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.
- 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.