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  1. Janus

    I "Dark matter exerts gravity according to the same rules that ordinary matters follows"?

    When examining the idea of dark matter, there are two things to consider, Its effect on the orbits of the stars in the galaxy, and its effect on itself. When we assume that it behaves just like other matter gravitationally, everything "fits together". We can work out how it would naturally...
  2. Janus

    I "Dark matter exerts gravity according to the same rules that ordinary matters follows"?

    Even if a single molecule of this "stuff" weighed as much as the Sun (effectively making it a black hole). The gravity it produces would be the same as the Sun. What he meant was that it's gravity would fall off by the square of the distance (not by the cube or some other value). It would...
  3. Janus

    I Center of Mass of a jetpack-Earth System

    The only way to move that body via propulsion is for it to throw mass in the opposite direction. The center of mass of expelled fuel and object will remain fixed. Since the center of mass of this system doesn't move, the center of mass of the whole system doesn't move,and the other body doesn't...
  4. Janus

    B Impression of relative velocities

    For a visual representation, the black lines are your field of vision. Your motion is "downward" in the figure. At first, both the distant and near object are in the center of your field of vision and along the same sight line. At some moment later, you have moved to a new position relative...
  5. Janus

    B Is one side of the Earth faster at night and slower at daytime?

    The center of the Earth orbits the Sun at ~30 km/sec A point on the equator moves relative to the center of the Earth at ~ 0.46 km/sec due to the Earth's rotation. Thus you get the following (with the sun located below the image): The point of the equator nearest the sun has its rotational...
  6. Janus

    B Is one side of the Earth faster at night and slower at daytime?

    The diameter of the Moon's orbit is 768000 km and it takes ~29.5 days to complete 1 orbit with respect the Sun. Thus it takes 14.75 days to move 768,000 km from leading the Earth in its orbit to trailing it. The Earth's orbital speed is ~30 km/sec, and thus would have moved 38232000 kn along...
  7. Janus

    B So what is the new definition of the kilogram?

    The new definition I found is Planck's constant divided by 6.62607015e-37 m-2s
  8. Janus

    I Move the Earth Away from the Sun

    Others have addressed the issue of how much mass it would take to produce such a change in the Earth's motion. But what I don't think you understand is that such a change would result in only an very small change in the size of the Earth's orbit. It's not a matter of giving the Earth a 1 mph...
  9. Janus

    B Gravity constant

    One point of interest. For certain bodies, such as the Earth, we actually know the value of the product GM to a higher accuracy than we know the value of either G or M. GM for the Earth is 3.986004418e14(8) while G is 6.67408(31)e-11, a fair difference in the number of significant digits.
  10. Janus

    I Inverse square law confusion

    1. We use telescopes with large apertures that can gather much more light than our eyes can. 2. We spend a lot of time gathering light from what our telescope is pointed at. The Hubble telescope can spend weeks looking at a single spot in the sky, slowly collecting light in order to build up an...
  11. Janus

    I Which object will hit the surface of a planet first?

    In this situation, both the mass of the Earth and 1 gram object are, for all practical purposes, insignificant compared to the 1 solar mass neutron star. So the easiest way to examine the scenario is to treat it as the Earth being a given distance from the neutron star and the 1 gram object...
  12. Janus

    I Dropped objects hitting the ground at the same time?

    If you drop the objects individually and at different times, then the more massive object will take slightly less time to impact. However, if you drop them simultaneously, and side by side, then they will impact at the same time. The Earth cannot have two different values of acceleration over...
  13. Janus

    B Lowest possible altitude for a Satellite

    And even for the same satellite, varying solar activity can have an effect on the Earth's atmosphere, which will cause the drag to change.
  14. Janus

    B Time travel for airplane

    That was exactly the point I was making, that tnich's example of leaving at 2 pm and arriving at 5 pm going one way and leaving at 2 pm and arriving at 3 pm going the other did not mean that the trip took 3 hrs going one way and 1 hr going the other.
  15. Janus

    B Time travel for airplane

    Note that in tnichs answer has you leaving and arriving San Francisco in Pacific time and leaving and arriving in Denver in Mountain time. You are moving from one time zone to another. MST is one hour ahead of PST, so taking off in at 2PM PST and arriving in Denver at 5pm MST means you had a...
  16. Janus

    B Why does GPS require an accurate clock?

