An uncharged metal sphere hangs from a nylon thread. when a positively chargedglass rod is brought near it, the sphere is drawn toward the rod. but if the spheretouches the rod, it suddenly flies away from the rod. explain why the sphere is firstattracted and then repelled.

Explanation:

When  a positively charged glass rod is brought near the uncharged sphere (which contains equal amount of positive and negative charge) then it started attracting metal sphere. This occurs because glass rod polarize metal sphere such that negative charge is induced at end near to the rod.

But when rod is touched to sphere it started to repel because some of the rod positive charge goes into metal sphere and thus similar charges repel each other.

measuring things 1 litre of water