Physics regards the physical aspects of the natural world. It includes topics that deal with forces on different bodies within the universe and phenomena that explain how the universe works.

243,037 Questions
Meteorology and Weather

Why is the sky blue?

Sunlight interacting with the Earth's atmosphere makes the sky blue. In outer space the astronauts see blackness because outer space has no atmosphere.

Sunlight consists of light waves of varying wavelengths, each of which is seen as a different color. The minute particles of matter and molecules of air in the atmosphere intercept and scatter the white light of the sun. A larger portion of the blue color in white light is scattered, more so than any other color because the blue wavelengths are the shortest.

When the size of atmospheric particles are smaller than the wavelengths of the colors, selective scattering occurs-the particles only scatter one color and the atmosphere will appear to be that color. Blue wavelengths especially are affected, bouncing off the air particles to become visible.

This is why the sun looks yellow from Earth (yellow equals white minus blue). In space, the sun appears white because there is nothing in between to scatter its white light.

At sunset, the sky changes color because as the sun drops to the horizon, sunlight has more atmosphere to pass through and loses more of its blue wavelengths. The orange and red, having the longer wavelengths and making up more of sunlight at this distance, are most likely to be scattered by the air particles.

The scattering of visible light by atmospheric gases is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering after Lord Rayleigh, who studied it in more detail a few years later. Rayleigh Scattering is where red, orange, yellow, and green are passed through and blue, indigo, and violet are "scattered" out creating the color.

Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

(*As for why the sky does not appear violet -- the wavelength most scattered -- see the explanation at the related link below.)

Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
The sky appears blue to us because of the scattering of the blue light component of the light from the Sun. Some alpine lakes also appear a quite light blue colour for the same reason, light is scattered by tiny suspended flakes of minerals in the water.
Because the water particles in the air split the light and Blue light is dispersed the furthest that's why it creates the illusion the sky is blue


The forces of attraction between two objects varies with what two factors?

The forces of attraction between two objects vary with mass and distance. The force is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.

Example Sentences

How do you use attenuation in a sentence?

After his heart transplant, he was given special medicine to prevent rejection, these drugs are to be taken daily in order for attenuation to take place and to attenuate the immune response and to keep the body from rejecting its new heart.

Installing this new piece of equipment will let through the lower frequencies, therefore achieving attenuation.


What device or instrument is used to measure air pressure?

There are several options depending on the type of air pressure being measured:

  • In the 1800's ships used a weather glass - a simple barometer - to measure changes in air pressure.
  • To measure atmospheric (ambient air) pressure, a barometer is used. (It may be either an aneroid or a mercury type of barometer.)
  • For tires, it's a tire pressure gauge.
  • In industrial processes and boilers the gauge is frequently called a manometer.
  • For pressure vessels it is a Bourdon gauge.

Which science is the most complex chemistry or biology or physics?

Chemistry is an important branch of Science and it discusses the reactions of chemical elements in human body. Learning Chemistry needs skills like attention, hard work and determination which an online Chemistry tutor inculcates in a student. For me science is basic and I truly appreciate science so I would state that the trouble level is about the equivalent in spite of the fact that science utilizes a considerable amount of maths and you have to comprehend ideas so as to utilize them while science is content substantial. having said that, science mark plans are exceptionally particular with watchwords and granting marks...

Math and Arithmetic

What is bigger 3.6 m or 36 cm?

3.6 m is bigger


What is the Example of digital quantity?

A digital quantity example is provided by any digital microprocessor display where a discrete number is provided and the actual value is an irrational number.

Some examples are the computer display of the values of pi, the square root of 2, the quotient of 3/19, or any other irrational number.

A computer basically works with the integers 0 and 1, so it is a "digital" machine (it cannot deal with 3/19, or pi, etc with perfect accuracy).

Social Sciences

What is the movement of organisms from one place to another?

Lentiviruses are very efficient at delivering genetic materials to both dividing and non-dividing cells. According to whether proviral DNA is formed in transduced cells, lentiviruses can be further divided into two groups: integrating lentivirus (ILV) and integrase-deficient lentivirus (IDLV). ILV is first developed and applied in scientific research and IDLV is constructed later through genetic modification.

Space Travel and Exploration
Science Experiments
Sound Waves

What is the speed of ultrasound waves in the air?

