Stars appears to move from east to west because
Answer: B
Because the earth rotates from west to east.
The apparent motion of the sky is the reflection of that rotation, so the stars rise in the east and set in the west just as the sun does. The whole sky appears to turn once in a sidereal day of 23 hours 56 minutes.
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Large astronomical telescopes always use as objective a
Answer: C
Large astronomical telescopes use a mirror as the objective.
A big lens can be supported only at its rim, so it sags under its own weight, and it also suffers chromatic aberration. A mirror can be supported across its whole back, is free of chromatic aberration, and needs only one surface figured. The largest refractor ever built is 1 m across; mirrors now exceed 10 m.
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Sound travels with a different speed in media. In what order does the velocity of sound increase in these media?
Answer: C
The order is air, water, iron.
Speed is \(v = \sqrt{E/\rho}\), and the elastic modulus rises far more sharply from gas to liquid to solid than the density does. Sound travels at about \(343\ \mathrm{m\,s^{-1}}\) in air, \(1{,}500\) in water and \(5{,}100\) in iron.
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Moment of inertia is:
Answer: D
Moment of inertia is a tensor.
For rotation about a fixed axis a single number will do, but in general the angular momentum is not parallel to the angular velocity and a \(3\times3\) inertia tensor is needed to relate them. Only about a principal axis do the two line up.
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Stars which appear single to the naked eye but are double when seen through a telescope are
Answer: B
Such stars are binaries.
A binary is a pair of stars bound by gravity and orbiting their common centre of mass, too close together to be separated by the eye but resolved in a telescope. More than half the stars in the sky are thought to belong to multiple systems.
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On a cold day when a room temperature is 15oC, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of 15oC, because
Answer: C
Because metals are good conductors of heat.
Both parts are at 15 °C, but the metal cap draws heat away from the finger far faster than the plastic body does, and it is that rate of heat loss the nerves register as cold. The sensation reports conductivity, not temperature.
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Sound waves in air are
Answer: B
Sound waves in air are longitudinal.
The air molecules oscillate along the direction of travel, producing compressions and rarefactions. This is why sound cannot be polarised: polarisation requires vibration perpendicular to the direction of propagation.
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The intensity of sound at a point is ______ its distance from the source.
Answer: D
Intensity is inversely proportional to the square of the distance.
A point source spreads its power over a spherical wavefront of area \(4\pi r^2\), so \[I = \frac{P}{4\pi r^2}\] Doubling the distance therefore cuts the intensity to a quarter.
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The absorption of ink by blotting paper involves:
Answer: B
Blotting paper works by capillary action.
Its loosely packed cellulose fibres leave fine pores between them. Ink wets the fibres, so adhesion exceeds cohesion and surface tension draws the liquid into the pores - the narrower the pore, the stronger the pull.
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Materials for rain-proof coats and tents owe their water-proof properties to
Answer: A
Waterproofing depends on surface tension.
The fabric is treated so that water does not wet it: the contact angle becomes obtuse, cohesion within the drop exceeds its adhesion to the fibre, and surface tension pulls the drop into a bead that rolls off instead of soaking through the pores.
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