Jeszcze bardziej Perfekcyjna Pani Domu … a może Pan

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Even more perfect housewife ... or maybe a perfect host?
The purpose of this article is to show that physics is all around us, and thus, it is worth
understanding what phenomena we are connected with it. Among others he article discusses
the phenomenon of inertia.
From time to time also men are engaged in housework. In many families traditionally men
beat carpets, remove and hang curtains. These are not my favourite activities, but since I
discovered that many interesting phenomena can be observed while performing these
mundane activities, I decided to improve them and I appointed myself Mr. Perfect Host.
What might be interesting in beating carpets?
We beat carpets with a carpet-beater. We use the rule of inertia, which means that bodies are
very reluctant to change the state they are in. Inertia is the resistance of any physical object to a
change in its state of motion or rest, or the tendency of an object to resist any change in its motion. At
school teachers often talk about this rule as Newton’ first law.
When we beat a carpet hanging on a hanging frame, it starts to move and dust particles
stay in place (or at least some of them) because of inertia. Another batch of dust, which moves
with the beaten carpet will explode at the time when it stops and starts to go back to the
starting position, also due to inertia, but this time because the particles move together with the
carpet so they tend to further movement.
I agree that beating carpets is a monotonous and tiring job. As Mr. Perfect Host I came to the
conclusion that a specialized company will be more efficient in carpet cleaning. You just need
to take your carpet there. But if you drive a car, where should you place the carpet? When we
get into the car we need to remember to wear a seat belt, because we know that it protects us
from the effects of sudden braking or a collision, again the consequences of inertia. In your
car you usually throw things casually where it is possible. We do not think that unprotected
they also can be a serious threat. At the time of braking they move with the speed before
braking – according to the principle of inertia, which works without exceptions. In turn, the
mass is the measure of inertia of a body, the greater the mass, the greater the inertia. This is
Newton's second law.
It is harder to set a heavier body in motion and to give it higher acceleration, but if it is
already in motion it is more difficult to stop. The effects are also more serious. According to
the National Road Safety Council even at a safe speed, eg 64 km / h a body weighting 84 kg
in the case of sudden braking hits with the same force as a body weighing 2.5 tons. So the
poorly secured carpet lying on the trunk on top of the car at high speed can fire as a torpedo
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during a collision. I realized that the mess inside the car is sometimes dangerous and objects
on the rear shelf usually hit the driver’s head. It may end up on fear, bump on the head and
shattered windshield but it could be fatal. Physics reclaims its rights.
The described examples show that the effects of inertia may be positive or negative. The
attached video shows an example of inertia of bodies. There are two cups on a handkerchief
laid on the table. They do not change its position relative to the table when the scarf is pulled
vigorously. If the scarf was pulled slowly, cups would move together with the scarf.
Film 1 Removing the handkerchief from under the cups.
Let’s get back to the cleaning. It’s time to remove curtains. Do not underestimate the task and
choose a stable stool to stand on. It’s very easy to lose balance and fall and the collapse may
end up really unfortunately. Why do we lose balance? Each body has the center of mass, also
called the center of gravity. This is the focusing point of the whole mass of the body or the
whole of its weight.
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The center of gravity of a man standing upright is roughly around the navel. The exact
location depends on the figure, because if a body has irregular shape, the center of gravity is
shifted toward the area where there is more mass. Thus, the center of gravity depends on the
shape. Men with wide, muscular chests will have it located in the upper parts of the body.
In some cases, the center of gravity is located outside the body. An example of this situation is
shown in another of the listed films - the center of gravity of the fork - spoon set is at the
fulcrum of the match on the cup.
Film 2 The center of gravity of the spoon - fork.
When you change the position of your body or anything else, we also change the
position of its center of gravity and the position of this point determines the stability of the
adopted position, which affects the stability of equilibrium. The body can maintain stability
(will not turn over) if it is supported or suspended at a point directly above or below the center
of gravity. On a moving bus, we intuitively spread our legs so that we do not fall over. Then
our center of mass is over the area bounded by feet.
When we take off curtains, then it is much safer to stand on a low stool with its legs set at an
angle greater than 90 °. Such a stool is more stable than a higher one with legs positioned
perpendicularly to the seat. The center of gravity of a lower stool is located lower and you
need much greater angle to overturn it.
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Below there are pictures showing how you can make a toy called a Humpty Dumpty.
It "rises" itself due to the fact that its center of gravity is lowered by placing a heavy ball at
the bottom of the doll.
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Photos 1-6 show what materials you need to make a Humpty Dumpty. The film closing the
series of images shows that the doll deflected from the equilibrium position returns itself to
the upright position.
As a break in strenuous activities and I recommend some physical exercises. I suggest you
check whether it is possible to perform slope when standing upright, with your back very
close to a wall. Is it possible to perform a slope without moving feet away from the wall. Can
you keep your balance? Where will our center of gravity be then? Is it still on the area limited
by feet? Is it possible to stand only on your left leg, when you stand with our left side close to
the wall? The left foot should touch the wall, too.
A man has a sense of balance, and if it is not damaged, the impulses reach the brain,
informing about the position of the body in space. The brain continually analyzes them and
tells us to take the position appropriate to the situation. In the described exercise the center of
gravity moves outside the designated area of our feet. The brain suggests that we should move
the foot from the wall, otherwise we will fall over.
I have already taken off the curtain and now it can be laundered. Washing ends with rinsing
spinning. Due to which phenomenon will water be removed from the curtains? The
phenomenon responsible for this has just been described in this article.
On the occasion of cleaning, you might ask yourself many more questions and when you
think about the answers, then you might like tedious tasks such as housework. Mr Perfect
Host assures you of that.
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Exercises:
1. Visit the National Road Safety Council:
http://www.krbrd.gov.pl/kampanie/pasybezpieczenstwa.htm and read the information
contained there.
2. The center of gravity is very important for the performance of many sports. If you like
riding a bike or swimming, search for the information what impact the center of gravity has
on the economic movement and how to get the most appropriate location?
Bibliography:
1. Paul E. Hewitt – „Fizyka wokół nas” Wydawnictwo Naukowe PWN, Warszawa 2003
2. http://pl.wikipedia.org/wiki/Trzepaczka_do_dywan%C3%B3w 19.02.2013
3. http://portalwiedzy.onet.pl/29683,,,,bezwladnosc,haslo.html 19.02.2013
4. http://merkuryusz.com/fizyka/rozdzial_XIX.html z dnia 19.02.2013
5. http://www.fizykon.org/statyka_osr_ciagle/srodek_masy.htm z dnia 02.03.2013
6. http://www.krbrd.gov.pl/kampanie/pasybezpieczenstwa.htm z dnia 10.03.2013
List of films:
Film 1. Removing the handkerchief from under the cups - the author’s own film.
Film 2. The center of gravity of the spoon – fork - the author’s own film.
Film 3. The author’s own film.
List of photographs:
Fig. 1-6 The author’s own photos.