**Second Newton Law. Concepts and Illustrative
Exercises**

**Newton's Second Law can be summarized as
follows: If F represents the net force applied to an object of mass m, then
its acceleration has the same direction as the net force and its magnitude
is proportional to the quotient F/m, i.e. **

**a represents the acceleration, m the mass
and F the net force. By net force it is understood the resultant vector sum
of all the forces acting on the body. **

**Let's see Newton's Second Law concepts of
Mass, Inertia, Acceleration and Force and an access to exercises on this Law.**

**Definition of Mass, Inertia.**

**What is mass? Newton himself used the term
mass as synonym of quantity of matter. This idea used by Newton is not very
precise. With more precision we can say that the mass is a measure of the
inertia of a body. The greater the mass of a body the harder is to change
its state of movement. It is hard to get moving starting from rest, or to
stop while moving, or to get moving to a side from a rectilinear trajectory.
A truck has much more inertia than a tennis ball moving at the same speed,
being very much harder to change the state of movement of the truck.**

**To measure the concept of mass a standard
should be defined. In the International System Units, the mass unit is the
kilogram. The present standard is a cylinder made of platinum conserved in
the International Bureau of Weights and Measures near Paris, whose mass, by
definition, is exactly equal to one kilogram. In cgs units, the mass unit
is the gram (g) and 1g = 10 ^{-3} kg. In the English System, the unit of mass
is called the Slug. **

**Mass should not be confused with weight.
The mass is a property of a body, is a measure of its inertia or its quantity
of matter. The weight is a force, the force that the Earth exerts on the body.
To clarify the difference, suppose we take an object to the Moon. There it
will weight a sixth of its weight on Earth, but its mass will remain the same.**

**Acceleration, Net Force**

**The First Newton Law states that in absence
of a net force on a body, this body remains at rest, or if it is moving, continues
moving with constant velocity (keeping magnitude and direction). But, ¿what
happens if a force acts over a body? The velocity must change, that is, a
net force produces an acceleration.**

**The relation between acceleration and force
can be found in daily experiences. Let us think pulling a supermarket cart.
The net force exerted on the cart is the force that I apply minus the friction
force on the wheels. If the net force is F, the acceleration will be a, if
the force is 2F, la acceleration will be 2a, and so on. Hence, the acceleration
of a body is directly proportional to the net force applied. The acceleration
depends also on the mass of the object. If I keep the net force F and the
total mass is doubled, the acceleration will be a/2.**

**Then, we can affirm that**

**The unit of force is chosen in such a way
that the constant of proportionality in
, be 1, and so, **

**a = F/m**

**Notice that with this second law we can
state a more precise definition of force: as an action able to accelerate
an object.**

**When the mass is in kilograms and the acceleration
in meters per second squared, la force unit is called Newton (N), 1 N = 1kgm/s ^{2}.**

**In the English System, the force unit is
the pound. It is defined as the weight (a force) of a body whose mass is 0.45359237
kg on some place on the Earth where the acceleration of gravity is 32.1734
feet/s ^{2. }**

**Exercises, Newton's Second Law:**

** Second
Newton's Law Exercises, Part One
Second
Newton's Law Exercises, Part Two**

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