Which of the four cars had the least net force applied to it when it was launched down the ramp

The least net force applied : Car 3(12 N)

Newton's 2nd law explains that the acceleration produced by the resultant force on an object is proportional and in line with the resultant force and inversely proportional to the mass of the object  

∑F = m. a  

Car 1 ⇒m=0.5 kg, a=36 m/s²

Car 2⇒m=0.8 kg, a=50 m/s²

Car 3⇒m=0.6, a=20 m/s²

Car 4⇒m=1, a=19~m/s²

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.