993).
The Law of Interaction implies, however, the existence of two equal forces of
opposite signs, applied to two separate bodies. If a person exerts a force on a stone,
the stone exerts an equal force in opposite direction on the person. In this case,
neither force is compensated for: the rock is accelerated in the direction of the force
applied by the person, and the person is accelerated in the direction of the force of
equal magnitude applied by the rock on the person. If the launching of the stone
happens on rough ground, the friction developed between the shoes and the ground
introduces new forces to the system that prevent the person’s movement. The
acceleration is then cancelled, so the true effect of the Law of Interaction is no
longer visible. However, if the throw occurs on an icy surface without friction, we
will see the person sliding in the opposite direction to that of the throw.
By the same token, the gases formed by combustion in a rocket motor expand,
exerting a force against the respective inner walls as they exert an opposite and
equal force against the gases, expelling them. The gases are forced to a downward
acceleration and by the principle of action-reaction they exert an opposite force in
the structure of the rocket, impelling it to an ascending acceleration.
In these examples, the two bodies involved are physically separated or separable,
insofar that one of them can accelerate in one direction and the other can accelerate
in the opposite direction. If the two bodies are coupled, e.g. a horse pulling a
wagonette, all this analysis is apparently more difficult to accomplish.
According to the Law of Interaction, the sleigh will also pull the horse in the
opposite direction with equal speed, although it is visible that these two bodies do
not accelerate in opposite directions but are together safely evolving in the same
direction. If the forces connecting the horse and the wagonette were the only ones
existing, there would be no movement. The frictional force exerted by the ground
surface (exerted by the Earth) induces the progressive movement of the
horse-sleight system. On an ice surface, without any frictional force, neither the
horse nor the sleigh would progress.
In real conditions, and in the presence of frictional forces, the sleigh system
exerts a frictional force on Earth and this, in turn, causes a frictional force on the
sleight system. As a result of this interaction, the sleight system is displaced and
progresses in a certain direction, while Earth is displaced in the opposite direction.
As the planet has much more mass than the system, we only notice the movement in
progression of the system.
These concepts of various forces in interaction in a system are exemplified in
Fig. A2.1 exemplified by the effort made by a person to move a sleigh. In fact, it
can be verified by the figure that the two forces mainly responsible for the
progressive movement of the person-sleigh system are the friction force F ps exerted
by the ground on the person and the force F pt exerted by the sleigh on the person.
When the ground exerts a force F ps on the person oriented in direction of movement
336
Annex A2: Basic Topics on Laws of Motion and Evaporation