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  1. Janus

    I Does Escape Velocity depend on a rocket's direction away from Earth?

    While rockets are initially launched straight up, they will begin to tilt over afterwards. Remember, most of the time, the objective is to put something in orbit around the Earth. This means that once it reaches orbital altitude, it needs to be moving at over 7 km/sec parallel to the Earth's...
  2. Janus

    I Falling in an O'Neill Cylinder

    While standing on the inside of a rotating cylinder, you are accelerating. Acceleration is a change in velocity, and velocity includes both speed and direction. Since the cylinder fall is forcing you to travel in a circle, and this causes you to constantly change direction, you are...
  3. Janus

    B Speed of light versus wavelength

    Think of it this way: Light is like a train and the wave length is the length of each individual car. If the train travels at 10 m/s and each car is 10 m long, then the frequency at which cars pass is 1 car/sec. If the cars are 5 m long, the same train speed produces a frequency of 2 cars/sec...
  4. Janus

    B How long it takes the Earth to fall halfway to the Sun--ellipse method

    Towards the end of this article is an equation which allows you to calculate this. https://en.wikipedia.org/wiki/Equations_for_a_falling_body
  5. Janus

    I Absolute and relative motion/rest

    You have to apply a force in order to go from moving with respect to the Earth to at rest with respect to it also. Now you might argue that you needed to use your engine to get moving relative to the Earth, and only had to apply brakes to come to rest, But you can also say that all the brakes...
  6. Janus

    B Why doesn't the Earth at the equator experience a sonic boom?

    When the first steam trains were introduced there was concern by some in the general populace that you wouldn't be able to breathe moving that fast, or that the vibrations would knock you unconscious. Of course these fears were unfounded, and anyone who put some thought to it, ( such as...
  7. Janus

    B Does Gravity decrease at a steady rate as we go away from the Earth?

    The equation for gravitational force is F = GMm/R^2. M is the mass of one object, m is the mass of the other object, and r is the distance between their centers. G is a the universal Gravitational constant. It's value depends on the units you are using for R, M&m, and time. If you use...
  8. Janus

    I Why don't we consider Earth's forces/movement in problems?

    Another thing to consider is how large the difference in the answer would be when taking into account all these other effects when compared to the uncertainty of your initial conditions. So for example, to find the fall-time for a 100 kg object over a given height, you need to know the...
  9. Janus

    B Hypothetical Gravity Question #2

    You might what to check this out: https://en.wikipedia.org/wiki/Gravity_train
  10. Janus

    I Gravity on Earth vs the Moon

    Yes, I meant the moon's current mass.
  11. Janus

    I Gravity on Earth vs the Moon

    Of course that is due to the fact that Saturn has a density that is 1/8 that of the Earth's. In the same vein, The Moon has a lower density than the Earth. If it had the same density as the Earth and the same mass, its surface gravity would be closer to 1/4 the Earth's rather than 1/6.
  12. Janus

    B Forces inside a moving elevator

    The real question is why you think there would even be a discernible difference just because you were moving vertically at a constant speed.
  13. Janus

    B Coriolis problem - Point mass movement upon release from Earth

    If you cancel gravity completely than the object travels in a straight line and fly off into space. From the frame of the person standing on the surface, this will appear to be a curved trajectory(Coriolis effect). To have it "hover" a fixed distance above the ground (assuming a spherical...
  14. Janus

    I Travel from Mars to Earth question.....

    Below is a diagram of orbital trajectories. Blue circle is Earth orbit, Red circle Mars orbit. The green ellipse is a Hohmann minimum energy transfer orbit. Your craft will intersect Earth's orbit at the Green arrow after ~258 days. What you want is a trajectory more like the violet...
  15. Janus

    B GMm/r^2, r→0?

    https://www.mathpages.com/home/kmath402/kmath402.htm
  16. Janus

    I How strong is gravity?

