What is Gravitational field: Definition and 369 Discussions

In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena, and is measured in newtons per kilogram (N/kg). In its original concept, gravity was a force between point masses. Following Isaac Newton, Pierre-Simon Laplace attempted to model gravity as some kind of radiation field or fluid, and since the 19th century, explanations for gravity have usually been taught in terms of a field model, rather than a point attraction.
In a field model, rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is what is perceived and measured as a "force". In such a model one states that matter moves in certain ways in response to the curvature of spacetime, and that there is either no gravitational force, or that gravity is a fictitious force.Gravity is distinguished from other forces by its obedience to the equivalence principle.

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  1. tom.stoer

    Geodesics in a constant gravitational field

    I want to interpret geodesics in a constant gravitational field. As a simple example I start with flat Minkowski spacetime ds^2 = -dT^2 + dX^2 + dY^2 + dZ^2 with a geodesic (in terms of coordinate time T) X^\mu(T) = (T, X=A, 0, vT) where A is an arbitrary constant and v ≤ c...
  2. T

    Gravitational Field Paradox Explanation?

    Gravitational Field Paradox Explanation?? Hi everyone. If electric and gravitational field has an infinite nature-meaning that the effect one charged particle has on another charged particle always remains even though it diminishes over distances but can never reach absolute 0-then how is this...
  3. U

    Need help with Gravitational Field Strength question

    Homework Statement A satellite orbites Earth at a distance of 3rEARTH, above Earth's surface. What is the magnitude of gravitational field strength at the point where the satellite is?Homework Equations Fg= Gm1m2/d^2 (m1=mass 1, m2=mass2) F1/F2 = D2^2/D2^2 (D1 is a variable, and so is D2, so...
  4. A

    Gravitational field strength of the earth

    Hi, Using the formula g = \frac{GM}{r2} I found 'g' to equal (9.792121683 x 106 \frac{N}{kg}) The value of g is 9.8 \frac{N}{kg}. I am wondering if I made an error in my calculations or if there is a method of simplifying to get 9.8. Thanks,
  5. P

    Acceleration in Gravitational Field

    Hi It is always said that in a vacuum two objects even if they have different mass, have the same acceleration to a gravitational point regardless of their mass. If I understand this right, it needs the same time for 1kg mass or 1000kg to fall from 100m to the gravitational center...
  6. C

    Quantum Mechanics: Particle in Gravitational Field

    Homework Statement The Hamiltonian for a particle moving in a gravitational field and under no other influences is H = (P^2)/2m - mgZ where P is the momentum in the Z direction. 1. Find d<Z>/dt. 2. Solve the differential equation d<Z>/dt to obtain <Z>(t), that is, <Z> as a function of t, for...
  7. X

    Lorentz contraction and gravitational field

    Here's a scenario: A rod shaped object 1m in diameter and 300m in length is moving through space at a velocity of .999994444429013c (picked that velocity arbitrarily for the 300:1 length contraction). The Lorentz factor is 300 for this problem. So the equation to figure length contraction is...
  8. H

    Change in Potential Energy levels of a body in Gravitational field.

    Homework Statement Now , I Will Begin by stating the calculation and i will post the query at the end. so here we begin, we know that If W is the work done by a conservative force, potential energy change is given by: ΔW= - ΔU ...(1) ie when work done by conservative force is positive...
  9. M

    Orbits in gravitational field of a toroidal planet

    Homework Statement We have a homogenous toroidal planet. Its cross-section is half of a square, so that the hypotenuse is parallel to the z-axis. Find the orbits of constant (especially lowest) energy around that planet. Homework Equations 1) Homogenous planet: \rho=const. 2) Potential...
  10. T

    Gravitational field vs. acceleration due to gravity

    "Gravitational field" vs. "acceleration due to gravity" So I'm pretty sure the following paragraph is all true. Do the citizens of PhysicsForums agree? Please confirm and/or correct and/or clarify, as needed. In electostatics, the electric field E is a completely different quantity than the...
  11. D

    Gravitational field strength for irregular object

    Hi all I'm trying to work out what the surface gravitational field strength of an irregularly shaped body would be (for example Mars' moon Phobos). I know that for a sphere, any point outside it can consider all the mass to be at a point inside it, but for something that's potato shaped, how...
  12. C

    Resultant gravitational field strength

    Homework Statement Consider the objects X and Y, each of mass 8000kg and 2000m apart where XYP forms an equilateral triangle. Determine the resultant gravitational field strength at point P due to objects X and Y. Homework Equations G= GM/R^2 The Attempt at a Solution
  13. J

    The total sum of forces in circular motion in gravitational field

    Hello, I am now studying centripetal force. and the problem is that centripetal force in gravitational field. Let's talk about Ferris-wheel ride, this ride has some rooms to carry people and fixed to the edge of the wheel. Since it is rotating periodically, its magnitude of net force...
  14. P

    Perpetual Motion In The Gravitational Field

    Can perpetual motion be possible in a gravitational field? If I take an object and into space from earth, the potential energy increases but as we go further away from the Earth the forces which contribute in the work done decreases, thus it seems to me as if energy is destroyed. Because the...
  15. M

    Work done by external force in a gravitational field.

