Answer:

Explanation:
Given:
- Three identical charges q. 
- Two charges on x - axis separated by distance a about origin
- One on y-axis
- All three charges are vertices
Find:
- Find an expression for the electric field at points on the y-axis above the uppermost charge.
- Show that the working reduces to point charge when y >> a.
Solution
- Take a variable distance y above the top most charge.
- Then compute the distance from charges on the axis to the variable distance y:
                                   
- Then compute the angle that Force makes with the y axis:
                                  cos(Q) = sqrt(3)*a / 2*r
- The net force due to two charges on x-axis, the vertical components from these two charges are same and directed above:
                                  F_1,2 = 2*F_x*cos(Q)
- The total net force would be:
                                 F_net = F_1,2 + kq / y^2
- Hence,
                                 
- Now for the limit y >>a:
                               
- Insert limit i.e a/y = 0
                               
Hence the Electric Field is off a point charge of magnitude 3q.
 
        
             
        
        
        
Step 1: list known info
distance(change in position (Δx))= 18m+22m= 40m
time= 20 seconds
Step 2 :solve for velocity
velocity= Δx÷time
v= 40/20= 2m/s
Answer: the velocity is 2 meters per a second (m/s)
        
             
        
        
        
C. Magnetic draws certain types of metal together.
        
                    
             
        
        
        
Answer:
 The potential energy stored in the spring is 0.018 J.
Explanation:
Given;
spring constant, k = 90 N/m
extension of the spring, x = 2 cm = 0.02 m
The potential energy stored in the spring is calculated as;
U = ¹/₂kx²
where;
U is the potential energy stored in the spring 
Substitute the given values in the equation above;
U = ¹/₂  x  90 N/m  x  (0.02 m)²
U = 0.018 J
Therefore, the  potential energy stored in the spring is 0.018 J.
 
        
             
        
        
        
Answer:C
Explanation:
Partially submerged block along with vessel is accelerated upwards .
Initially the block weight is supported by buoyant force such that it is in equilibrium.
when the system start accelerating upwards then the effective gravity will be 
g+a where a is the acceleration of the system.
so only net gravity is increased so block will not ascend or descend.
Mathematically 


where  density of liquid
density of liquid
V=volume of object inside the water