BUNYI

Benda-benda yang bergetar dapat menghasilkan bunyi. Pada saat sebuah benda bergetar, benda tersebut memberikan energi kepada partikel-partikel di sekitarnya. Energi ini menyebabkan partikel-partikel tersebut ikut bergetar. Dan dalam bentuk rapatan (daerah yang pertikelnya rapat) dan renggangan (daerah yang pertikelnya kurang rapat), getaran itu merambat meninggalkan sumber bunyi. Rangkaian gerakan rapatan dan renggangan disebut gelombang longitudinal. Bunyi dihasilkan oleh benda yang bergetar, merambat dalam bentuk gelombang longitudinal.

Gambar 1 Drummer CN Blue memainkan drum yang menghasilkan bunyi

Sebagai contoh, dapat dilihat pada gambar di atas. Drum adalah contoh benda yang menghasilkan bunyi.

Ø Medium Perambatan Bunyi

Bunyi dapat merambat melalui zat padat, cair dan gas (sebagai contoh kayu, gelas, baja, air, udara). Pengeras suara dalam radiomu sebenarnya menggerakkan udara di depannya sehingga partikel-partikel udara itu bergetar. Jika tidak ada udara (medium) di depan pengeras suara itu, tidak akan terjadi bunyi. Tanpa medium untuk merambatkan getaran, tidak akan terjadi bunyi. Di permukaan bulan tidak ada atmosfer, sehingga tidak ada medium untuk merambatkan gelombang. Jadi tidak ada bunyi di bulan maupun di ruang hampa di tempat lain.

Ø Cepat Rambat Bunyi

Gambar 2 Alat musik yang dimainkan bersamaan akan terdengar dalam waktu yang sama

Jika seruling dan gitar dimainkan bersamaan, kedua gelombang bunyi itu akan sampai di telinga pendengar dalam waktu yang sama. Oleh karena itu dapat dikatakan bahwa cepat rambat bunyi tidak bergantung pada jenis sumber bunyinya. Cepat rambat bunyi bergantung pada dua hal, yaitu jenis medium yang dilalui gelombang bunyi dan suhu medium. Udara merupakan medium yang paling sering dilalui gelombang bunyi. Gelombang bunyi dapat merambat melalui berbagai jenis medium. Zat cair dan zat padat merupakan penghantar yang lebih baik daripada udara sebab partikel-partikel di dalam zat cair atau zat padat saling mempengaruhi lebih kuat daripada partikel-partikel udara. Hal ini mengakibatkan perpindahan energi gelombang bunyi di dalam zat padat atau zat cair menjadi lebih mudah daripada di udara. Tabel berikut merupakan cepat rambat bunyi pada berbagai jenis bahan:

Tabel 1 Cepat rambat bunyi pada berbagai bahan

No.	Jenis Bahan	Cepat Rambat Bunyi
1	Udara	344
2	Air	1.500
3	Batu bata	3.650
4	Kayu	4.700
5	Besi	5.100
6	Kaca	5.000 s/d 10.000

Suhu medium juga merupakan faktor penting dalam menentukan cepat rambat bunyi. Pada saat suhu zat meningkat, molekul-molekulnya bergerak lebih cepat sehingga frekuensi tumbukan antar partikel lebih banyak. Meningkatnya tumbukan molekul ini akan lebih banyak memindahkan energi dalam waktu yang lebih singkat. Ini memungkinkan gelombang bunyi berpindah lebih cepat. Bunyi merambat melalui udara dengan cepat rambat 344 m/s, pada suhu 20° C, namun hanya dengan cepat rambat 332 m/s, pada suhu 0° C.

Cepat rambat bunyi untuk medium tertentu dan suhu tertentu besarnya tetap. Gerak dengan kecepatan tetap ini disebut gerak lurus beraturan (GLB). Seperti yang telah kamu pelajari pada tentang gerak, dalam GLB hubungan antara cepat rambat dengan jarak tempuhnya adalah:

cepat rambat=(jarak tempuh)/waktu

atau dapat ditulis dalam bentuk persamaan:

v=s/t

dengan v = cepat rambat bunyi (m/s)

             s = jarak sumber bunyi ke pendengar (m)

             t = selang waktu yang diperlukan bunyi untuk merambat sampai ke pendengar (s)

Seperti halnya berlaku untuk gelombang lain, pada gelombang bunyi juga berlaku persamaan:

v=λf

dengan v = cepat rambat bunyi (m/s)

            λ = panjang gelombang bunyi

            f  = frekuensi bunyi (Hz)

IMPULSE AND MOMENTUM

Pada permainan sepak bola akan berlaku hukum kekekalan momentum. Apa itu impuls dan momentum? Yuk! sama-sama kita pelajari makna kedua kata yang saling berhubungan ini.

