please, give the question properly.
 
        
             
        
        
        
The weight of an object is taken to be the force on the object due to gravity. The weight ( W ) is the product of the mass ( m ) of the object and the magnitude of the gravitational acceleration ( g ).
On Earth: g = 9.81 m/s²
m = 20 kg
W = m · g = 20 kg · 9.81 m/s² = 196.2 N
        
             
        
        
        
Rubbing both pieces cause each piece to have a negative charge.
When two parts have the same they repel each other, so holding one piece up tot he end of the other piece would push it away. 
Because one piece is held in the middle by a string, it would rotate the piece in a circle.
 
If they held the piece to the other end of the one held by a string it would start to rotate in the opposite direction.
 
        
             
        
        
        
One side of the wave changes speed before the other side, causing the wave to move