Answer:
Spring constant of the spring will be equal to 9.255 N /m          
Explanation:
We have given mass m = 0.683 kg 
Time taken to complete one oscillation is given T = 1.41 sec
We have to find the spring constant of the spring 
From spring mass system time period is equal to  , here m is mass and K is spring constant
, here m is mass and K is spring constant 
So 

Squaring both side 


So spring constant of the spring will be equal to 9.255 N /m 
 
        
             
        
        
        
#1. A. Waxing crescent. 
#2. 1. 
#3. C.
#4. C.
        
             
        
        
        
Explanation:
Charges,

The distance between charges, r = 10 cm = 0.1 m
We need to find the magnitude and direction of the electric force. It is given by :

So, the required force between charges is 36 N and it is towards positive charge i.e. +8 μC.