Hey it's professor Dave, let's discuss forces.
There are certain words that are commonly used in the
English language which are very
important in physics.
These are words like work, energy, and
force. In everyday parlance they can mean
a variety of things, but in physics they
have very specific definitions, so we
will need to learn these definitions if
we are to learn about classical
mechanics. We already went over
kinematics, which is the branch of
mechanics that studies the motion of
objects without referring to forces, so
now it is time to learn about dynamics,
which is the study of forces and their
effect on motion. We will refer to a
force as any interaction that if
unopposed will alter the motion of an
object. A force can cause an object with
mass to start moving, stop moving, move
faster slower or in a different
direction.
A person can exert force by pushing or
pulling, an object can exert force by
falling or rolling, a planet or star can
exert force through its gravitational
pull. Any force must have both magnitude
and direction, so forces will always be
represented by vectors when we draw free
body diagrams. If you push a shopping
cart through the grocery store, you are
applying force on the cart in order to
generate a forward acceleration. This
force vector, which begins at the center
of the cart's mass, points in the forward
direction and also has a particular
magnitude, depending on how quickly you
want to get your shopping done, and this
magnitude will be represented by the
length of the vector. Some forces are
called contact forces because they arise
when two objects touch or collide.
Pushing a cart forward is an example of
a contact force. The same goes for when
you throw or kick
a ball, go bowling, and even when you sit
on a chair, pushing down on it with the
force of your weight. Some forces are
called noncontact forces. These involve
some action at a distance, like objects
that fall to the earth because of its
gravity. In classical physics, non-contact
forces are sometimes called field forces
because they typically involve some kind
of a field, like a gravitational field or
electromagnetic field. In fact, contact
forces that describe macroscopic motion
are just manifestations of field forces
happening on the microscopic level. If
you push an object, it moves because of
the repulsion between the electrons in
the atoms in your hand and the electrons
in atoms in the object, so even though we
will discuss many different types of
forces in this course, in actuality there
are only four fundamental ones. Those are
the strong nuclear force, the weak
nuclear force, the electromagnetic force
and gravity. All other forces are just
some variation or application of one of
these four, which we will get to a bit
later. Given that any force must have
magnitude, we must have a unit of
measurement to communicate these
quantities. That unit is called the
Newton, and it is equal to the amount of
force that will cause a one-kilogram
mass to accelerate by one meter per
second squared. Since these are all SI
units, the Newton is the SI unit of force.
The Newton is named after Sir Isaac
Newton, who put forth the first extensive
study of forces in the 17th century. His
studies are summarized in the Principia
where he describes the laws of motion
that deal with force and mass. These laws
changed the world and are considered
among the most important accomplishments
in the history of science, so let's learn
about them next.
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