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2015-09-26T11:42:37+05:30

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To solve the average  force exerted by the wooden plank on bullet use the given formula :-
                   
                              v^{2} = u^{2}+2as

We don't know the average force  by the bullet , so let it be m/s²


⇒ Final velocity of bullet = v = 0 

Initial velocity of the bullet = u = 75 m/s

Distance = s = 5cm = 
 \frac{5}{100} = 0.05

⇒ Now , let us substitute the values in the formula :-
 
                      
 v^{2} = u^{2} + 2as
 
                      
 0^{2} =   75^{2} + 2 a ( 0.05)
 
                      0=5625+0.1a
 
                      
-5625 = 0.1a
   
                      
 \frac{-5625}{0.1}=a
 
                     -56250 = a

→ Therefore , we get acceleration as -56250 m/s²

⇒ Now , let us find the force :-
 
                     f=ma
 
                     f =  0.02 kg × ( - 56250 )
 
                     f = -1125 Newton 

Hence the average force is -1125 N
 

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2015-09-26T22:47:54+05:30

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M=20 g=20/1000=.02 kg
v=0 m/s,since the bullet comes to rest
u=75 m/s
s=5 cm=5/100 m=.05 m
a=?
To find  we can use v^2-u^=2as
0-75^2=2*a*.05
-5625=.1a
a=-5625/.1=-56250
to find the resistive force  use this formulae
f=ma
f=0.02*-56250=-1125N
therefore the average resistive force is -1125 Newton

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