Three laws of Newton for friction
Answers
Newton's First Law:
Newton's first law, , tells us that if no other
body acts on a body, it will remain indefinitely moving in a straight line with
constant speed (including the state of rest, which equals zero velocity).
As we know, the movement is relative, that is,
it depends on which is the observer who describes the movement. Thus, for a
passenger on a train, the interventor is walking slowly down the train
corridor, while for someone who sees the train passing from the platform of a
station, the intervener is moving at a high speed. Therefore, a reference
system is required to refer to the movement. Newton's first law serves to
define a special type of reference systems known as Inertial Reference Systems,
which are those reference systems from which it is observed that a body on
which no net force acts moves with constant velocity.
Actually, it is impossible to find an inertial
reference system, since there is always some kind of force acting on bodies,
but it is always possible to find a reference system in which the problem that
we are studying can be treated as if we were in a Inertial system. In many
cases, assuming a fixed observer on Earth is a good approximation of the
inertial system.
Newton's Second
Law:
Newton's Second Law is responsible for
quantifying the concept of force. It tells us that the net force applied on a
body is proportional to the acceleration that the body acquires. The constant
of proportionality is the mass of the body, so that we can express the relation
as follows:
F = m a
Both force and acceleration are vector
magnitudes, that is, they have, in addition to a value, a direction and a
direction. In this way, Newton's Second Law must be expressed as:
F = m a
The unit of force in the International System is
Newton and is represented by N. A Newton is the force that must be exerted on a
body of one kilogram of mass to acquire an acceleration of 1 m / s2, that is,
1 N = 1 Kg · 1 m / s 2
The expression of Newton's Second Law that we
have given is valid for bodies whose mass is constant. If the mass varies, such
as a rocket that burns fuel, the relation F = m · a is not valid. We are going
to generalize Newton's Second Law to include the case of systems in which mass
can vary.
To do this we first define a new physical
quantity. This physical quantity is the quantity of movement that is
represented by the letter p and that is defined as the product of the mass of a
body by its speed, that is to say:
P = m · v
The amount of movement is also known as the
linear momentum. It is a vector magnitude and in the International System it is
measured in Kg · m / s. In terms of this new physical quantity, Newton's Second
Law is expressed as follows:
The Force acting on a body is equal to the
temporal variation of the amount of movement of that body, that is,
F = dp / dt
In this way we also include the case of bodies
whose mass is not constant. In case the mass is constant, recalling the
definition of momentum and how a product is derived:
F = d (m · v) / dt = m · dv / dt + dm / dt · v
As the mass is constant
Dm / dt = 0
And remembering the definition of acceleration,
we have
F = m a
As we have seen previously.
Another consequence of expressing Newton's
Second Law using momentum is what is known as the conservation principle of
momentum. If the total force acting on a body is zero, Newton's Second Law
tells us that:
0 = dp / dt
That is, the derivative of the momentum with
respect to time is zero. This means that the amount of movement must be constant
over time (the derivative of a constant is zero). This is the principle of
conservation of momentum: if the total force acting on a body is zero, the
amount of movement of the body remains constant over time.
Newton's third
law:
As we said at the beginning of Newton's Second
Law, forces are the result of the action of some bodies on others.
The third law, also known as the Principle of
Action and Reaction, tells us that if a body A exerts an action on another body
B, it performs on another equal and opposite action.
This is something we can check daily on numerous
occasions. For example, when we want to jump upwards, we push the ground to
propel us. The reaction of the soil is what makes us jump up.
When we are in a pool and push someone, we also move
in the opposite direction. This is due to the reaction that the other person
makes about us, even if they do not try to push us.
It should be noted that, although the action and
reaction pairs have the same opposite value and meanings, they do not cancel
each other out, since they act on different bodies.
Second law:In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant – see below.)
Third law:When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.