a metal sphere is suspended from a nylon thread .initially metal sphere is uncharged .when a positively charged glass rod is brought close to the metal sphere,the sphere is drawn towards the rod.but if sphere touches the rod ,it flies away.explan why sphere is first attracted then repelled?

well this is bcoz when the uncharged ball is brought near the positively charged rod a negative charge is induced in the sphere which causes attraction but when the rod is touched to the sphere the sphere also becomes positively charged due to flow of electron and thus repulsion takes place

doesnt the flow of e- neutralise the rod in this case?

this is bcoz initially an equal and opposite charge is introduced inboth of the objects due to induction.

but when the glass rod touches the surface of the sphere then there is an equal charge of same magnitude and chagre is produced in both the objectsdue to which they repell each other.

This is because due to electostatic induction a negative charge develops on the near end of the sphere and a positive on the farther end. Unlike charges attract each other so the sphere is drawn towards the rod. When both bodies are in contact the free electrons from the sphere move towards the rod giving the sphere an overall positive charge. like chargs repel eachother so the sphere moves away from the rod.