>dbromage@metz.une.edu.au (David Bromage) wrote:
>>Terry Flynn (terry@cclru.unsw.edu.au) wrote:
>>>Craig Haber <albatross@harnessnet.com.au> wrote:
>>>
>>>>
>>>>Terry Flynn wrote:
Actually, tvarious peoples' comments were nested about 4 deep here
>>And just going off topic for a second, a steam locomotive can pull a
>>heavier train than it could ever start. Opposite to a diesel, a steam
>>locomotive has more power at speed than when starting.
Bit of confusion here between power and tractive effort.
It is true that the power expressed by a steam loco rises with the
speed (but not indefinitely, of course). But "pulling" is dependent
on tractive effort and, in common with diesel, this falls with speed
(though not as fast as with diesel and, for the 1st 10-20 mph it
doesn't fall much at all.)
The formula for power (P), in terms of tractive force (F) and velocity
(V) is
P = F * V
For the diesel, F falls off almost hyperbolically with V and so the
product of F and V is a constant.... a consequence of the constant
power development under these conditions.
For steam, F falls off at a rate that is less than hypebolic (almost
but not quite linearly over a wide range) and so power rises (if F IS
linear, the the graph of P versus V is then a parabola)
But, at any rate, F ALWAYS falls with both types as speed rises.
On the other hand, static friction ("stiction") may be higher than
kinetic friction and so it may be possible for any loco (steam or
diesel) to haul a train it cannot start (without taking up slack,
etc.). But this has nothing to do with the relationship between speed
and power/force- and it applies to any tractive unit.
Geoff Lambert