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# Tag Archives: distance

## Doubt in Law of Gravitation

According to law of gravitation

F=g*(m1*m2)/r^2

If we bring two objects close to each other say our hands then the distance will be zero and the force of attraction will become infinite because r will we zero. Therefore F will become infinite but this is not the case.

When we touch our both palms together we can easily pull then away.

Asked Ankit Sharma

**Answer**

When we consider the distance, it is between the center of mass of the two bodies under consideration. When we bring our hands together the distance between their CM is not zero.

Other wise, the two particles under consideration must be point masses.

Then if you consider two atoms or two nuclei, the forces that come into play will be different if we decrease the distance between two atoms or nuclei. The attraction will turn into strong repulsion **when the limits are exceeded.**

## Problem from Kinematics

The speed of a train changes from36km/h to 72km/h in 10sec. calculate the distance travelled during this time

**Answer**:

u=36km/h = 10 m/s

v=72 km/h=20 m/s

t=10 s

S=?

use **v = u+ at** to find a

a = 1 m/s^{2}

Use **v ^{2}-u^{2}=2aS** to find S

S = (400-100)/2 = **150 m**

## Ron writes on Light and Relativity

Let me start by saying I am NOT a physicist. That much should be painfully clear soon enough. Please bear with me as I am not certain I have the language and in depth **knowledge** to explain myself adequately.

A few years ago I began experiencing parallels between the world of human behavior and **physics**. At the **time** it seemed simple enough. That is until I started teaching myself **physics**. But at its core it seemed to make sense, the rules of life recycle themselves in different forms. Out of simple ideas comes complexity. Newton’s laws, thermodynamics… pascals principle, snells law, red and blue shift, wave **particle** duality and on and on… all seem to have their parallels in human behavior and on the surface seem to follow the same mathematical equations. Then I ran into the most famous of **physics** equations E=MC^2. It makes sense in human behavior. The **energy** (work) we can get out of an individual is relative to that individuals’ **mass**. The more **matter** we attach to the individual, **knowledge**, life experience etc (therefore increasing their **mass**) the more we can get out of them in the form of **energy**… but then the problem of the **speed** of **light** squared. The only known constant (**light** **speed**) is a problem in human behavior. At least to this point I know of no known behavioral constant.

I juggled the **idea** of it being a relative constant. Constant for the individual but relative as it would differ for everyone. The **speed** of cognition, or thought **speed**, would remain the same in potential throughout life of a given individual. Or that it was just a theoretical potential that humanity had yet to obtain. I even juggled the **idea** of it being a collective ability, but all of these options cause problems with the original equation. This has forced me to contemplate the nature of **light** and left me with questions I simply lack the understanding of **physics** to answer.

Is the **speed** of **light** truly constant or is it only constant as it relates to the big picture? As in: our **perception** of **time** as it relates to all **time** that has ever existed would appear as a single point in **time**. The older we get the more **time** seems to “fly by” if we as humans could continue to live for 13 billion years would the **perception** of an hour become so perceptively small that we wouldn’t even know it has passed? Is it therefore possible we can only understand and therefore measure the **speed** of **light** at a specific point in **time**, even if we try to come back and remeasure and compare the **speed** of **light** now with the **speed** of **light** fifty years from now the difference between the two would be imperceptivity small as a result of the displacement in **time** as it relates to the whole of **time**?

If the four dimensionality of **time**/**space** is linked shouldn’t **time** expand as **space** expands? And vice versa… if the **speed** of **light** is to remain constant as measured under such conditions isn’t **light** actually slowing down/speeding up over **time** as it relates to the whole of existing **time**/**space**? It’s a **distance** displacement problem…. If points A and B are actually farther apart but **light** travels the same **distance** in the same **time** the “**speed**” may seem constant but **time** has actually expanded to give the **light** more “**time**” to cover that **distance**. The **speed** of **light** would therefore be constant as a **relation** to **perception** and not as it relates to physical principles. Like the fact that the **perception** of **time** changes as we get older even though the actual measure of **time** remains the same.