    The satellites are not in geostationary orbits, which means they are over different points of the Earth at different times . To know where a particular one is when using it to get a position fix, you have to know when it is.
  17. Janus

    I Why do rockets turn horizontally so soon after launch?

    Let's look at two different ways of putting a rocket into orbit. First let's consider your idea. straight up until you reach orbital altitude and then horizontal to go into orbit. We will assume an orbit at an altitude of 300 km. First you will have to get up to enough speed to reach that...
  18. Janus

    Why can’t gas support a solid body?

    For an an object to be buoyant in a gas, it's density must be less than that of the gas. The lightest metal is lithium and the heaviest known gas is Tungston Hexafluoride. At anything over 41 atm of pressure, The Tungston Hexaflouride would be denser than the lithium and thus a hunk of...
  19. Janus

    B Gravity and how does the surface get it ?

    And to add to what's already been said: Writing a number as 5.972 x 1024 is called "scientific notation" or "exponential notation". (10^24 is the same as 1024) It is a shorthand way of dealing with very large or very small numbers without all the zeros ( a number like 0.000000003 would be...
  20. Janus

    I Michelson Morley experiment: relativistic explanation

    This animation might help:
  21. Janus

    B Can this plane take off?

    But isn't that point? In the case where the treadmill moves at the same speed as ground speed, the results are the same for car and plane. When it's the same speed as tangential speed of the wheel, then obviously the car doesn't move relative to the ground as it is propelled by the wheels...
  22. Janus

    B Can this plane take off?

    The problem I have with that interpretation of the intent of the question is the equivalent of asking that if you put a car on a treadmill that travels backwards at the same speed as the car is moving forward relative to the ground, can the car move forward? It is a bit nonsensical, because the...
  23. Janus

    B Can this plane take off?

    The problem is that there are scenarios that if they are reduced too far, lead to nonsense results.
  24. Janus

    B Can this plane take off?

    Bandersnatch has already touched on the key issue of this problem. If you assume ideal conditions, the instant the jet tries to move forward, the conveyor belt speed and rpm of the wheels would become infinite. It suffers from the problem many scenarios do when you try to apply ideal...
  25. Janus

    I Can 2 spheres orbit each other say inside ISS?

    Assuming a lead sphere at a density of 11,340 kg/m3 and a density of 5520 kg/m3 for the Earth, the Roche limit for the lead sphere is ~0.99 Earth radii, or below the surface of the Earth. So this means that a small object sitting on the surface of the lead sphere would be gravitational held to...
  26. Janus

    I Can 2 spheres orbit each other say inside ISS?

    The radius of the hill sphere for a Kg mass at the orbit of the ISS is less than 2.5 centimeters, or put another way, well inside the radius of the lead ball. P.S. rechecked my post and found a unit error, which I corrected.
  27. Janus

    I Comparison of tidal forces acting on the Moon vs Enceladus

    There are a number a factors to consider. Enceladus is less dense than the Moon and thus likely not as rigid. As a comparison, consider what would happen if you were rapidly squeezing and releasing a foam rubber ball, vs. doing the same, with equal force to the ball made of a rigid material...
  28. Janus

    B How do Neutrons and U235 Nuclei get out of the fuel rod

    With the Chernobyl graphite reactor design, the graphite was used in the same way heavy water is; to slow neutrons so that non-enriched natural uranium can be used for fuel. The water serves two purposes, it carries away energy that is used to run the turbines for power production while it...
  29. Janus

    B How do Neutrons and U235 Nuclei get out of the fuel rod

    You seem to be talking about a heavy water reactor. This is a reactor that uses heavy water as the moderator so that you can use non-enriched or lightly enriched uranium for the fuel. The heavy water moderator and the control rods are two separate things. The moderator slows fast neutrons...
  30. Janus

    B Effect of the Sun's Gravity on the Moon

    As already pointed out both the Earth and Moon are being pulled on by the Sun by pretty much the same amount. I say "pretty much" because at Full moon, the Moon is slightly further from the Sun than the Earth is and during the new moon is is slightly closer. This produces a small difference in...
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