The figure describes a chain placed at the edge of a table. Half the chain is lies on the table and the other half is hanging down.
Assume the mass of the chain is uniformly distributed. When the chain is released, the portion lying on the table is not affected by the gravitational force of attraction of the earth; the portion hanging down however is pulled down with a force equal to m*g where m is the mass of the portion hanging down.
As the portion lying on the table is connected to that hanging to the side, the acceleration due to the force is equal to `(m*g)/(2*m) = g/2` .
The initial acceleration of the chain when it is released is g/2 or 4.9 m/s^2