Question

why is it difficult to push a glass tumbler down into water

Answer 1

The density of air inside the glass tumbler is less than that of of water.so it experience an upthrust called buoyant force which makes it difficult to push the glass tumbler

Answer 2

Air is present in the tumbler and air exerts pressure and also requires space.Air pressure acts on the water surface and does not allow it to go down into the water.