# Amr Morsi

Senior Members

272

1. ## Equations! Find the smallest value...

timo, have you considered the first equality in your calculations? I think we should use 4,6,8 for a,b,c and 4 for x.
2. ## Length of spiral

asinh(theta) contains a ln function which cannot be solved with "theta * sqrt (1 + thetaˆ2)". So, I think the inverse function will not be a closed form.
3. ## String theory

For sure, strings are interacting with each others and are exchanging energy with each others. But, I don't think that this is the main reason. I think it has to do with the medium (or the surroundings); radiation and potential energy.
4. ## what does c-squared have to do with mass?

The main equation is E=sqrt((p*c)^2+(m*c^2)^2). When the particle is at rest, p is zero, and then you get E=m*c^2. It is called the Rest Energy. And it is the total energy confined in mass m. When m is in Kgs and c in m/s, the unit for E is Joule.
5. ## Simple momentum question

So as not to get confused, make the equation as follows: Total momentum before=Total momentum after. Note that it is not a must for the change in momentum to be negatively equal. They only moved together.
6. ## Related Rates Question

O.K. Bignose. Sorry for that.

Thanks.
8. ## Uniform continuity

"Compact interval" is not involved here. It is a separate point.
9. ## completeness in R

It uses the definition of a Bounded Sequence.
10. ## Another limit

But, you shouldn't go the first step directly. You must use Left and Right limits (for both x and y).
11. ## Uniform continuity

Check the following: http://en.wikipedia.org/wiki/Uniform%5Fcontinuity Go to the last point in "Properties" section.
12. ## photons mass and energy

O.K. ajb. Do you know any other deduction for the zero "mass" of the photon? I thought the previous was the reason.
13. ## Related Rates Question

area=0.5*base*height=0.5*base*sqrt(S^2-(9.5)^2), where S is the length of the side. Then, d(area)/dt=d(area)/dS*dS/dt= 0.5*19*S/sqrt(S^2-(9.5)^2) * 2 ft^2/min At height=8 ft, S=sqrt(8^2+(9.5)^2) ft. Substitute with S in the rate of change of area , and you will get the result.
14. ## completeness in R

Since the sequence is bounded from above, therefore it reaches a certain number (Ao), as n tends to infinity. Then, lim A(n), as n tends infinity, equals Ao. Therefore, it has a limit.
15. ## Another limit

1. You will have to check the Left Limit for the variable x. It will be lim f(x,y)=xo*y 2. Check the Right Limit for the variable x. It will be also lim f(x,y)=xo*y. 3. Therefore, the Left Limit of f(x,y), as x tends to xo, is equal to the Right Limit. 4. Repeat the previous, as y tends yo. And, you will get the same results. 5. Then lim f(x,y), as (x,y) tends to (xo,yo), is equal to f(xo,yo). Note: There are some simple functions that can be considered continuous, unless it is required to prove so....... such as f(x)=x, f(x)=x^2.
16. ## Uniform continuity

Sorry, another factor is remaining which is lim f(x), as x tends to infinity, exists and is equal to zero. And, we have the theorm "If a real-valued function f(x) is continuous on (0,infinity) and its limit, as x tends to infinity, exists (and is finite), then f(x) is uniformly continuous."
17. ## Ax = b question

You are right, timo. A is not invetible and so A^2. Sorry for that error.
18. ## photons mass and energy

The reason that photon does have a zero rest mass is that its gamma is infinity. Therefore, mo=m/infinity=zero.
19. ## Einstein Field Equations

But, as you said, Einstein Tensor depends upon the Metric Tensor, therefore the metric (which is sufficient to describe the spacetime curvature) is defined in The Einstein Field Equation. Then, the basic background of gravity is defined in " The Einstein Field Equation". It is, then, what is remaining, is the way that spacetime curvature is acting upon the mass situated at certain position.
20. ## Uniform continuity

That a function to be continuous on an interval, it has to be continuous at every point in the interval.
21. ## Another limit

O.K...... I was talking like that because f(x,y) is obviously continuous, and lim f(x,y), as (x,y) tends to (xo,yo), is f(xo,yo). If you need an exact proof, then find the left limit which will be f(xo - epsilon , yo - epsilon). And, find the right limit which will be f(xo + epsilon , yo + epsilon). Then, since both are equal then the limit is found.
22. ## Einstein Field Equations

You misunderstood me ajb. I agree with you that it's indirectly implied. But, we can't say that "this energy is not included in the Einstein Field Equations".
23. ## Einstein Field Equations

How is gravitational energy not involved in Einstein Field Equation?! It is totally built upon the effect of Mass on the spacetime curviture, which in turns define gravity. Gravitational energy is implied in the Einstein Tensor in the Equation. But, don't forget. There is another part that is remaining which is "An object in a gravitational field follows the shortest distance between two points". It is completing the description of the gravitaional effect on masses.
24. ## wave energy conversions

For sure, many of the properties you stated about energy waves are correct. But, there is a difference between Probability Wave and Energy Wave. PW is describing the probability of existence of a particle under certain potential. Of course, the particle has energy, but, it is the wave nature of the particle not the energy. Energy waves, however, can be described by the motion of mediators, which are particles with special specifications. And, I agree with you, these mediators can be described quantatively by Quantum Field Nature according to the properties of the source of the field. When gravitons interact with a mass, this doesn't imply a change at the source. But, as this mass is affected, its own field of cource changes (at a velocity of c).
25. ## PCB Power Supply

You can check it with a small lamp..... Intensity of light is proportional to the voltage applied.
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