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Newton's law: the rate of cooling is proportional to the temperature excess above surroundings: $dT/dt = -k(T - T_s)$. Solution: $T(t) = T_s + (T_0 - T_s)e^{-kt}$ (exponential approach to $T_s$).

For JEE, the practical average form is used: $(T_1 - T_2)/t = k \cdot [(T_1 + T_2)/2 - T_s]$. This approximation works well when the temperature interval is not too large. Common problem type: given one cooling interval, find the time for the next. Two intervals can determine both $k$ and $T_s$ (divide the two equations to eliminate $k$).

The law is derived from Stefan's $T^4$ law by linearization: when $T \approx T_s$, $T^4 - T_s^4 \approx 4T_s^3(T - T_s)$, giving the linear dependence. This is why Newton's law fails for large temperature differences (where the $T^4$ nonlinearity matters).

A cooling curve (T vs t) shows rapid initial cooling that gradually slows, asymptotically approaching $T_s$.