  Newton's force law $\frac{d {\bf P}}{dt} = {\bf F}$ is derived from the
Schr\"odinger equation for isolated macroscopic bodies, composite states of
e.g., $N\sim 10^{25}, 10^{51}, \ldots$ atoms and molecules, at finite body
temperatures. We first review three aspects of quantum mechanics (QM) in this
context: (i) Heisenberg's uncertainty relations for their center of mass (CM),
(ii) the diffusion of the C.M. wave packet, and (iii) a finite body-temperature
which implies a metastable (mixed-) state of the body: photon emissions and
self-decoherence. They explain the origin of the classical trajectory for a
macroscopic body. The ratio between the range $R_q$ over which the quantum
fluctuations of its CM are effective, and the body's (linear) size $L_0$, $R_q
/L_0 \lesssim 1$ or $R_q/ L_0 \gg 1$, tells whether the body's CM behaves
classically or quantum mechanically, respectively. In the first case, Newton's
force law for its CM follows from the Ehrenfest theorem. We illustrate this for
weak gravitational forces, a harmonic-oscillator potential, and for constant
external electromagnetic fields slowly varying in space. The derivation of the
canonical Hamilton equations for many-body systems is also discussed. Effects
due to the body's finite size such as the gravitational tidal forces appear in
perturbation theory. Our work is consistent with the well-known idea that the
emergence of classical physics in QM is due to the environment-induced
decoherence, but complements and completes it, by clarifying the conditions
under which Newton's equations follow from QM, and by deriving them explicitly.
