Two blocks, A and B, of masses m and 3m respectively are accelerated along a smooth surface by a force F applied to block A. What is the magnitude of the force exerted by block A on B during this acceleration?

Question

silverlark2 · Answer

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mathslover · Answer

well first of all we are going to use Newton's second law of motion . Do you know what it states?

mathslover · Answer

@Silverlark2 ?

anonymous · Answer

The force should be $$F \div4$$ according to newton's third law. Got a question like in CIE MCQ papers....

silverlark2 · Answer

@mathslover yeah I do. F=ma. But how to apply?

mathslover · Answer

Total mass = 3m + m = 4m 

put that : F = ma = (4m) a 

a = F/4m

mathslover · Answer

Then apply Newton's Second Law specifically to block B to find the force acting on B. This gives:: 

$F_B = m_B a_B$ 
$F_B = 3m * F /4m$ 
Just remember that for block A, the forward force is  but according to Newton's Third Law, block B exerts a force of  on it. Therefore, its resultant force is , which keeps the acceleration of both blocks the same.

mathslover · Answer

I took the answer from : http://www.thestudentroom.co.uk/showthread.php?t=370704 Check that there. it is nicely explained.