Question

A 70 kg man pushes a 50 kg man by a force of 15 newton by what force has the 50 kg man pushes the other man

Answer 1

is really interesting question .
everyone think that here we have to use   e.g., action and reaction are of equal in magnitude but directed in opposite direction. 
this way answer should be 50N.
means, 50kg man pushed the other man by 50 N force .but it's wrong .

Here we have to use .
here 70kg person pushes a 50kg man by a force of 50N . 
Let F = 50N 
now, normal reaction act on 50 kg man = mg = 500N { see figure } 
we also know, friction is directly proportional to normal reaction then, 
fr = kN , k is coefficient of static friction and N is normal reaction.
now, for balancing fr = F 
so, 50 = kN = k × 500
k = 0.1 , hence coefficient of static Friction = 0.1

now, Let F' force act by 50kg man on 70 kg man 
then, F' = static Friction act on 70kg man
= coefficient of static friction × normal reaction act on 70 kg man
= 0.1 × 70 × 10 [ g = 10 m/s² ] 
= 70N 

hence, answer should be 70N

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