On Mar 17, 5:00*pm, Bob Cook wrote:
At 18:16 17 March 2009, The Real Doctor wrote:

Ian,

YOU did use the term "power" correctly.

What I didn't like about it was that the question I posed referred to
force. *(OK, so you gave us some additional information)

I agree that a sailplane, in gliding flight, in still air, *has no
"power" at all. *(Although, as you said, some wrongly believe that
gliders are "gravity powered")

By some of the responses, I think I am correct in assuming that some
confuse power, energy, and force.

So I again ask, (not to Ian, but to some of the others who answered my
question with "gravity",) how can gravity alone, a force which acts
vertically downward, impart forward motion to a glider or anything for
that matter?

Cookie



I don't. I used "power" because I meant "power"!

Ian



Sigh. No a glider moving through still air even at a constant rate
expends power. A glider has gravitational potential energy, the rate
it converts that potential energy to overcome drag is an expenditure
of power and you can talk about that as horsepower or kW etc. We plot
these things now as polars, just with different display units.

As I explained, Gravity provides the energy but the forward motion
comes from the forward component of the lift vector. The large part of
the lift vector is resisting the pull of gravity and as a side effect
the horizontal part moves the glider forward, and conveniently the
glider needs to move forward to create lift. Lucky thing that.

Analogous to the chicken sliding down a ramp, gravity there also only
acts through the center of mass yet the chicken does not fall
vertically. I think this is obvious to any chicken sliding down a
ramp. You need a chicken and a ramp and do the experiment then ask the
chicken to explain why.

Darryl