Science Que-Ans (Each of 2 Mark )

Neils Bohr proposed a model of atom in 1912 to remove the drawbacks of Rutherford's atomic model and to explain the structure of an atom in detail. The features of Bohr's model of atom are given below:-
i) An electron revolves in the orbit of atom with well-defined energy.
ii) Energy of orbits increases from the inner shell to the outer shells i.e. energy of orbit nearest to the nucleus is the lowest.
iii) If energy is supplied to an electron, it moves from a lower orbit to a higher orbit. When an electron moves from a higher orbit (energy level) to a lower orbit (energy level), energy is radiated as electromagnetic waves.
iv) Each orbit or shell represents an energy level. Such orbits are represented as K,L,M,N,O (named starting from the centre of an atom).
v) The shell or orbits are associated with a certain amount of energy and energy of orbits/shells increases from the nucleus towards the valence shell.

a. a stone of the same size will have more inertia than a rubber ball.
b. A train will have more inertia than a bicycle.
c. A five rupees coin will have more inertia than a one-rupee coin.
Explanation: Inertia is associated with mass. Inertia is directly proportional to the mass of a body. Objects having more mass have more inertia.

If the time gap between the original sound and reflected sound received by the listener is around 0.1s, only then the echo can be heard.
Velocity of sound in air - 344 m/s.
Time interval=0.1 s
Therefore, The minimum distance travelled by the reflected sound wave for the distinctly listening to the echo Velocity of the sound Time interval 344 x 0.1-34.5 m
Since, the girl is sitting in middle of a park of dimension 12 mx12 m, Therefore, The distance travelled by the sound reflected from the building and then reaching to the girl (6+6) 12 m, which is much smaller than the required distance. Therefore, Girl will not hear any echo of sound.

In a pressure cooker, pressure is raised by preventing the vapours formed inside to escape from the closed cooker. This causes water to boil at much higher temperature than its normal boiling point. Due to this reason, vegetables can be cooked bener and faster in a pressure cooker.

In such a case when the density of solid is exactly equal to that of the liquid, it will remain in equilibrium and keep floating in it The solid will float in such a way that the entire part of it will remain within the liquid, its upper surface coinciding with the liquid surface.

The mass of the balloon filled with helium is less than the mass of the air displaced by it. Hence upthrust acting on the balloon is more than its weight. As a result the balloon experiences a net upthrust which makes it rise. As the balloon rises it experiences lesser and lesser upthrust due to the fact that with height the density of air decreases. At a certain point the weight of the balloon may be completely balanced by the upthrust acting on it. Thereafter the balloon stops rising.