    Escape velocity is the speed that an object would have to have at some distance "R" from the center of mass, in order to never fall back. So imagine you have your two in-falling apples, and instead of hitting, they started with just enough sideways motion to just skim past each other with...
  17. Janus

    I How strong is gravity?

    As mentioned by Ibex, the escape velocity for an apple is really, really, really small. This, in turn, means that the collision speed between the two apples, even if they start 10 million light years apart, is going to be of similar magnitude. Not even enough to produce any significant...
  18. Janus

    Why there is no disturbance in orbital paths?

    Small additional forces cause small changes in the orbit, for major changes, you need large influences. So for example to cause the Earth to leave the Solar system, you would have to apply enough force for a long enough time to increase its orbital velocity by some 12 km/sec. To cause it to...
  19. Janus

    I Hypatia, Elliptic orbits, conic sections

    As already pointed out, Kelper's laws came before Newton. Kepler law's were based on careful measurement of the planets. He observed that planets followed certain rules as they orbited the Sun. Newton later came along to give a theoretical basis for these observations Kepler made about...
  20. Janus

    B Thought Experiment from a Non-Physicist

    Some additional points that might be of interest to the OP. If we we assume a spherical Earth of constant density, and using a radius of 6378 km for the Earth, then the round trip would take 84 min 28 sec, or 42 min 14 sec one way. If we drill our hole such that it doesn't pass through the...
  21. Janus

    B Thought Experiment from a Non-Physicist

    You'd be in free fall the whole way and the tidal gradient wouldn't be anything to worry about. As long as you don't hit a wall on the way through (at the center you'd be moving at better than 7.9 km/sec), you'd be fine.
  22. Janus

    B Thought Experiment from a Non-Physicist

    Here's a graph comparing how gravity would change with depth according to three different models for its interior: constant density, Density that increases linearly as you move towards the center, and Our estimate of what is is for the real Earth (Preliminary Reference Earth Model,or PREM)
  23. Janus

    B Grokking the 1/2 * a * t^2 product

    Just to add to what Vanadium 50 has already said. Consider the two graphs below. Both are for velocity over time. The one of the left is for an object at constant velocity and the the one of the right for an object starting at 0 velocity and under a constant acceleration. In both cases...
  24. Janus

    I Forces and escape gravity

    The thing to keep in mind is that escape velocity at any given distance from the Earth's surface decreases with the distance. So if you were to climb away from the Earth at 100 m/s, once you reached a distance of ~80,000,000 km from the Earth, you will be moving at escape velocity for that...
  25. Janus

    I Devil's Racetrack

    They do according to a study done by Scripps Institution of Oceanography, UC San Diego, which placed motion activated GPS units on rocks in the area in 2011. In December of 2013 these units registered the rocks moving.
  26. Janus

    I Instability of a Rigid Untethered Ring Around a Planet

    In 1859 James Clerk Maxwell addressed this problem in connection with the structure of Saturn's rings. He proved that such a solid ring would be unstable, and thus the rings must be made up of many separate bodies.
  27. Janus

    How long would it take two planets to collide?

    It also wouldn't take any amateur astronomer too long to note that the Moon was no longer moving relative to the background stars, since this motion is about its angular size per hour.
  28. Janus

    How long would it take two planets to collide?

    I assume that you mean that if the Moon were falling straight towards the Earth, you wouldn't see a noticeable increase in its size for the first 24 hrs. This is likely true, as in the first 24 hrs, its angular size would only increase by ~3%. And assuming that it started at it's average...
  29. Janus

    Orbit of Cube planet

    http://arxiv.org/pdf/1206.3857.pdf
  30. Janus

    How rockets take curved paths in space (absent gravity)

    Yeah. sometimes just getting to the right point isn't enough, you have to get there at the right moment too. Say you are going to use a gravity-slingshot to get you to your final destination. If you don't arrive at the slingshot point at the right time, you won't get the right boost in the...
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