How can **light** exist forever? At the **speed** of **light** we theoretically freeze **time** for that **photon** but that would also require an **infinite** amount of **energy** to obtain and maintain. So even a subatomic **particle** with a lifespan of a nanosecond would appear to exist for all eternity but in actuality would still only exist for a nanosecond. Because quite simply…it can’t have **infinite** **energy** and if it can I don’t understand how. Thermodynamics: no system is a perfect system and will experience **energy** loss, Newton: equal and opposite reaction, if something begins it has to end to balance the equation.

Is it therefore possible that **light** is born of the fourth dimension… we experience it in the dimensionality of **space** as long as it loses its **energy** to the three **dimensions**. A **photon** folds and pushes its way through **space**/**time** The initial **energy** of the **photon** is high and generates bigger leading waves which resist the **photon** holding it from passing the “**speed** of **light**” as the **energy** dissipates/**photon** begins to die those folds restricts the **photon** less allowing it to maintain the **speed** of **light**. We experience **light** because of its “ripples” in three-**dimensions** **light** dissipates as the waves of **space** become less folded in front of the **photon**. But this would mean the **photon** eventually loses enough **energy** that it can no longer be perceived in the third dimension…. So what happens to it?? Imagine : A man running through a corn **field** has to exert the **energy** to push the stalks aside but over **time** if the corn stalks are slowly spaced out even as he loses **energy** he can maintain his pace, because there is less impeding his path. Our **perception** of **light** would be like being in a helicopter looking down on the **field**. In the begging the **field** is densely packed with cornstalks and they slowly spread out until there is none. We only know the man is there while he is running through the corn because he pushes the stalks aside and we see that movement but once there are no stalks or he lacks the **energy** to continue running we no longer have a way of measuring his presence.

That brings me for some reason to a theory of Dark **matter**. Why? Well what happens to **photons** that no longer move with enough force to be visible. Like that guy running from a cornfield into an empty **field**. How would we know he is still there? If it was an **infinite** number of guys all stopping in that empty **field** we would know they are there by the depression their weight leaves in the **field**. Or more accurately by the stones in the **field** rolling towards a depression we cant actually see. The thought: An **infinite** number of “massless” (or perceived **mass** less) subatomic particles would still have **infinite** **mass**. Infinite **mass** would supply more then enough gravitational pull even spread out over **infinite** **distance** to cause the continued and speeding expansion of **space** as more and more visible **photons** “die” contributing its “dark masslessness”. Any dark **matter** existing within the universe would act as force… a moving invisible **mass** existing only on the 4th dimension pushing upon any objects in its way. But because **energy** propagated internally expanding outward in all **dimensions** would compound on the outside as those **energy**’s converge.

I hope you followed those questions. I don’t know what these ideas would do to theoretical **physics** but it would allow for the relativity of cognitive **speed** between individuals and reopen the door to “what the hell is dark **matter** in **relation** to human behavior” but that’s something different all together.

## A Numerical for Practice

**Akshay** posted a numerical:

1. A disk of mass m=50g slides with zero initial velocity down an incline plane set at an angle 30 degrees to the horizontal. Having traversed the distance of 50 cm along the horizontal plane, the disc stops. Find the work performed by friction forces over the whole distance. Assume friction coefficient is 0.15 for both horizontal and incline planes.

Answer:- (-0.05J)

## A Problem based on Friction

an unloaded bus can be stopped by applying **brakes** on straight road after covering a **distance** x. Suppose, the passenger add 50% of its **weight** as the load and the braking **force** remains uncharged, how far will the bus go after the application of **brakes**? (**velocity** of bus in both case is same)

## Problem from Kinematics

**Devika** posted:

There are two particles A and B moving with same velocity u such that initially there velocity are perpendicular to each other and distance between them is L.the velocity of A is always directed towards B.find the distance between them after a long time.

Ans will be posted soon

## Average Speed

Naman Asks:

“A car travels from A to B with a speed of 30km/h and returns back to A with a speed of 50km/h.Find distance,displacement,average speed and average velocity ifdistance between A and B is 100km.”

Answer:

Distance 200 m

Displacement 0

Average velocity 0

Average speed = = 37.5 kmph

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