physics about velocity

1. Since you did not give the question, I suppose car A is travelling at a uniform speed. You want to find the acceleration of car B such that it would catch up with car A before the finishing line.

Use equation of motion: s = ut + (1/2)at^2
Car A: 300 = 20t
Car B: 400 = (1/2)at^2
solving the two equations for a gives a = 3.56 m/s^2

30C. Acceleration of the 3N block = g.sin(60) N = 8.66 m/s^2
Take the acceleration due to gravity g to be 10 m/s^2

43. The given answer, option D, is correct.