How much tension is there along two strings connecting objects when one is pulled by a force 
You are given three objects with mass $M$, $2M$ and $4M$ which are connected through strings as shown in the shape. A constant force $F$ is being applied to the object of mass $M$ on the right. There is no friction. Calculate the tension $T_1$ and $T_2$, of the two strings with respect to the applied force $F$.
I have tried looking up the definition of tension and this is what I have found https://www.physicsclassroom.com/class/newtlaws/Lesson-2/Types-of-Forces
I think that the tension to these objects is $F$, as the force $F$ is being applied to all of them. I am not sure though. I have tried looking it up to work it out on my own, however I am just getting confused. Could you please tell me if my reasoning is correct and if it isn't, why it isn't and what is the correct answer and reasoning?
 A: The tensions are not the same because although the acceleration of all the masses is the same, the net force on each mass is different because the masses are different, making the tension different.
Since all the masses are connected together their acceleration is the same, which you can obtain from Newton's second law using $F$ and the sum of the masses.
The net force on each mass can be obtained with a free body diagram for each and applying Newton's second law to each. To determine the tensions in the two strings, you need only perform a FBD on the first and second mass and apply Newton's second law to each. That will give you the tension in the two strings in terms of $F$. The net force on the third mass is simply the tension of the string between the second and third mass.
Hope this helps.
A: No , you are doing a mistake.
If you take tension in each string to be equal to $F$ then for sure $M$ will not accelerate since $F$ gets balanced by the tension (say $T_1$) . Now $2M$ experiences $T_1$ and $T_2$ ($T_2$ from the other string) and according to you both have same magnitude $F$. So it should also not move.  Now coming to the body of mass $4M$ , it has only one force acting on it $T_2$ (which is towards $2M$) , so it will accelerate towards $2M$ for some moments and the moment string becomes loose , it stops accelerating and will collide with $2M$ and eventually both $4M$ and $2M$ collides with $M$ which are not mentioned in the question.
The main point here is that since the bodies are connected by strings (inextensible) they must move with same acceleration otherwise it would break or become loose and since these are not mentioned in the question , so we should not consider them .
Now you know all the masses move with the same acceleration , so first of all you can calculate their common acceleration by taking all three as a system of mass $7M$ and then finally apply Newton's second law to each of those bodies considering all the forces on them.
Now you should be able to proceed since I can't give complete answer to such questions.
Hope it helps ☺️.
