Question

A net force of magnitude 36 N gives an object of mass m1 an acceleration of 6.0 m/s^2. The same net force gives m1 and another object of mass m2 fastened together an acceleration of 2.0 m/s^2. What acceleration will m2 experience if the same net force acts on it alone?

Answer 1

This is a beautiful problem to test whether a student actually understands
Newton's 2nd law of motion . . . Force = (mass) x (acceleration). 

That simple law is all you need to solve this problem, but you need to
use it a few times.

m₁ alone:
                     Force = (mass) x (acceleration)

                       36 N = ( m₁ ) x  (6 m/s²)

                         m₁ = (36 N) / (6 m/s²)   

                         m₁  =  6 kilograms .

m₁ and m₂ glued together:
                     Force = (mass) x (acceleration)

                       36 N = (6 kg + m₂) x (2 m/s²)

                     6 kg + m₂  =  (36 N) / (2 m/s²)  =  18 kilograms

                               m₂  =  12 kilograms .

m₂ alone:
                     Force = (mass) x (acceleration)

                       36 N = (12 kg) x (acceleration)

                     Acceleration = (36 N) / (12 kg)

                     Acceleration  =   3 m/s²