Unbalanced
forces cause objects or systems to move, change direction or speed; once this happens, work
is done.
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| W = F||d |
W = work done (Joules, J; 1 J = 1 Nm)
F|| = force parallel to the direction of movement (N)
d = distance |
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Energy is the ability to do work. It comes in many forms, solar,
chemical, mechanical, heat, electrical, etc. Though each is a form of energy, each
is defined slightly differently. We are mainly concerned with these types:
Kinetic Energy (KE) and Potential
Energy (PE).
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If the forces
do not result in movement or change in direction or speed, NO work is
being done! The strain you feel comes from the upward force you exert to balance the
downward force of gravity.
The work being done in moving an object from
one position to another does not depend on the path it took; regardless of how an object
gets from A to B, the same amount of work is done.
|
This is true
only if the forces are conservative (energy is not lost through them).
Example, spring, gravity are conservative and friction is not.
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The
work-energy principle is a general statement that relates the work done on an object due
to a change in its energy. the net work (Wnet) done o an
object is the sum of all work done by conservative forces (Wc) and
non-conservative forces (Wnc). |
Wnet
= Wc + Wnc |
since |
Wnc =  KE = 1/2 mvf2 - 1/2 mvi2
and
Wc = PE =
mghf - mghi |
So, |
| Wnet = KE + PE If every kind of energy was considered (from both conservative and
non-conservative forces) in the work being done to an object, the total energy before and
after in that object is the same; this is The Law of Conservation of Energy.
The total energy of any system that is
isolated from the rest of the Universe remains constant, even though energy may be
transformed from one kind to another within the system.
Sum of all energy before = Sum of all energy types after. |
