Newton's laws of motion

Question

a horizontal force F pushes a 4 kg block which pushes against a 2 kg block as shown the blocks have an acceleration of 3M per second square to the right there is no friction between the blocks and the surface on which the slide what is the net force be exerts on A

Answer 1

Here I rewrite the problem with some corrections and also present a possible answer.

Question 

A horizontal force F pushes a 4kg block A which pushes against a 2kg block B. The blocks have an acceleration of 3m/s^2 to the right and there is no friction between the blocks and the surface on which they slide. What is the net force that B exerts on A?

Answer

Let  B exerts force f on A then A also exerts same force on B. 

Hence considering the net force on A we can write   F - f = 4×3 N

Again considering the net force on B we can write   f =  2×3 N= 6 N

Hence B exerts 6 N force on A

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