Applying the first law of thermodynamics to the lumped object gives d U d t = − − Q ˙ ˙ , where the rate of heat transfer out of the body, Q ˙ ˙ , may be expressed by Newton's law of cooling, and where no work transfer occurs for an incompressible material. Thus, d T ( t ) d t = − − h A C ( T ( t ) − − T env ) = − − 1 τ τ Δ Δ T ( t ) , where the time constant of the system is τ τ = C / ( h A ) . The heat capacitance C may be written in terms of the object's specific heat capacity, c (J/kg-K), and mass, m (kg). The time constant is then τ τ = m c / ( h A ) .