Let’s analyze this physically. When b = 0 the response is a sinusoid. Damping is a frictional force, so it generates heat and dissipates energy. When the damping constant b is small we would expect the system to still oscillate, but with decreasing amplitude as its energy is converted to heat. Over time it should come to rest at equilibrium. This is exactly what we see in (3). The factor cos(ωdt − φ) shows the oscillation. The exponential factor e−bt/2m has a negative exponent and therefore gives the decaying amplitude. As t ∞, the exponential goes asymptotically to 0, so x(t) → also goes asympotically to its equilibrium position x = 0. We call ωd the damped angular (or circular) frequency of the system. This is sometimes called a pseudo-frequency of x(t). We need to be careful to call it a pseudo-frequency because x(t) is not periodic and only periodic functions have a frequency. Nonetheless, x(t) does oscillate, crossing x = 0 twice each pseudo-period. ..