Physics and life

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Physics and life
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Funny questions and answers
Q1 Referring to the temperature scale proposed
by Isaac Newton in 1701, do you think it is good
to use the normal human body temperature as one
of the fixed points?
A The human body temperature may vary in
different health conditions. Therefore, it is not
accurate for use as fixed point.
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Q2 Imagine that you were a doctor and you were
considering what type of clinical thermometer to
use in your clinic. Which type of clinical
thermometer would you choose?
A Digital clinical thermometer should be used as
it can measure temperature accurately.
Q3 Can a glass of hot water have the same
internal energy as another glass of cold water?
A glass of hot water can have the same internal
energy as a larger glass of cold water.
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Q4 If we keep stirring a cup of hot coffee, do
you think the coffee will heat up or not?
A As there is a temperature gradient between the
coffee and the surrounding air, the coffee will
cool down. In fact, stirring increases the rate
of evaporation and hence the coffee will cool
down at a faster rate.
Q5 A tile floor usually feels colder than a
carpet floor, even though they are of the same
temperature. Why?
A As tile conducts heat better than carpet, heat
conducts away from our feet more readily through
tile than carpet. Hence, a tile floor feels
colder.
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Q6 The photo shows a man walking quickly across
hot coals without wearing shoes. He can finish
the walk without burning his feet. Why?
A (Never try this on your own! Your feet may be
seriously burnt!)The man does not get hurt
because coals are poor conductors of heat.
Besides, the man wets his feet before the walk.
Heat is absorbed when the water changes into
vapour.
Q7 Imagine that if air is a good conductor of
heat, what would we feel in a room of temperature
25? Would we feel colder or hotter?
A Colder. Heat will transfer to the cooler
surroundings more readily if air is a good
conductor of heat.
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Q8 The warmth-keeping ability of a cotton quilt
decreases when it has been used for a long time.
Can you explain why? Can you suggest a way to
recover its warmth-keeping ability?
Reasons for the decrease of warmth-keeping
ability 1.The amount of trapped air
decreases. 2.The cotton absorbs moisture from
human body and from the surroundings. The
moisture then fills up part of the air spaces
within the cotton. Fluff the quilt to remove
clumps inside the quilt and redistribute the
cotton more evenly.
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Q9 Could a candle keep burning on a spacecraft
in outer space?
A No. Due to the lack of gravity, no convection
current is formed when the candle burns.
Therefore, the carbon dioxide produced by the
flame will not be carried away by convection, and
instead suffocate the flame.
Q10 Suppose there is a cup of hot coffee and you
intend to drink it five minutes later. To keep
the coffee as hot as possible, should you pour in
the cream now, or when you are ready to drink the
coffee?
A You should pour in the cream now. The colour
of the coffee becomes paler when milk is added.
As a result, it emits less radiation and hence
cools down slower.
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Q11 When snow begins to melt, the temperature of
the air is often lower than the time when it is
snowing. Why?
A When snow melts, it absorbs latent heat of
fusion from the surroundings. As a result, the
air temperature is lowered.
Q12 Steam burn is often more damaging than a
burn from boiling water of the same temperature.
Why?
A The energy released when steam condenses
(latent heat of vaporization) is much more than
that released when boiling water cools down.
Therefore, steam burn is more damaging than a
burn from boiling water.
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Q13 Have you ever seen a stadium wave? It is
produced when people in a stadium stand up and
down in a special sequence. Do you know the way
in which it is similar to and different from a
wave we learnt in physics.
A Both stadium waves and the waves in physics
transmit a disturbance from one place to another.
However, a stadium wave does not carry energy
from place to place.
Q14 Do you notice that water waves always arrive
at a beach nearly parallel to it? Could you
explain this phenomenon.
A When a water wave approaches a beach at an
angle, the end in shallower water travels slower
than the end in deeper water. The wave bends
until the two ends are at the same depth. As a
result, the wave will finally arrive at the beach
parallel to it.
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Q15 In a storm, why do we always see the
lightning before we hear the thunder?If you hear
the thunder 3 seconds after you see the
lightning, can you estimate how far away the
lightning is? Do you need to know the speed of
sound, or of light, or of both in your
calculation?
A We see the lightning before the thunder
because the speed of light is far greater than
that of sound. The time taken for light to travel
is negligible when compared to that taken by
sound. Therefore, we only need to know the speed
of sound in the calculation.Suppose the speed of
sound in air is 340 m s-1. Distance speed x
time 340 m s-1 x 3 s 1020 mThe lightning occurs
at a distance 1020 m away. Microwaves cannot
heat up substances such as plastic, glass and
ceramics. Based on this property, discuss why in
general, microwave ovens can cook food more
quickly than electric ovens. As little energy is
wasted in heating up the container, a microwave
oven can cook food quickly. Besides, microwaves
can penetrate the food and therefore can heat up
the food from both inside and outside. In
contrast, an electric oven can only heat up the
food from the outside.