The 'normal' speed of sound is 340 m/s in dry air at room temperature and pressure.

From your question, it appears we should differentiate the terms "ultrasonic" and "supersonic".

Ultrasonic relates to sonic frequencies higher than 20 KHz, i.e. beyond audible range. For a given gas, the speed of sound is independent of the frequency of the sound measured and also independent of the density of the gas.

Supersonic relates to a speed of an object greater than the normal speed of sound (340 m/s in air at STP) and usually the phenomena associated with it.

The speed of sound is also a functionof the medium through which it's passing. For example, the speed of sound through water is 1,500 m/s, and is slightly over 5,000 m/s in iron.

So to answer your question, the speed of ultrasound in airis340 m/s for the reasons given.


Which term describes the tendency of an object to resist a change in motion?

An object's tendency to resist a change in its state of motion is called inertia. This is the basis of Newton's Laws of Motion; "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.". The state of motion refers to the object's velocity, which is the the speed and direction. One quantifies inertia as the quantity of mass of an object. On can say that the mass of an object is a measure of how much an object resists change in its motion. The more mass an object has, the more inertia it has. That's why it is harder to push a 3 ton box as opposed to a soccer ball, or something lighter.

Botany or Plant Biology

Science investigatory project of avocado leaves?

You can try to find a use for avocado leaves like observing if it is good at repelling mosquitoes or if it will make a fire last longer or something like that.

Thunderstorms and Lightning

Is it safe to take a shower during a thunderstorm?

No—in fact, the CDC encourages avoiding all water in your house during a thunderstorm. Your plumbing and the water coming out of it can conduct electricity if lightning strikes your home, and that can lead to you getting seriously hurt. Granted, this is a pretty rare occurrence, but experts still advise against it as an easy way to avoid something potentially fatal. You should wait half an hour after hearing the last boom of thunder before hopping into the shower.

Chemical Bonding

Why does water have a high and specific heat capacity?

In simple words, water has high heat capacity because it has a lower molar mass, and specific heat is inverse to a substance's mass. Another reason for high specific heat is its strong hydrogen bonding.


What kind of a simple machine is a toothbrush?

A toothbrush is not a simple machine, it does not work on the principle of levers, effort, and load.

It's not a machine at all in mathematical terms. A "machine" is that which performs "work", i.e. transfers or converts energy.

You arm moving the toothbrush is a machine by this definition, however.

Nuclear Physics
Particle Physics

Matter attracts other matter does antimatter will repel the antimatter?

Antimatter observes and obeys the same fundamental forces that matter does:

  • gravity
  • electromagnetism
  • weak interactive
  • strong interactive

A positron, which is the anti-particle of the electron, for example, has the same mass as an electron and experiences the same attraction to all other matter (gravity) as an electron. That same positron is repelled by positively charged particles and attracted to negatively charged particles (electromagnetism).
Stephen Hawking

What is Stephen Hawking's IQ?

Although Professor Hawking discounts the value of numerical IQ's, he admitted that his own is likely very high (it has been suggested as 160 or more). He also said that "People who boast about their IQ's are losers". (see link to 2004 interview)

---a possibly dramatized anecdote from Introducing Stephen Hawking (2006)---

Near the end of his term at Oxford and no doubt beginning to feel the effects of ALS, Hawking took a terrible fall down a staircase in the university hall. As a result, he temporarily lost his memory. He could not even remember his name. After several hours of interrogation by his friends, he finally returned to normal but was worried about possible permanent brain damage. To be sure, he decided to take the Mensa test for individuals with superior intelligence. He was delighted to find that he passed with flying colours, scoring between 200 and 250!

Math and Arithmetic
Weight and Mass

What is mass divided by weight?

Answer #1:

It's (1) divided by (the acceleration of gravity in the place where that mass has

that weight):

weight = mass x g (where g is the acceleration due to gravity)

⇒ mass/weight = mass/(mass x g)

= 1/g

On the earth, g ≈ 9.81 ms-2 ⇒ mass/weight ≈ 1/9.81 ms-2 ≈ 0.102 m-1s2

On the moon, g is approx 1/6 that of the earth, ⇒ mass/weight ≈ 6/9.81 ms-2 ≈ 0.612 m-1s2

If the questioner really meant weight divided by mass it gives the acceleration

due to gravity in that place otherwise I'm not sure of a use of knowing the

reciprocal of the acceleration due to gravity that the questioner asked.


