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# In newtonian mechanics, why is the answer so?

Imagine there are two square masses A and B touching each other from one side mutually.

Mass A is of 1kg and Mass B is of 2kg.

A force is exerted on Mass A perpendicular to its surface and of magnitude 6N.

Calculate the overall acceleration of the system.

I found it to be 2, and agree with the answer.

Calculate the force A exerts on B and thus vice versa.

As shown in the image, the worked answer considers mass of B multiplied by the common acceleration.

This is of 4N.

But if I were to pick mass A... that would be 1×2=2N.

But by Newton's third law they should be both equal. So what is wrong please?

I really appreciate the help.

### 1 Answer

- Demiurge42Lv 76 years agoFavorite Answer
First off, you need to define a positive direction. Otherwise any answer you give is meaningless. If you define positive to be the direction the force is applied then you get an acceleration of 2 m/s. 2 is not an acceleration.

The force exerted on mass A is 6N.

Since mass B accelerates at 2m/s the force from mass A on mass B must be 4N.

By Newton's 3rd law the force from mass B on mass A is -4N. The net force on Mass A is 6N - 4N = 2N which is what is expected.