If the box is moving at constant velocity, net force must be zero, so:
F + fr = 0
fr = -F
<u>fr = -40 N</u>
 
        
                    
             
        
        
        
Answer:
12500W
Explanation:
Given parameters: 
Work done  = 250000J 
Time taken  = 20s 
Unknown: 
Power of the crane = ? 
Solution: 
Power is the defined as the rate at which work is being done; 
   Mathematically; 
         Power =  
 
  insert the parameters and solve; 
       Power  =  = 12500W
   = 12500W
 
        
             
        
        
        
They create energy ……………….