ENERGY IS ALWAYS CONSERVED--if you find a case where you think
it is not conserved then you are missing something.
Remeber that friction causes things to heat up,
which is a transfer of energy.
When using conservation of energy to calculate quantities (i.e. v=?):
Calculate the initial energy you start with:
What is h? => P.E. = mg h
What is v? => K.E. = 1/2 m v^2
Where did this energy go? What is your final energy
What is new h?
What is new v?
Was there heat generated? Heat = Friction x distance
Since energy is Always conserved-- you set the initial total energy (P.E. + K.E.)
equal to the final energy (P.E. + K.E. + Heat) and solve for what you
want.
Remeber that sometimes you ignore friction (no heat is generated).
Other times another force is present that increases the kinetic energy.
For example if I were pushing on a box with a constant force (F)
over a distance (d) I would increase the kinetic energy of the box
by an amount = F x d.
Things are not always cut and dry--the important thing to remember
is that energy is conserved and you are just transforming from
one form to another.
Forces are the means by which the transfers occur.
In the Example above, where I'm pushing the box and increasing its kinetic energy--
Where is this energy coming from? Is it a transfer of energy? How?
Demo:
The car and coffee can-- You use conservation of energy to find out the velocity
of the car as it leaves the table. How? What type of energy does it start with? What
type of energy is it converted to? After it leaves the table you find out have far
it goes by using x=vt. To use the equation you need to know t, the time it travels in
the air before hitting the ground. For this you use the equation y=1/2at^2.
When do you use d=vt and when do you use d=1/2at^2? Why do I use the equation
y=1/2at^2 to find the time, and what is a?