Infinite weight? :p

Sorry (but since it was a

prefect world and all). Maybe not the best starting point. I would think it depends on how fast you want it to get to a certain speed. Not really my forté(?), I'm not sure how to calculate this. I think it would be dependent on things like wheel radius too.

T = F * L (in my simple mind at least, also I found it is equal to angular acceleration * inertia).

also

F = m * awhere:

T = Torque

L = arm length where force is applied

F = Force

a = acceleration

m = mass

so

a = F/m, up to a certain speed for your motor. Except minus some friction resistance, which is the big question as to how to get. Once you get that somehow, its probably just a matter to juggle the equations a bit to get a ballpark torque for your motor?

Or something like this. I'm sure a google could provide better answer. Or maybe a

Wolfram Alpha.... and since this was interesting, I googled up

Rolling resistance According to this, it is:

F = C_{rr} * N_{f}And

F = N_{f}*b / rand also

b = C_{rr} * rwhere

C_{rr} is the dimensionless rolling resistance coefficient or coefficient of rolling friction (CRF)

N_{f} is the force normal to the surface.

F is the rolling resistance force

r is the wheel radius

b is the rolling resistance coefficient or coefficient of rolling friction with dimension of length

(The above basically taken from wikipedia.)

Now I don't know anything about what kind of wheels you use, the radius or substance they roll against, so this is a guesstimate. According to the rolling friction table at the above link, I guess on a C

_{rr} of about 0.005 for your wheel thing.

It would then requre a force of:

5 lbs = 2.268 kg

F = 0.005 * 2.268 kg * 9.8 m/s/s = 0.111 N

This is just to be moving (rolling) horizontally. If you got 4 wheels I suppose they would add up to the same. Plus a bit for friction on the bearings.

That is part 1... Then part 2; how fast do you want to get up to a certain speed? Which is back to the F = m * a formula

Say 5 seconds to get to a speed of 1 m/s...just to pick some numbers. This is probably too slow an acceleration though.

Which indicates an acceleration of:

v = a * tso

a = v/t = 1/5 m/s/s

Where

v = speed

a = acceleration

t = time

requiring a force of

F = m * a = 2.268kg * 1/5 m/s/s = 0.454 N

Add this to the rolling resistance force required:

F

_{tot} = 0.457N + 0.111N = 0.565 N

Now for a wheel radius of say 5 cm (just to pick another number..), you would need a torque of:

T = F

_{tot} * L = 0.565N * 0.05m =

0.0283NmSeems like a pretty low number, but I also picked a pretty low acceleration. At least I think so.

[glow]Be warned:[/glow] I got virtually no experience with motors and the like.. I don't even know if this is a practical number for torque, or indeed if this is correct-ish. But if it is, maybe it could be a "starting point" to fill in your actual numbers.