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  • 0 posts edited
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  • 599 votes cast
Jan
28
comment Rotational Kinematics on a Clock
Can you convert hours to seconds? It is $2\pi $ radians per revolution.
Jan
19
comment Does all hydrogen originate from the Big Bang?
Wikipedia has charts of the atomic composition of the human body, both by weight and by number. By number, hydrogen looks is around $62\%$. By weight it is about $10\%$.
Jan
19
comment As The World Turns 2
@Floris: I hadn't seen that,but the Wikipedia page you link to says scientific consensus is against. Even so, increasing the radius to $102\%$ will decrease $g$ by $4\%$. That will swamp the decrease due to rotation speed. We still have less than $5\%$ decrease overall. Not a big effect if you are doing mechanical design of animals.
Jan
18
comment classical gravity wave
@Robbydcm: Newtonian mechanics, including gravity, assumes a universal time coordinate everywhere and that gravitational effects travel at infinite speed. It also presumes that the speed of light is influenced by source velocity. You can't pick and choose. You could try to formulate an alternate gravity theory that has finite transmission speed, but you would have to be careful not to cause a contradiction. Probably you have seen the derivation of all of special relativity from the constant speed of light. It is all interrelated.
Jan
18
comment classical gravity wave
There are two senses of classical here. You are using classical as an opposite to quantum, and in that sense general relativity is a classical (smooth, not quantized) theory. The other sense is classical Newtonian mechanics and gravity as opposed to general relativity. In that sense, there are no gravity waves in classical mechanics because gravity propagates instantaneously. I think OP was using classical in the second sense.
Jan
18
comment What is the effect of constant wind on a hot air balloon?
@ispirato: I said it in the first sentence. The balloon speed will approach the air speed asymptotically. It is the same as if the balloon were magically started moving at the wind velocity in still air. It will slow down, appoaching zero speed asymptotically. The two situations are equivalent-the balloon starts with a certain velocity relative to the air and that velocity decays away.
Jan
17
comment Two mass spring system
What makes the problem hard is we are using the wrong variables. If we define $x_1=y_1-y_2, x_2=y_1+y_2$ the equations decouple. The $x_1$ equation uses all three springs and the $x_2$ equation ignores the middle spring. These are the two modes of the system.
Jan
17
comment Two mass spring system
You are correct that you don't know from the start whether the middle spring is in tension or compression, but you should be able to see that its force is $k(y_2-y_1)$, then reason that if $y_2-y_1$ is positive the spring is in tension. That will get you the sign for both masses. If the middle spring is in tension, which means $y_2-y_1 \gt 0$ it produces a downward force on $m_1$, so adds to $\ddot y_1$ and produces an upward force on $m_2$, so subtracts from $\ddot y_2$
Jan
17
comment Two mass spring system
I don't know any way other than being careful. You look at the direction the force is applied and make sure the sign of the acceleration is correct. When you pull $m_1$ down, the force on it is up, so the acceleration is negative, and so on.
Jan
16
comment Why do unused tires lose their pressure?
Another assumption is that identically designed and mounted tires will have the same leak rate. I think variability between the items and installation can drive a difference in leak rate. OP did not specify the difference in pressure.
Jan
11
comment How to relate heat build up in an open pipe to sound frequency and specific heat capacity of the pipe's material.
The speed of sound in air increases as the square root of the absolute temperature. I suspect the heat buildup inside the instrument is much more due to the warm air supplied by the musician than due to acoustic losses inside the instrument. The amount of energy in sound waves is quite small, so it is hard to get any reasonable heating that way. Transferring the heat to the wall will reduce the effect even more as the wall is so much more massive than the air.
Jan
11
comment How does the magnetic field get induced in car's tire?
Many types of steel become magnetized in processing. It is likely the steel belts in the tire and/or your lug nuts are magnetized.
Jan
11
comment Does quantum mechanics play a role in the brain?
My reading of "The Emperor's New Mind" is that they ascribe brain function not only to quantum mechanics, but to quantum gravity. As the density of matter in the brain is not very different from water, this makes even less sense to me.
Jan
10
comment What will hapeen if we remove air from a pipe
I gave you the height of the water in the pipe. To get the volume you just multiply by the area. The temperature only changes the vapor pressure. Are you thinking about flow rate instead of the volume that flows?
Jan
10
comment Baryon - Anti Baryon scattering
@KimPeekII: the gluons are electrically neutral. They interact with the quarks and other gluons through the strong (color) force, not electromagnetism. At very high energies (short distances) the gluons and quarks are free, so scattering can happen with just the exchange of one quark or one gluon. This leaves the proton and antiproton colored, which has to be neutralized as they fly apart, leading to the exchange of some more quarks or gluons. Eventually the exchange has to add up to be colorless.
Jan
10
comment How to determine the radius of curvature of a convex lens?
Yes. That is a purely geometric property of the surface. As the light does not pass through the mirror, the focal length does not depend on the index of refraction.
Jan
10
comment How to determine the radius of curvature of a convex lens?
@AccidentalFourierTransform: OP has found the formula, applied it correctly, and finds the result counterintuitive. I think it is a fine question.
Jan
10
comment What will hapeen if we remove air from a pipe
@Floris: that is true. At 20C it is $0.023$ atm, so the column height will be reduced by about $23$ cm
Jan
9
comment Heating and Retaining of Heat in Air
@Floris: that is yet another way to understand adding extra $CO_2$ to air and you are correct. We need to know what is being done before we can answer the question.
Dec
25
comment How apply $\mathbf{F} = m\mathbf{a}$ to a whole pulley system?
@DJohnM: yes. You can just do a force balance. The only external forces are gravity on the masses and the support force on the pulley. The accelerations of the masses are altered by the tension in the rope, but that is internal.