Answer #2:

If you ask a scientist, that's true answer in the sense that a mass M experiences

a gravitational force Mg and if you measure weight in units of force (which

nobody does). But anyone else would be surprised to learn that a mass M (say

10 grams) would have a weight of anything else but M grams (10 grams).

Sometimes expressed as "grams weight" often just grams for short. If you pick

up a Kilogram, even a scientist would say "its weight is 1 kilogram". The

gravitational force on it is 1g, so if you let it go it will accelerate at a rate force

over mass, which is g. So the answer depends on your units of mass and weight.

That's why science lessons tend to avoid use of "weight". In outer free space

mass would be measured by (say) tension in the string if you whirl it on the end

of it around your head, but the weight (measured by a spring balance) would be

zero (precisely as described in the first answer above, with g=0).


Answer #2.1:

The problem with discussing mass and weight in the same units, and the reason that this masked contributor is waging a one-man battle to make the distinction recognized and acknowledged by users of this website, is the new problem that

you have now that the space age is here.

As long as we were all irrevocably bound to the Earth, one kilogram of mass would always weigh one kilogram, if you like it that way. We could afford to be sloppy about it, with hardly one out of ten men-on-the-street knowing or caring about the difference, and nobody ever had a problem with it.

But now that some of us have already slipped these surly bonds ... and among

the general population, the younger you are, the better the chance that you will

do so one day before you're done ... those who ignored the distinction begtween

mass and weight all through school, or never even encountered it there, are

poised to step into an inconvenient pile. Because as soon as you pack for your

trip to anywhere else away from Earth, and take along your lucky kilogram,

you're due for a shock when you step out at your destination: Your kilogram

doesn't "weigh" a kilogram there. It weighs something else. If you're on the

moon, for example, your kilogram weighs 0.165 kilogram ! That's the

shock I'm trying to avoid, because if you think the straight dope is too complex

for people to handle now, you haven't seen anything yet.

Math and Arithmetic

If a mineral has a mass of 100G and a volume of 50Cm3 what is its density?

The mineral has a density of 2 g/cm3

Newtons Laws of Motion

What is the formula for calculating acceleration?


There are a few. The most famous is a = F/m, where F is the net force applied to a mass, m.

Acceleration is also the change in velocity, (Delta-V), divided by the change in time, (Delta-t). So, a =Δv/Δt.

For example, if an object's velocity changes from 10 meters per second to 20 meters per second in five seconds, its acceleration is (20-10)/5 = 2 meters per second per second, or 2 meters per second squared (m/s2).

For circular motion, centripetal acceleration is v2/r, where v is the linear velocity of the rotating object and r is the radius of its circular path.

Equations in a nutshell

Constant Accelerationa = Δv/Δt = (vfinal - vinitial) / (tfinal - tinitial)

a = (v2-u2)/2s

a = 2(s - ut)/t2


a=acceleration (m/s2)

v=final velocity (m/s)

u=initial velocity (m/s)

t=time (s)

s=distance (m).



a=acceleration (m/s2)

v=final velocity (m/s)

vo=initial velocity (m/s)

t=time (s).

Newton's Second Law

F = ma, thus, a = F/m

Centripetal Acceleration

ac = v2/r

Warning: Calculus Speak:

Acceleration is the second derivative of position with respect to time: d2x / dt2, which makes it the first derivative of velocity: dv / dt. Therefore, the acceleration is the slope of the curve on the velocity-versus-time graph.


a = dv / dt = d2x / dt2

Acceleration is a quaternion with real and vector parts:

a= (V^2/R - cDel.v)) + (dcv/dR + cDelxv + V^2/R r)

a= (V^2/R - cV/R cos(v)) + (dv/dt + cv/R sin(v) + V^2/R r)

where R=ct and dR=cdt.

cv/Rcos(v) is the Centrifugal Acceleration a part of the real accelerations in the first parenthesis. The second parenthesis contains the vector accelerations.

Acceleration = F/m, where F is the net force applied to a mass, m.


acceleration in terms of velocity.

a = v - u/t Delta Velocity divided by Time.

