Forces affecting m 1 : m 1 g − − T = m 1 a Forces affecting m 2 : T − − m 2 g = m 2 a and adding the two previous equations yields m 1 g − − m 2 g = m 1 a + m 2 a , and the concluding formula for acceleration a = g m 1 − − m 2 m 1 + m 2