Two masses accelerate towards each other

[Janek]

Homework Statement

Hey, I have problem which I have no idea how to solve:
https://ibb.co/5n8JX6m
As you can see: There are 2 equal masses m connected by a massless string with length 2l. Now a constant force F attacks at the center of the string (at P) and pushes the string upwards so that the masses accelerate in x direction towards each other. Find a formula for the acceleration a depending on the time. Thank you for hints and solutions :)

Relevant Equations

F=ma

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[kuruman]

According to our rules, to receive help, you need to show some credible effort towards answering the question. How about telling us what you do know and how you would approach this problem? F=ma is not enough.

Please read, understand and follow our homework guidelines, especially item 4, here
https://www.physicsforums.com/threads/homework-help-guidelines-for-students-and-helpers.686781/

 

[haruspex]

Start with a diagram of the general position, not the initial position.
But I see no way to get a solution in closed form. Try to get an ODE at least.