# Which takes less energy? Pulling a load or carrying it?

Which would require more energy: A vehicle pulling a load behind or carrying the same load on it's own chassis? Would it depend on the
terrain? i.e. hard ground versus soft ground?
Does anyone know of any published studies in this regards?
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http://en.wikipedia.org/wiki/Travois ... 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
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per Mr. Smith's analysis
.......For vehicles, only the rolling friction of *the assembly* will make a difference in the energy required.........
I agree
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.
cheers Bob
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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?
Thanks,
Larry
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Dear Larry767:
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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 competition.

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
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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.
Larry767 wrote:

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