A = ΔV ÷ T Acceleration is worked out by (final speed - initial speed)/ time taken for change in speed a = v2-v1/ t2-t1 Strictly you should say velocity ie the speed in a certain direction. Youalso have the formula f=ma which tells you that the force needed to get something moving will be the mass of the object multiplied by the accelertion you want to achieve; so from this formula if you know force and mass you can work out acceleration. The formula for acceleration is: Vf-(Vi)/t ie. change in velocity per unit time. Instantaneous acceleration in its differential form is d2x/dt2 where x is a function of time t.

Acceleration is the time rate of change of velocity.

That is, acceleration =dv/dt (v - velocity ; t - time)

Or simply acceleration = change in velocity / time


What instrument measures temperature?

Most commonly, a thermometer.

Related Information:

Thermometers are used to measure the increase or decrease in the temperature of a system as it gains or loses internal energy.

An alcohol-in-glass thermometer has been the most common personal instrument used to measure temperature. Mercury thermometers are still around but are no longer offered for sale. Today, digital devices are available that scan the forehead or ear. Some other devices used to measure temperature are:

Radiation pyrometer, for extremely high temperatures;

Glass thermometer: mercury or alcohol;


Thermistor (thermal resistor)

Bimetallic strip;

Bimetallic spiral;

Platinum resistance thermometer (a resistance detector);


A mercury in glass thermometer uses mercury liquid contained within its glass structure to be subjected to heat.

the heat causes the mercury fluid to expand along the glass tube

and the total amount of expansion can be seen as a measure

along the accurate scale of indication.

This is a direct indication of the effects of temperature.

A more complex method of measuring temperature could be a

thermocouple measuring device.

A thermocouple consists of 2 dissimilar types of metal materials in the form of wires , which are joined at 1 end by weld/ fusion.

this single joined end is called the hot junction. The other end of the 2 wires are then terminated at separate junctions;

as in a electronic terminal block. This end of the 2 wires can be called the cold junction.

For most accurate temperature measure, I think temperature sensor is the first choice.

Atoms and Atomic Structure

What is the temperature of a lighter flame?

The temperature of a Bic lighter flame is 1977 C or 3590.6 F.

Solid State Physics

What is dislocation density?

Dislocation density is the areal density of dislocations intersecting a plain, usually the free surface, given as number per cm2.

It may also be the volume density of dislocation line segments, given as the total length of dislocations divided by the containing volume (also 1/cm2), but this is rarely used in semiconductor physics, and more frequently found in engineering.

Dislocation density is typically measured by etching the free surface to form pits around the location at which the dislocation breeches the surface, and is termed etch pit density, or EPD.

Quantum Mechanics

Why are leptons divided into families?

Leptons are divided into three families with 4 particles (2 particles, plus their two anti-particles) in each family. In the electron family we have the electron, positron, electron neutrino and electron anti-neutrino. Each family has a higher mass than the one before it so the tauon is heavier than the muon which is heavier than the electron. The physical reason for there being three families is completely unknown and will probably win you a Nobel prize if you can figure it out!


Why are light and heat not considered matter?

What i understand is that there is noting call "Light" or "Heat" its just the "Space",. "Space" created between sub atomic particles and atom we understand and call it "Light" and our eye recognizes it - and "Space" created between atom, molecules and cells we feel it and call it "Heat" - our cells / skin feels it

IF "Light" is a "particle" or "wave" then you should not see the same star from just one feet away -

your eye should loose the particle as it should have gained a little space during its million kilometer journey.

Rajeev. Colombo.

OLD reply >>

Light (including all forms of electromagnetic radiation) is not considered matter because it has no rest mass. Heat doesn't have rest mass either, in fact it is nothing other than the random motion of atoms. These things have energy. They interact with other stuff. But they aren't matter because they aren't what material objects are made up of.

Alternatively: Light and heat are not tangible things that you can see, touch or weight. One can not chemically isolate atoms of heat or light.

There is no official answer to this question because everyone just takes is as a given fact.

Nuclear Physics
Atoms and Atomic Structure
Particle Physics

Are quarks nucleons?

No, quarks are not nucleons. A nucleon is a term (in physics) that is given to either of the two component particles of an atomic nucleus: the proton and the neutron. Both protons and neutrons are composite particles from the family of hadrons, and hadrons are made up of quarks.


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