Momentum adalah derajat kesulitan untuk menghentikan gerakan suatu benda yang didefinisikan sebagai hasil kali massa (m) dan kecepatannya (v), atau secara sistematis dapat dituliskan:

p = m x v

Sedangkan impuls adalah perubahan momentum yang dapat dirumuskan sebagai hasil kali gaya yang bekerja pada sebuah benda dengan selang waktu gaya tersebut bekerja. Secara sistematis dapat dituliskan:

I = F x dt

Nah, apa benar pada permainan sepak bola juga berlaku konsep impuls dan momentum?

Pada saat bola belum dikenai gaya, dalam hal ini bola belum ditendang, maka momentum awalnya adalah nol (karena benda yang diam tidak memiliki kecepatan atau nol sehingga jika dikalikan dengan massa bola momentumnya juga akan bernilai nol). Ketika bola ditendang oleh pemain sepak bola, bola dikenai sebuah gaya, berarti bola mendapatkan impuls (ingat definisi impuls). Pada saat itulah bola bergerak dan memiliki kecepatan, sehingga momentumnya tidak sama dengan nol lagi.

Sehingga dengan kata lain, benda bergerak karena diberi impuls, dan benda yang bergerak mempunyai momentum.

Bagaimana dengan permainan bola yang lain, apakah juga berlaku konsep impuls dan momentum di dalamnya? :-)

HWAITING!!! 

EXPANSION

When you enter the hot water into the glass, suddenly the glass will be crack. What do you think about that? Any relation with some physics phenomena?

This phenomenon is one of the physics phenomena that called expansion. The glass can be crack because the content in glass expand early than expansion in the outside of glass. Expansion of an object will be happen if the object heat by a heater. In heat process, particles run with the others, so the object will expansion. When the object will be cold, the particles will close with the other, so the object will be shrinking. Expansion can happen in solid, liquid or gasses.

Solid Expansion

If something have heat, the things will expand in all of direction. The size of length, area and volume will increase (happening length expand, area expand and volume expand). The length and area of solid are limited, we can only concentrate in length direction. So it called in condition length expands. If solid have not been heat in steam form but in flat form, such as circular flat we can only concentrate in area expand. When they have heated, they will have area larger than before. If solid have been heated not in stem or flat form, but in cubs, ball, or box form, we can only measure the volume of them.

Liquid Expansion

The main characteristic of liquid is following the form of the place where it will be placed. For example if we place the water into the glass, the form of water will follow the form of glass. Because of this characteristic, liquid only have volume expand.      Volume expand of liquid will bigger than volume expand of solid.

When the glass has been heated, the liquid in glass will expand. But the volume expand of liquid bigger than volume expand of glass (solid). Because of it, the liquid height will increase.

Gas Expansion

Like solid and liquid, gas also expand when it is heated by heater. Because gas haven’t form, so gas only expand in volume.

Some Problem that Caused by Expansion

Ø Installing of glass window

The car glass window can crack because park under the sun light directly at 2 hours. Because the temperature of glass will increase, so it will be expand, when expand, the frame will opposite with the glass expand. So, the glass will be crack. It better if the car park under sun light directly, not closed more, give same space for expand.

Ø Connection of railway track of train

Railway track of train expand in hot day. Because of it, between connections of railway track always give one space, in order expansion of railway track not caused the railway track will be curve.

Ø Telephone wire and electric wire

Installed telephone wire or electric wire is slack, in order at in hot temperature the wire will not break off.

MEASUREMENT

To measure a physical quantity, we need some instruments; for example, the quantity of length can be measured by instruments such as a ruler, vernier caliper and micrometer screw gauge.

