## Once light is generated at the source, where does it go ?

Once light is generated at the source, where does it go and when does it stop being light ?

Asked Laurence Simons

Answer:

Light is an electromagnetic wave. Once it is produced, it will travel out for ever if not absorbed or scattered. In vacuum there is nothing to absorb or scatter the light. So it will travel forever in vacuum. When it enters a medium it may be absorbed, reflected/scattered or transmitted.

You may refer to these links for more info

https://www.wired.com/2015/02/5-things-every-human-know-light/

## DC Motor

My question is that is my book it’s written that when the coil is in vertical position once it rotates the first time . There is no force acting on it and it’s balanced but when i search it up online they say that the coil does not stop abruptly and it’s moves because of inertia. and in book it’s written we can make it move continuously by changing the direction of current. But when i search it up online it’s written that the direction of current does not change only the side of the loop change.

## Work Done by resistance force and applied force

In my book there is a question that is, A box is pushed throgh 4 m on a horizontal surface and surface offering resistance of 100 N. Find the work done by (1)applied force (2) resisting force?

now, solution is also given in the book, both answer is same but only difference of sign.One is positive and one is negative. solution are given as follow-:

(1)work done by applied force=f×s×cos(theta)

= 100×4×cos 0 degree

=400joule

(2)work done by resisting force=f×s×cos (theta)

=100×4×cos180 degree

= -400joule

I understand the second answer but i dont understand the 1st answer how work done by applied force is 400 j,since how in case of 1st force is 100 N. The resisting force is 100N, not applied force how in the 1st case applied force is 100N

simply my question is – in the case of 1st how force is taken as 100N i think here we should take value of applied force and we are taking the value of resisting force.

Asked Md Sadique

Answer:

For enabling motion the applied force must at least be equal to the resistance force in opposite direction.

## Hawking Radiation

I know this is an odd way to ask a question but I found that giving what I do understand about the subject will make it easier to correct where I went wrong and what it is I don’t understand or what I understand incorrectly. From what I understand Hawking radiation is a theory created by Stephen Hawking in order to among other things retain in his astronomical model the laws of conservation of matter and energy. These are the concepts that dictate that matter is never created or destroyed but always conserved. It can be changed from one element to another or converted into energy but for every atom in the system at the begining there will always be the exact amount of energy and mass released or present at the end. According to this almost universally accepted law all matter and energy was created in the big bang and has only been manipulated and changed from Mass to energy and vice versa perhaps many times and will always comprise all matter and energy in the universe. This law is not only applicable to situations we encounter every day such as when we burn a piece of paper; for every atom in the piece of paper before burning there will be an exact corresponding amount of ash, smoke, heat, and light released. This law is also applicable and extremely useful on the astronomical scale. When a black hole consumes matter, be it meteors, planets, stars, quasars, or even entire galaxies with it’s indomitable avaricious gravitational appetite, there must be some relatively massive corresponding amount of energy and mass created somewhere in the universe to maintain the conservation of matter. Since as far as we can tell that which has been consumed by a black hole is effectually now nonexistent although can be added to it’s infinite mass so if that matter were not replaced over astronomical time periods there would eventually be no more mass or energy to compose the universe. Among British physicist Paul Dirac’s many accessions to modern science, he predicted that, if enough energy could be concentrated, such as in and around a black hole, an antielectron (always accompanied by an electron in order to preserve the overall electrical charge and net conservation) could in theory be produced where none had existed before. When this process occurs on the edge of a black hole at what is known as the event horizon the antimatter component will often be unable to escape the event horizon while the matter component is able to escape, allowing for a net addition to the system or universe as a whole, which over millennia will theoretically make up for all of the matter consumed by the black hole.?? First off why is it that when these matter antimatter pairs are created is it that an equal number of antimatter particles do not escape causing many to annihilate coming in contact with the escaped matter particles and the rest drifting into the surrounding universe but still leaving a net change in the total mass to zero leaving the mass consumed by our black hole unaccounted for. That is my understanding of Hawking radiation and I am very aware this simplification will no doubt infuriate many, if someone capable to help me to gain a better understanding even in these oversimplified terms I would be most appreciative

Paul

## Alternating current

An inductor and a bulb are connected in series with an a.c source. How would the brightness of the bulb change when..

a)the number of turns in the inductor is reduced?

b)an iron rod is inserted in the inductor?

c) a capacitor of reactance Xc=Xl is connected in series in the circuit?

Induchoodan Ramesh

## Vehicle in a turn

Hello. This is not a homework question, but as i was doing my seminar work, i came to a question that it has been bugging me for a while now. I do not need this for my project, but it came to my interest as it isn’t specified anywhere, but there must be some complex physics behind it. question: i’m doing construction engineering and i have been researching into importance of good road and surface design for maximum amount of friction between a car and tyre. What doesn’t add up are coefficient of friction of tyre, and rules and regulations for building roads. In the latter, coefficient of 0.037 was used(which i calculated backwards from a formula), but the tyre manufacturers state their coefficient of friction to be between 1 and 0.5(depending of size, model, intended use,… of tyre). So if i use coefficients of tyre of manufacturers in the formula for banked turn with friction, i get unrealistic results. So i’m thinking, how would i, at least approximately calculate that to get realistic results? Do i have to include many various different factors, from height and length of the car, suspension, air drag, weight, centrifugal and centripetal force, angle, as well as a safety factor, for when snow falls on the surface. So how would a formula like that look like? It seems to me it could get very complicated, that’s why i am asking as i do not have a deep physics knowledge. Thank you for your time.

Kristjan

## Visualisation paradox of tension

My son and I were discussing tension in a handheld rope with a 10 lbm weight on the end. The hand exerts an upward force of 10 lbf and gravity exerts a 10 lbf downward (net = zero). With these vector forces acting in opposite directions they would appear to ADD to 20 lbf of tension in the rope rather than 10 lbf. Where is the conceptual fallacy? How can I explain this clearly? Asked Hecket Answer When you consider the two points where force vectors are drawn 1. At hand and 2. On the hanging weight Tension of 10 lbf is produced throughout the rope. See the diagram