... but you didn't ask about creatures ...
For vehicles, only the rolling friction of *the assembly* will make a
difference in the energy requried. If you add more weight to have two
self-supporting structures, and do nothing about rolling resistance,
then you simply require more energy to offload the load from the
vehicle to the trailer.
Beyond this, terrain, load placement, and design flexibility will
affect stability, safety, and the ability to get people to buy.
David A. Smith
per Mr. Smith's analysis
.......For vehicles, only the rolling friction of *the assembly* will
difference in the energy required.........
so if you add the weight to the vehicle vs adding a trailer....the
amount of energy usage increase would depend on which configuration
had higher total rolling resistance.
Now a trailer behind a pickup truck would have increase (slightly) the
total skin drag over a truck alone but I would guess that it increase
would be small.
It's a matter of where the energy is lost....
frontal area drag, rolling resistance, skin drag......(I think these
are in order of relative contribution, but it's been a while since
I've done these calcs)
If one looks at transportation schemes..."trailers", in one form or
another seem pretty popular, but that might be a labor saving device
more than an energy issue.
I guess I'm thinking more in the case of a farm implement. For years
we've been buying big tractors, filling the tires with fluid and
adding large weights to them so we have enough traction to be able to
drag a heavy implement behind. In this case aerodynamic drag is
irrelevant due to the slow speeds. By removing the weights from the
tractor's wheels and replacing it with an implement mounted right on
the tractor rather than being dragged behind the whole machine should
move exactly the same but take less energy to do so (because there is
no additional load due to friction and we lose whatever additional
weight was added for traction). Does this make sense or am I not
looking at this right? If I am wrong why has everyone moved to self-
propelled combines versus pull-type?
I've been trying to think of a real life analogy. It does indeed seem
easier to pull a child in a sleigh 6 blocks than it would be to carry
her. But is the actual energy expended any less when pulling? Could
it be that my body just isn't built to hold the weight of the child
and walk 6 blocks and so I am under more stress but not necessarily
running out of energy?
Liquid-filled tires tend to reduce rolling resistance (I think), but
do horribly increase "unsprung weight"... which won't matter for any
vehicle under 15 mph.
The implement may or may not have wheels. There is friction there as
well. You simply move where it affects the system.
Lazy. Increase the apparent value, to "get a leg up" on the
No, it is very likely more. But your body does a significant amount
of work in simply walking that is not associated with road friction.
*Still* running out of energy. Add a child-equivalent amount of
weight to your body, say around the belly. You get used to it.
Walking friction is not large, but the effort to raise and control
lowering of limbs, and joint friction is where the extra energy is
expended. Walking friction can be measured by where you center of
mass is vs. where you are in contact with the ground. Energy expended
in producing this motion is always greater.
Draw a box around your tractor with weights and your tractor with
implement. How does it interact with the world outside this box?
David A. Smith
No need for a cryptic engineering repsonse here. What you're saying
makes perfect sense. By removing the weights you're moving less
mass. By putting the implement (assuming the implement had wheels) on
the tractor, you're decreasing rolling friction. Comparing a rolling
machine to a human body is not really an ideal analogy just because of
the differences in geometry, also there is no need to make the
comparison to begin with. If you can get the job done with less mass
moving, you win.
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