Solution

Both boxes are being accelerated forwards by an unbalanced force of 12 N.

First, calculate the acceleration caused by this unbalanced force:

F = ma

12 = 6a you are calculating the acceleration of the whole system i.e. (2 + 4)kg

a = 2 m s-2

Next figure out the unbalanced force that is pulling the 2 kg box forwards, this is the tension between the two boxes:

F = ma

F = 2 x 2 = 4N final answer - B

Note: If you calculate the unbalanced force acting on the 4 kg only then it will be 4 x 2 = 8 N. In other words, the total unbalanced force adds up to the original 12 N given in the question. So in any situation like this, whether it involves two objects or more, calculate the combined acceleration of all the objects first; they are joined together so they will move together with the same acceleration. This value can then be used to calculate the individual tension between each of the joined objects – the sum of which will be the total unbalanced force pulling on the objects.