Answer:
By the use of slow motion camera. 
Explanation:
Visually, it is very hard to differentiate between an ac and dc power supply. But Since, we that In Ac supply polarity changes 100 times in a second ( because frequency of ac supply is 50 Hz generally). Whereas, Dc gives a steady power supply. So, in slow motion camera we can easily capture the flickering light tubes which won't happen in case of dc supply. 
 
        
                    
             
        
        
        
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Answer:
50 N. 
Explanation:
On top of a horizontal surface, the normal force acting on an object is equivalent to the force of gravity acting on the object. That is: 

The mass of the block is 5 kg and the given force due to gravity is 10 N/kg. Substitute and evaluate: 

In conclusion, the normal force acting on the block is 50 N. 
 
        
             
        
        
        
Answer: dakdadakdadakdadakda
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