Download Document

4 Motion Station
Inertia and Mass
All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some
objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist
changes in its state of motion is dependent upon mass. Inertia is that quantity which is solely dependent upon mass.
The more mass which an object has, the more inertia it has - the more tendency it has to resist changes in its state
of motion.
Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a
table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position
because of the absence of a force; rather it is the presence of a force - that force being the force of friction - which
brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the
same speed and direction - forever! A force is not required to keep a moving book in motion; in actuality, it is a
force which brings the book to rest.
Newton's first law of motion is often stated as:
An object at rest tends to stay at rest and an object in motion tends to stay in
motion with the same speed and in the same direction unless acted upon by an
unbalanced force.
Balanced forces do not cause motion, they cause accelerations that cancel out.
Motion Station
Forces
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there
is an interaction between two objects, there is a force upon each of the objects. Forces always come in pairs known as "action-reaction force pairs." Identifying and describing action-reaction force pairs is a simple matter of
identifying the two interacting objects and making two statements describing who is pushing on who and in what
direction. Unbalanced forces cause motion unless resisted by a fixed object.
For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:


contact forces, and
forces resulting from action-at-a-distance
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Tensional Force
Normal Force
Air Resistance Force
Applied Force
Spring Force
Gravitational Force
Electrical Force
Magnetic Force
Motion Station
Work-Energy
Work is defined as a force acting upon an object to cause a displacement. The final total energy of a system is
always equal to the initial energy of the system plus the external work done on the system.
Kinetic Energy
Kinetic energy is the energy of motion. An object which has motion - whether it be vertical or horizontal motion has kinetic energy. There are many forms of kinetic energy - vibrational (the energy due to vibrational motion),
rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to
another).
Potential Energy
An object can store energy as the result of its position. For example, the heavy ram of a pile driver is storing energy
when it is held at an elevated position. This stored energy of position is referred to as potential energy.
Mechanical Energy
An object which possesses mechanical energy is able to do work. In fact, mechanical energy is often defined as the
ability to do work. Any object which possesses mechanical energy - whether it be in the form of potential energy or
kinetic energy - is able to do work. That is, its mechanical energy enables that object to apply a force to another
object in order to cause it to be displaced.