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Vertical motion
More on the energy balance
From the values we have
determined
for the light plastic ball,
we may observe that it returns to us
with a speed of about 14 m/s,
which is considerably less then
the speed of 20 m/s
which it had been given at the beginning of its motion.
Consequently, quite some kinetic energy has been lost during the trip
of the light plastic ball.
Remember that kinetic energy is quadratic in the velocity.
Now,
142≅200,
whereas,
202=400.
So, during its trip
the plastic ball lost half of its initial kinetic energy.
This is what one may expect as the effect of air resistance.
Namely, air resistance, which is a
frictional force,
always works opposite to the direction of motion.
So, whenever present, it only takes away mechanical energy.
Such interactions are
non-conservative forces
which do
depend
on the
path of displacement
that you choose to go from one point to the other.
But, how does that observation agree with the concept of
energy conservation?
The answer is that in the present case the
work of friction
is converted into heat.
This phenomenon we may observe every night
when small meteorites enter the atmosphere at high speed.
The air resistance reduces their velocity.
But the work of that friction is converted into heat
which often let the small meteorites obtain such high temperatures
that they shine as bright as a star,
a shooting star.
We may conclude by stating that the total amount of energy is conserved,
but that mechanical energy got lost in the trip of the plastic ball.
One should remember that every single day
millions
of events are studied
in
laboratories
all over
the world.
Not even one event has ever been reported
in which
energy was
not conserved.
dynamics