Nigel,

There are also 2 positrons formed in the fusion reaction. These are particles which have the mass of an electron and a positive charge. That accounts for all 4 of the positives and as you pointed out, charge is conserved. Actually, hydrogen fusion was first produced on earth 50 years ago. It's called a hydrogen bomb, and the problem is it takes the detonation of an atomic bomb to start the reaction. So I guess our real difficulty is producing a _controlled_ hydrogen fusion reaction which can be sustained, because controlled fusion has also been produced in the laboratory. Not much help because the reaction lasts much less than one millionth of a second and takes more energy to start than it produces.

jbgood,

Good point, I was referring to _available_ energy. Of course, all mass is equivaent to energy but we can't avail ourselves of it (yet).

Kim,

This is another slightly more complicated situation (like the toy car and friction mentioned earlier) because gravity is involved. Try thinking of it like this.

Action: you push dumbbell. Reaction: dumbbell pushes you. The mass will _ALWAYS_ push back when you push it, even if there is NO gravity.

When gravity is involved, it must be overcome before an object can be lifted. You grasp the 10 pound dumbbell and start to pull - 1 pound, 2 pounds, 3 pounds ... - the dumbbell doesn't move. When your force (pull) builds up to a tiny amount over 10 pounds (we need that little bit extra to overcome the inertia of the DB), the DB starts to move, and any additional force you apply will continue to _CHANGE_ the motion of the DB - in other words it will accelerate. If you _maintain_ the 10 pounds of force, then the TOTAL force on the DB will be zero - 10# up (by you) plus 10# down (by gravity) - and it will rise at constant speed.Remember, the reaction force doesn't enter into this at all, because it is not acting on the DB.

If we take gravity away the situation is simpler (one less complication to worry about) - and only a very tiny force is needed to move the 10# weight. A larger force will simply move it faster (more acceleration). This is why astronauts orbiting the earth can't get their exercise by weight lifting. In orbit the _effects_ of gravity are negated, so even the tiniest force will move the largest mass. While working on the construction of the space station, astronauts routinely push around large structures they couldn't budge if they had to overcome the effects of gravity first.

To summarize: Applying 11 pounds of force to a 10# DB on earth produces a certain motion. If I can eliminate the effect of gravity, I can get exactly the same motion of the 10# DB with only 1 pound of force.