Question

Explain the following : 1.telephone wires sag in summer

2.iron rims are heated before they are fixed on wooden wheels

3. The are left between the successive rails on a railway track

4. A cement floor is laid in small pieces with gaps in between

5. A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle

Answer 1

#.iron rims are heated before they are fixed on wooden wheels
Ans: Metals expand on heating and contract on cooling. This expansion and contraction of a metal is a reversible change. The metallic rim used is slightly smaller in diameter than the wheels. On heating, the rim expands. And can easily lie on the wheels. When the rim cools, it contracts and presses tightly on to the wheels.
#. They are a gap left between the successive rails on a railway track
Ans: In traditional railway construction, rails are laid down over sleepers, clamped to them and then fastened to one another by means of fishplates. These fishplates not only help the whole track structure to maintain its integrity but also ensure that a certain (small) distance between rails is kept. This is due to thermal expansion.

#.A cement floor is laid in small pieces with gaps in between
Ans: A cement floor is laid in small pieces with gaps in between to allow expansion in summer. As objects expand on heating.If the cement floor is laid without gaps then in summer floor will get cracking...

#.A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle
Ans:
Glass expands and contracts due to thermal changes. When heat is applied to the neck of the bottle that area expands slightly. This loosens the stopper. So it becomes possible to remove the stopper which is stuck in the neck of the bottle.When there is an increase in temperature every linear dimension increases by the same percentage. Even if the metal lid and the glass jar are heated equally the lid will become slightly loose because the thermal expansion coefficient of metal is greater than that of glass.