Answer:
Option A. 39.2 m/s
Explanation:
From the question given above, the following data were obtained:
Initial velocity (u) = 0 m/s
Acceleration due to gravity (g) = 9.8 m/s²
Time (t) = 4 s
Final velocity (v) =?
v = u + gt
Since the initial velocity (u) is 0, the above equation becomes:
v = gt
Thus, inputting the value of g and t, we can obtain the value of v as shown below:
v = 9.8 × 4
v = 39.2 m/s
Therefore, the velocity of the ball at 4 s is 39.2 m/s. 
 
        
                    
             
        
        
        
Answer:
Efficiency is the percent of work put into a machine by the user (input work) that becomes work done by the machine (output work).
Explanation:
It is a measure of how well a machine reduces friction.
 
        
             
        
        
        
ThIs is the same type of problem
find out the time value 
3 = 1/2*a*T^2
6/10 = t^2
t = 0.77 seconds
and the distance is given 5 m 
thus speed ,= distance/time
speed = 5/0.77
= 6.45 m/s
        
             
        
        
        
Explanation:
It is given that,
Voltage of the battery, V = 12 V
Current, I = 100 ampere-hours
Energy stored is given by the product of power and time taken. So, 

P is the power, 

P = 1200 watts
This power can be used for 1 hour or 3600 seconds
Energy, 
E = 4320000 J
So, the energy stored in this battery is 4320000 J. Hence, this is the required solution.