1.    Measurement of Length

To measure the quantity of length, we can use a ruler, vernier caliper or micrometer screw gauge. However, those instruments mentioned have their own strengths and weakness.

a.  Ruler

Ruler is an instrument to measure length which usually has smallest scale of 1 mm. But there are also rulers whose smallest scale is bigger than 1 mm, for example 1 cm. 

b.  Vernier Caliper

Vernier caliper is an instrument to measure length which is more accurate than a ruler, since it has the smallest scale of 0.1 mm.

c.   Micrometer Screw Gauge

A micrometer screw gauge is an instrument to measure length which is more accurate than a ruler or a vernier caliper, because it has smallest scale of 0.01 mm.

2.    Measurement of Mass

To measure the quantity of mass, we can use balances. There are several balances that are often used, such as cross-bean balance, spring balance, and digital balance.

3.    Measurement of Time

Life time is between birth and death. The SI unit for time is second (s). One standard second is the time needed by Cesium atom to vibrate 9,192,631,770 times. Based on this atomic hour, during 300 years the result of measurement will not miss more than a second. For long events, time is stated in larger units such as minute, hour, day, month, year, century, etc.

1 day = 24 hours

1 hour = 60 minutes

1 minute = 60 seconds

But for a fast event, you can use millisecond (ms),microsecond (μs), and nanosecond (ns).

4.    Temperature

Temperature is one of the fundamental quantities. During the day we feel hot, but cool at night. Fire is hot, but ice is cold. A hot object is said to have high temperature, but a cold object has low temperature. Thus temperature shows the degree of hotness and coldness of an object. Your palm can feel the hotness or coldness of a thing.

The common measuring tool for temperature is a thermometer. A thermometer consists of a long glass tube with a narrow column, called capillary pipe, containing liquid, alcohol or mercury. But, the upper part is vacuum.

5.    Volume

Big and small bucket can contain different amount of water. The big bucket contains more water than the small one does. This is related to the space occupied by that object,called volume. The bigger volume takes more space than the small one.

Volume is a derived quantity, from the fundamental quantity of length. For regularly shaped object, you can measure the length, width, and height and multiply them. If you measure the length, width, and height of a rectangular block in cm, the volume is cubic centimeter (cm3).Likewise, if the unit is meter, the volume is cubic meter (m3).

How can you measure the volume of a liquid? The volume of a liquid may be measured by pouring it into a measuring cylinder. When the liquid is poured into the measuring cylinder, the space occupied by the liquid is its volume.

UNITS

In 1960, some scientists organized international conference which agreed standard units called International System Units (SI). This system replaces all of another systems found in the countries which use own metric system. Metric system agreed using meter for length unit, kilogram for mass, and second for time. So, International System Units (SI) is units by using basic units of meter, kilogram, and second (MKS).

Meanwhile another units the value of which are smaller and often used are CGS system units, those are centimeter as the unit of length, gram as the unit of mass, and second as the unit of time.

The differences of MKS unit system (metric system), CGS unit system, and FPS unit system (British System of Units) can we see in table below.

Table 1. The differences of MKS, CGS, and FPS units system

No	Physical Quantities	The Units Used
		MKS	CGS	FPS
1	Length	Meter	Centimeter	Feet
2	Mass	Kilogram	Gram	Pound
3	Time	Second	Second	Second

Since units are the references or standards of physical quantities, the unit holds the rules as follows:

a.    Have constant values

b.    Are general in characteristics

c.    Can be converted into other unit system

INTERNATIONAL SYSTEM UNITS (SI)

In 1960, physicists organized general conference about weight and measure. In the conference there was agreed that the length of one meter is equal to 1,650,763.73 times of wavelengths of reddish-orange light emision from atom Krypton-86 in a vacuum. In 1983, meter standard was reconverted to be one meter is equal to the distance traveled by light in a vacuum for 1/299,792,458 second.

Mass standard of one kilogram in SI is fixed to be equal to the mass of a platinum-iridium cylinder stored in Internationa Bureau of Weights and Measures in Paris, France.

Time unit in SI is expressed in second. In the beginning, one second is defined as 1/86,400 of average day in the Earth’s rotation. But, the Earth’s rotation period from year to year changes, so the measure of second becone not precise. In 1967, physicists fixed that 1 second is equal to the time required by the cesium-133 atom to make vibration of 9,192,631,770 times. 

