fisher@dvinci.DEC (03/01/85)
<Go ahead, atomize this line and make my day...>
uwvax!derek asks
>~"how does energy get transferred from the photon to the sail..."
The problem with your explanation is that it does not consider the vector
nature of momentum. Rather than deal with vector equations, let's consider
a one-dimensional system so I can deal only with + and -.
The photon is plowing along with momentum +mv (m=mass of photon, v=velocity of
photon). It smashes into a stationary sail (momentum 0) in a totally inelastic
collision and bounces back, now with momentum -mv. Since momentum is
conserved, the total {photon, sail} system must still have a total momentum of
+mv. The momentum difference between the photon after the collision and the
total system before the collision is (-mv)-(+mv) or -2mv. Thus the solar sail
(the only other component of the system in this simple model) must now picked
up a momentum of +2mv, and thus a velocity of +2mv/M (M is the mass of the
sail).
Note that if the photon "sticks" to the sail (ignoring kinetic->heat
conversion), we have a much different situation. The total sail/photon system
still has a momentum of +mv, but now, since they are travelling together, the
velocity change of the sail is only mv/(M+m), (slightly) less than half as
much!
Now (wave hands) consider a two or three dimensional system, and you see that
if the photon is absorbed, the sail can only change velocity in the direction
that the photon was moving. If the photon is reflected, you can "tack" by
tilting the sail and forcing the photon to bounce off at a non-pi angle, thus
generating a velocity change in the sail at an angle.
Ta da! Not bad for having not dealt with this stuff (except in sci-fi books)
since freshman physics!
Burns
UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher
ARPA: decwrl!rhea!dvinci!fisher@{Berkeley | SU-Shasta}al@ames.UUCP (Al Globus) (03/04/85)
> > Now (wave hands) consider a two or three dimensional system, and you see that > if the photon is absorbed, the sail can only change velocity in the direction > that the photon was moving. If the photon is reflected, you can "tack" by > tilting the sail and forcing the photon to bounce off at a non-pi angle, thus > generating a velocity change in the sail at an angle. > Actually, it turns out that the accelleration of the sail is ALWAYS normal to the plane of the sail. If I could draw on this thing (UNIX) the explination would be obvious, but I'll try anyway. This explanation is intuitive for me but may miss some basic physics (I never took it). The result is correct however. Assume an elastic collision. When the photon hits the sail it imparts momentum in the direction the photon is going. When the photon leaves the sail, it imparts momentum by 'action-reaction' opposite to the direction it leaves the sail in. If you sum the two vectors you get a vector normal to the plane of the sail.