Mass of the earth has been determined through

• use Kepler's T²/R³ constancy law

• sampling the density of earth's crust and using R

• Cavendish's determination of G and using R and g at surface

• use of periods of satellites at different heights above earth's surface

A man runs towards a mirror at a speed 15 m/s. The speed of the image relative to the man is

• 15 ms^-1

• 30 ms^-1

• 35 ms^-1

• 20 ms^-1

The period of moon's rotation around the earth is nearly 29 days. If moon's mass were 2 fold its present value and all other things remained unchanged, the period of moon's rotation would be nearly

• $29\sqrt{2} \mathrm{days}$

• $\frac{29}{\sqrt{2}} \mathrm{days}$

• 29 × 2 days

• 29 days

24.

Observers on 10th, 5th and ground floor of a tall building measure the velocity of certain rain drop by some accurate method. Surprisingly the velocity of rain drop measured by the three observers is found to be same. This is because

• there is no gravitational force acting on the drop

• gravitational force on the rain drop is balanced by force produced by surrounding air

• gravitational force on the rain drop is balanced by upward force of attraction produced by the cloud

• data is insufficient to predict

The colour of the +ve column in a gas discharge tube depends on

• the type of glass used to construct the tube

• the gas in the tube

• the applied voltage

• the material of the cathode

Cathode rays are produced when the pressure is of the order of

• 2 cm of Hg

• 0.1 cm of Hg

• 0.01 mm of Hg

• 1 µm of Hg

When the displacement is one-half the amplitude in SHM, the fraction of the total energy that is potential is

• $\frac{1}{2}$

• $\frac{3}{4}$

• $\frac{1}{4}$

• $\frac{1}{8}$

When a stationary wave is formed then its frequency is

• same as that of the individual waves

• twice that of the individual waves

• half that of the individual waves

• $\sqrt{2}$ that of the individual waves

A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of 18 km/h. The speed of sound in air is 345 ms^-1. The number of beats heard by him is

• 4

• 6

• 8

• 3

A uniform plank of Young's modulus Y is moved over a smooth horizontal surface by a constant horizontal force F. The area of cross-section of the plank is A. The compressive strain on the plank in the direction of the force is

• $\frac{\mathrm{F}}{\mathrm{AY}}$

• $\frac{2\mathrm{F}}{\mathrm{AY}}$

• $\frac{1}{2}\frac{\mathrm{F}}{\mathrm{AY}}$

• $\frac{3\mathrm{F}}{\mathrm{AY}}$