Commercial airplanes are sometimes pushed out of the passenger loading area by a tractor. (a) The 1800 kg tractor exerts a force of 1.90 104 N backward on the pavement, and the system experiences forces resisting motion that total 2400 N. If the acceleration is 0.130 m/s?, what is the mass of the airplane in kilograms? ____kg (b) Calculate the force (in N) exerted by the tractor on the airplane, assuming 2100 N of the friction is experienced by the airplane. ____N