INTERNATIONAL SYSTEM UNITS (SI) PREFIXES

          The 20 SI prefixes used to form decimal multiples and submultiples of SI units are given in table below.

Factor	Name	Symbol
1024	yotta	Y
1021	zetta	Z
1018	exa	E
1015	peta	P
1012	tera	T
109	giga	G
106	mega	M
103	kilo	k
102	hecto	h
101	deka	da

Factor	Name	Symbol
10-1	deci	d
10-2	centi	c
10-3	milli	m
10-6	micro	µ
10-9	nano	n
10-12	pico	p
10-15	femto	f
10-18	atto	a
10-21	zepto	z
10-24	yocto	y

For example 10^-6 kg = 1 mg (one milligram). The other example is consider the earlier example of the height of the Washington Monument. We may write h_W = 169 000 mm = 16 900 cm = 169 m = 0.169 km using the millimeter (SI prefix milli, symbol m), centimeter (SI prefix centi, symbol c), or kilometer (SI prefix kilo, symbol k).

QUANTITIES

    DEFINITION QUANTITIES, UNITS AND MEASUREMENT

In daily life, certainly you often hear the terminologies such as length, width, height, time, temperature, and so on. Those terminologies are included into quantities. In physics, quantities are very closely related to measurement.

The definition of quantities, units, and measurement in physics can be expressed as follows.

v Quantities are something that can be measured and expressed in values.

v Units are the standard of comparison in the measurement.

v Measurement is a process of comparing something measured with another as measures made as standard.

FUNDAMENTAL QUANTITIES AND DERIVED QUANTITIES

In physics there are two kinds of physical quantities, those are fundamental quantities and derived quantities. Fundamental quantities are the physical quantities which the units are predetermined, while the derived quantities are the physical quantities which the units are derived from fundamental quantity units.

There are seven fundamental quantities; those are length, mass, time, electric current, temperature, luminous intensity and the amount of substance.

Table 1 Fundamental quantities and their units

No	Fundamental quantities	Units	Symbol
1 2 3 4 5 6 7	Length Mass Time Temperature Electric Current Luminous Intensity Amount of Substance	Meter Kilogram Second Kelvin Ampere Candela Mole	m kg s K A Cd mol

Meanwhile, the derived quantities is other quantities are derived from these fundamental quantities such as volume, density, velocity, force, work and many others.

Table 2 Examples of derived quantities and their units

No	Derived Quantities	Units	Symbol
1 2 3 4 5 6	Area Volume Density Velocity Energy Power	Square meter Cubic meter Kilogram/cubic meter Meter/second Joule Watt	m2 m3 kg/m3 m/s J W

SCALAR QUANTITIES AND VECTOR QUANTITIES

Most of the physical quantities encountered in physics are either scalar or vector quantities.  A scalar quantity is defined as a quantity that has  magnitude only.  Typical examples of scalar quantities are time, speed, temperature, and volume.  A scalar quantity or parameter has no directional component, only magnitude.  For example, the units for time (minutes, days, hours, etc.) represent an amount of time only and tell nothing of direction.  Additional examples of scalar quantities are density, mass, and energy.

A vector quantity is defined as a quantity that has both magnitude and direction. Magnitude of vector  is  also referred to as the vector's "displacement."  It can be thought of as the scalar portion of the vector and is represented by the length of the vector. By  definition,  a  vector  has both magnitude and direction.  Direction indicates how vector is oriented relative to some reference axis.

Vectors have magnitude and direction, scalars only have magnitude. There are some quantities, like speed, which have very special definitions for scientists. By definition, speed is the scalar magnitude of a velocity vector. A car going down the road has a speed of 50 mph. Its velocity is 50 mph in the northeast direction. Another example is mass and weight. Weight is a force which is a vector and has a magnitude and direction. Mass is a scalar. Weight and mass are related to one another, but they are not the same quantity.

The forces which operate on a flying aircraft, the weight, thrust, and aero dynamic forces, are all vector quantities. The resulting motion of the aircraft in terms of displacement, velocity, and acceleration are also vector quantities. The scalar quantities include most of the thermodynamic state variables involved with the propulsion system, such as the density, pressure, and temperature of the propellants. The energy, work, and entropy associated with the engines are also scalar quantities.