|When a skateboard rolls to a stop, is most of its
Kinetic Energy (KE) lost as heat in the ground,
the wheels' surface, or in the ball bearings...of
the wheel. Also, if two skateboards are in motion
(A and B), and the wheels, ball bearings,.. of B
have a higher heat capacity than A, would this
mean that skateboard B would roll longer?
There are actually two separate questions here.
First, how fast do the skateboards lose energy
and, second, how fast do the wheels, bearings,
etc., change temperature? The rate of energy loss
of the skateboards -- how fast they stop -- is
determined by the frictional force between the
bearings and the wheels and perhaps the ground if
the skateboards aren't rolling perfectly (I am
assuming that you're pushing the skateboards with
the same speed to start with). The more friction,
the faster the skateboard will stop. As you've
noted, the energy lost from the skateboard goes
into the wheels, bearings, etc., and is converted
to heat energy. What the heat capacity does is
determine the change in temperature of the wheels,
etc., given a certain amount of heat energy --
which used to be the kinetic energy of the
skateboards. So, even though the skateboards
might lose the same amount of kinetic energy in
the same time, the wheels and ball bearings of one
would get hotter than the other. But the
skateboards would stop at the same time as long as
the friction is the same!
Try to come up
with an experiment to test this!
No, the heat capacity is just a measure of how
much the temperature of the bearings, etc. will
change for a given amount of heat. So, if the
friction coefficients are the same, they will
roll the same distance, but A will be a higher
temperature at the end. Now, if the temperature
difference is enough to boil off the bearing
grease, then that changes things.
Most of the heat is in the ground, although on a
practical surface, there is substantial energy
loss in the form of media grinding -- i.e. sand
being subdivided and mixed into the viscous
pavement. (This mostly ends up in the substrate as
heat). Another substantial loss is sound
generation. The ball bearings have relatively
lower loss -- roll the skate board on a smooth
surface vs. a rough one, measure the distance --
then try with different bearings, i.e. sleve...
Sleve bearings won't last long, but will prove the
point. If the bearings had much lower heat
capacity, then by definition, the frictional heat
would cause a much larger change in operation
temperature. This would impare their function and
increase the friction substantially. However, it
is often the case that the operating temperature
range is more important than the heat capacity.
For example, car brake disks and drums are
invariably made from cast iron despite aluminum's
much greater heat capacity -- aluminum weakens at
much lower temperatures than cast iron, and at
high temperatures, the heat loss from convection
in the atmosphere is the primary means of
cooling. (Steel is not used because it has a
tendency to "creep" at such temperatures, which is
largely eliminated by the large crystals in cast
iron.) Upshot, if the skate board is run fast
enough to heat the bearings substantially, it is
more important to make sure that they operate at
that temperature than that they could store the
heat internally as added heat capacity. This would
not be true if heat superconductors existed at
This is a great question because it is difficult
to answer and raises several important issues.
It's not obvious to me, and it would be difficult
to measure, whether more energy is lost as heat
in the wheels, ground, or bearings. The primary
agent that dissipates energy is friction. Rub
your hand on something smooth like a desktop and
something less smooth like a denim pant-leg. The
rougher the rubbing, the more heat is generated.
With this in mind, the answer to your second
question is No. Heat capacity is NOT a measure of
how much energy a material will absorb. Given an
amount of added energy, heat capacity is a measure
of how hot a material will get. Thus, if
skateboard B has a higher heat capacity, it would
not necessarily absorb more or less energy than
skateboard A, but it would become less hot. A
skateboard rolling on a bumpier surface with older
(less smooth) bearings will not roll as long
because there is more friction. Whether more
energy is lost in the bearings or the ground
probably varies with the particular circumstances
of the experiment.
There are other outlets
for energy as well: The friction of air and the
skateboard, and the displacement of air by the
skateboard. Also, any deformation (bending or
denting) of the wheels, the ground, or the
bearings is a loss of kinetic energy. I can
imagine several different classroom skateboard
experiments with different wheels on different
surfaces carrying different loads.
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