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Q16 At sunset, why does light from the Sun
appear as a column when reflected from the water
surface?
A When there are water waves, the parallel light
rays from the Sun can be reflected to the
observer's eye at a number of points on water
surface. Hence, a column is observed.
Q17 If you use an auto-focus camera to take a
picture of yourself in a mirror, the picture is
usually blurred. Can you explain why?
A The image formed in the plane mirror is
located behind the mirror. However, the
auto-focus camera focuses on the mirror and
therefore the picture is blurred.
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Q18 If a plane mirror on a wall is just large
enough for you to see yourself from your head
down to your waist, can you see your entire body
by moving away from the mirror? Try to test this
at home and explain your observation.
A No.
Q19How many fishes are there in this aquarium?
Why?
A There is only one fish in the aquarium. Two
images are seen in the photo because light rays
undergo refraction when they pass through the two
sides of the aquarium that face the camera.
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Q20 In an optical fibre, why should the hair -
thin glass fibre be surrounded by a material of
lower refractive index?
A The glass fibre is surrounded by a cladding of
lower refractive index so that light can undergo
total internal reflection and travels the length
of the fibre without escaping.
Q21 If the refractive index of air was higher
than that of glass, what would happen to parallel
light rays that pass through a convex lens? Would
the rays be converged or diverged?
A The light rays would be diverged.
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Q22 Suppose there are two convex lenses of the
same shape, one is made of diamond whereas the
other is made of glass. Which lens has a shorter
focal length?
A Diamond has a larger refractive index than
glass. Therefore, the lens made of diamond
converges light rays more and has a shorter focal
length
Q23 It is often seen in movies that when a
spacecraft travels in space, a loud sound is
heard. Do you think the filmmaker has made a
mistake?
A The filmmaker has made a mistake. A sound
should not be heard because sound cannot pass
through the vacuum in space.
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Q24 Suppose the radio system of an astronaut on
the moon is out of order, can you think of a
possible way for him to communicate with other
astronauts?
A The astronauts can move closer so that their
helmets touch each other. Sound can then travel
through the helmets.
Q25 In a 100 m swimming race, is the winner the
one having the greatest average speed or the one
having the greatest average velocity? Why?
A In a 100 m swimming race, the winner is the
one having the greatest average speed but not the
greatest average velocity, because the swimmers
return to their starting position at the end of
the race and their average velocity is zero.
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Q26 There are three controls in a car that can
accelerate the car, what are they?
A Esteering wheel (allows for a change in
direction)    Eaccelerator pedal (allows for a
change in speed)    Ebrake pedal (allows for a
change in speed)
Q27 The terminal velocity of a feather falling
in the air can be as small as 0.1 m s-1. Why is
it so small? Can you explain why a hammer falls
much faster than a feather in air?
A The terminal velocity of a feather is small
because it is light (i.e. gravitational force is
small) and has a large surface area (i.e. air
resistance is large). A hammer falls much faster
because it is much heavier.
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Q28 Referring to the Physics and us , what would
be the tension between any two compartments in an
MTR train if all the compartments are motor cars?
A The tension between the compartments would be
nearly zero if all the compartments are motor
cars. Each compartment provides its own driving
force.
Q29 Although both the helicopter and the rocket
work on the principle of Newton's third law, the
rocket can work in a vacuum while the helicopter
cannot. What is the difference?
A The rocket expels hot gas out from its body
while the helicopter exerts force on the
surrounding air. In vacuum, there is no air and
so the helicopter cannot function, but the rocket
can still function by expelling its own gas.
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Q30 Sometimes, the motion of a fan appears to be
slowed down, stopped or reversed when illuminated
by a fluorescent tube, why?
A A fluorescent tube flashes at a regular
frequency and therefore acts like a stroboscope.
On the other hand, the rate at which a fan
rotates changes when the fan is switched on or
switched off. As a result, the motion of the fan
will appear to be slowed down, stopped or
reversed when the fan rotates at certain rates.
Suppose we observe a fan with 3 blades rotating
at a rate f. The fan will appear to be 'frozen'
when the flashing frequency of the fluorescent
tube is equal to f1/3,f/21/3 , f/31/3 etc.
When the flashing frequency is slightly lower
than any of these frequencies, the fan appears to
rotate forward slowly. When the flashing
frequency is slightly higher than these
frequencies, the fan appears to rotate backwards.
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Q30 Suppose you see your friends playing table
tennis. How can you make a rough estimate on the
velocity and hence the kinetic energy of the
ping-pong ball?
A Use an electronic balance to measure the mass
of the ping-pong ball. You can have a rough
estimate on the speed of the ball by timing
successive hits. The speed can then be estimated
by dividing the length of the table by the time
elapsed.
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Thank you