In a study comparing the efficacy of two diets, the mean weight loss for group A ($n_1=30$) is $\bar{x}_1 = 5.2$ kg, and for group B ($n_2=30$) is $\bar{x}_2 = 4.1$ kg. If the pooled standard deviation $s_p = 1.5$, calculate the Cohen's $d$ effect size.