6.A bullet of mass 4g when fired with a velocity of 50m/s,can enter a wall up to a depth of 10cm.How much will be the average resistance offered by the wall?

This problem they are asking force of wall...I mean opposite force..for that u need acceleration of wall..use 3Rd law of equation 2as=v^2 -u^2 S= 10 cm and convert this in to meter you will get 0.001m Here v=0 and u=50 m/s Substitute this value in 3Rd equation you will get acceleration of wall that is a=-125000 this negative acceleration and Come to newton 2nd law f=ma Here m=4g =4/1000 =0.004kg a=-125000 you will force F=0.004x-125000 F=-500N

see,  mass = 4 g
        velocity= 50 m/ s 

we will find the momentum of the bullet i.e. momentum= product of mass and velocity 
                             
therefore                                          momentum= 4×50
                                                                              = 200g m/ sec


so the average resistance offered by wall is 200g m/sec 

the reason behind this is the third law of motion of newton i.e. to every action there is an equal and opposite reaction . 
   

conservation of momentum

sorry u have to convert gram into kg so the answer will be 0.2 kg m/sec

0.2 kgms

I think that the answer should be 50N