Why does a person feel weightless inside a freely falling lift ?
Answer: C
Correct answer: The lift and the person fall with the same acceleration, so the floor exerts no reaction
Weight is felt through the normal reaction of the floor, and when both fall together at \(g\) that reaction drops to zero. Gravity continues to act throughout, which is precisely why they are falling.
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Heat is associated with
Answer: A
Heat is associated with the kinetic energy of the random motion of molecules.
Temperature measures the average of that random kinetic energy. Orderly motion of the whole body is not heat but bulk kinetic energy: a moving cricket ball is not hotter for moving.
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A woman’s voice is shriller than a man’s voice due to
Answer: C
Because of the higher frequency of a woman's voice.
Pitch is determined by frequency. Women's vocal cords are shorter and thinner, so they vibrate faster - typically 165–255 Hz against 85–180 Hz for men. Amplitude governs loudness, not shrillness.
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Persistence of vision is the principle behind?
Answer: A
Persistence of vision is the principle behind cinema.
An image lingers on the retina for about a sixteenth of a second after the light is cut off, so still frames shown faster than that merge into apparently continuous motion. Film runs at 24 frames a second for this reason.
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On a stationary sail boat, air is blown from a fan attached to the boat. The boat
Answer: B
The boat does not move.
The fan pushes air backwards and the air pushes the fan - and hence the boat - forwards; but the moving air then strikes the sail and pushes it backwards with an equal force. The two internal forces cancel, and with no external force the boat stays put. Removing the sail would let it move.
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Force of attraction between the molecules of different substances is called
Answer: A
This is adhesive force.
Adhesion acts between molecules of different substances - water and glass, ink and paper. Cohesion acts between molecules of the same substance. Whether a liquid wets a surface or beads up depends on which of the two is stronger.
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When a body falls in viscous medium, its velocity first increases and finally becomes constant. The constant velocity is called
Answer: D
This constant speed is the terminal velocity.
The body accelerates until the viscous drag and buoyancy together balance the weight, after which the net force is zero. For a sphere obeying Stokes' law, \[v_t = \frac{2r^2(\rho - \sigma)g}{9\eta}\]
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The orbital speed of a satellite depends on its mass.
Answer: B
False. Orbital speed does not depend on the satellite's mass.
Equating gravitational and centripetal force, \[\frac{GMm}{r^2} = \frac{mv^2}{r} \Rightarrow v = \sqrt{\frac{GM}{r}}\] The satellite's mass \(m\) cancels, so it depends only on the earth's mass and the radius of the orbit.
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Why does the sea appear blue in colour?
Answer: C
The sea looks blue because of scattering of sunlight by water molecules.
Scattering follows \(I \propto 1/\lambda^4\), so short blue wavelengths are scattered far more than red ones. Water also absorbs red light strongly within a few metres, which deepens the effect.
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Which of following in not a property of cathode ray
Answer: C
Cathode rays do not make materials cool - they heat them.
Being fast electrons, they give up kinetic energy on striking a target, which appears as heat and, from a heavy target, as X-rays. They do ionise gases, penetrate thin metal foil, and travel in straight lines while remaining invisible themselves.
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