It really doesn't matter whether the two masses are joined by a light string, a light reinforced concrete and steel frame, a light coating of strong glue, or a layer of scotch tape that's one atom thick. They look like a single mass of 10 kg.
Part a). Force = (mass) x (acceleration)
The acceleration of 10 kg of mass pulled with 100N of force is
100 N = (10 kg) x (acceleration)
Divide each side by (10 kg):
Acceleration = (100 N) / (10 kg)
Acceleration = 10 m/s²
Part b). Now look at just the rear 16 kg. It stays hooked onto the front 4 kg, and everything moves together. So both pieces must have the same distance, velocity, acceleration etc. We know that the acceleration is 10 m/s² .
Force = (mass) x (acceleration)
Force on the rear 16 kg = (16 kg) x (10 m/s²)
Force = 160 Newtons
