n in the elastic. It is impossible to have one
force alone, there must be a pair. You can't push hard against a body
that offers no resistance. Whatever force you exert upon a body, with
that same force the body must react upon you. Action and reaction are
always equal and opposite.

Sometimes an absurd difficulty is felt with respect to this, even by
engineers. They say, "If the cart pulls against the horse with precisely
the same force as the horse pulls the cart, why should the cart move?"
Why on earth not? The cart moves because the horse pulls it, and because
nothing else is pulling it back. "Yes," they say, "the cart is pulling
back." But what is it pulling back? Not itself, surely? "No, the horse."
Yes, certainly the cart is pulling at the horse; if the cart offered no
resistance what would be the good of the horse? That is what he is for,
to overcome the pull-back of the cart; but nothing is pulling the cart
back (except, of course, a little friction), and the horse is pulling it
forward, hence it goes forward. There is no puzzle at all when once you
realise that there are two bodies and two forces acting, and that one
force acts on each body.[16]

If, indeed, two balanced forces acted on one body that would be in
equilibrium, but the two equal forces contemplated in the third law act
on two different bodies, and neither is in equilibrium.

So much for the third law, which is extremely simple, though it has
extraordinarily far-reaching consequences, and when combined with a
denial of "action at a distance," is precisely the principle of the
Conservation of Energy. Attempts at perpetual motion may all be regarded
as attempts to get round this "third law."

[Illustration: FIG. 57.]

On the subject of the _second_ law a great deal more has to be said
before it can be in any proper sense even partially appreciated,
but a complete discussion of it would involve a treatise on
mechanics. It is _the_ law of mechanics. One aspect of it we must
attend to now in order to deal with the motion of the planets, and
that is the fact that the change of motion of a body depends solely
and simply on the force acting, and not at all upon what the body
happens to be doing at the time it acts. It may be stationary, or
it may be moving in any direction; that makes no difference.

Thus, referring back to the summary preceding Lecture IV, it is
there stated that a dropped body