    Homework Statement Spheres of masses m1 2.53kg and m27.16kg are fixed at a distance 1.56m apart centre to centre.A m3 212 g sphere is positioned 42cm from the 7.16 kg fromcentre of 7.16 kg sphere along the line of centres.How much work must be done by ext agent to move thesphere of 212 g...
  16. D

    MHB Motion of Particles in Uniform Gravitational Field

    Two particles move in a uniform gravitational field with an acceleration $g$. At the initial moment the particles were located at one point and moved with velocities $v_1 = 3 \text{ms}^{-1}$ and $v_1 = 4 \text{ms}^{-1}$ horizontally in opposite directions. Find distance between the particles...
  17. Y

    Gravitational field Strength Inside the Earth

    Since the Gravitational Attractive Force is given by, F_g=(Gm_1 m_2)/r^2 Let m_1be the minor mass of a small body (e.g. human) let m_2 be the massive mass of a huge spherically symmetric body (e.g. Earth) Let r be the...
  18. S

    Free Fall and gravitational field strength

    Hello. For free fall to occur, does the rate at which you fall have to equal the gravitational field strength at that point above the surface? If I'm in an airplane and accelerate towards the ground at an acceleration of 9.8ms^-2 equal to that of the Earth's gravitational field, then I...
  19. andrewkirk

    Why is a gravitational field negative energy ?

    Why is a gravitational field "negative energy"? The idea about the universe having zero net energy, as explained for instance in http://en.wikipedia.org/wiki/Zero-energy_universe, seems to be that gravity has 'negative energy', which offsets the positive energy of all the matter and radiation...
  20. A

    Stress-Energy Tensor for Gravitational Field

    I'm trying to understand the way that the stress-energy tensor for a gravitational field is derived and I've run into a few problems. It seems that there are two main avenues which are kind of similar. One derivation involves looking at g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu} where \eta_{\mu\nu} is...
  21. P

    Gravitational field strength equals zero?

    There is a point between Earth and moon where the gravitational field strength is zero. What about the same thing with three or more objects? With three of more objects, is there always at least one point where the gravitational field strength due to the objects is zero? Why / Why not?
  22. S

    Moving charge in gravitational field.

    An accelerated charge particle radiates. So, in the light of principle of equivalence, does a charge particle also radiate when it is in a gravitational field? Sorry if the question doesn't make any sense.
  23. D

    Universal gravitation 6-determine strength of gravitational field

    Homework Statement The moon has a mass of approximately 7.34x1022kg and a radius of about 1785 km. Determine the strength of the gravitational field on or near the surface of the moon. Homework Equations I have made a list of equations that are relevant for this entire module on...
  24. E

    How High Is a Meteor When It Accelerates Toward Earth?

    Homework Statement A 12 kg meteor experiences an acceleration of 7.2 m/s^2, when falling towards the earth. a.) How high above the Earth's surface is the meteor? b.) What force will a 30 kg meteor experience at the same altitude? Homework Equations Fg = ma , Fg =Gm1m2 / r^2...
  25. E

    Universal Gravitation and Gravitational Field Question

    Homework Statement The Earth has a mass of 5.98x10^24 kg and the moon has a mass of 7.35x10^22 kg. The distance from the centre of the moon to the centre of the Earth is 3.84x10^8 m. A rocket with a total mass of 1200 kg is 3.0x10^8 m from the centre of the Earth and directly in between Earth...
  26. Y

    Force as measured at different heights in a gravitational field.

    Say we have identical masses side by side low down in a strong gravitational field. We attach ideal massless cords to each mass and suspend them from spring balances higher in the field. One spring balance (with a longer cord) is at a much higher altitude (h2) than the other at h1. Will the...
  27. N

    Gravitational field caused by a charged particle

    I am new to General Relativity, so this question might sound silly. I know that the source of gravitational field is the stress-energy tensor. What I wonder is how do I find the spacetime metric, gmn caused by a stationary charged particle? What is the stress-energy tensor of a stationary...
  28. Z

    Help Solving Calculus/Mechanics Question on Particle in Gravity Field

    I'm really struggling to get my head around some of the mechanics applications in my calculus module and would really appreciate it if somebody could help me get my head around it. I have this question: A particle P of constant mass m has a position vector: r=x(t)i+y(t)j, and moves...
  29. J

    Finding the gravitational field of a rod

    Homework Statement A homogeneous rod with even thickness has mass M and length L. a) Find the gravitational field g(x) at a distance x from the rod on an axis normal to the midpoint of the rod. b) Find the gravitational field g(y) at a distance y from the rod's midpoint on an axis through...
  30. P

    How Do You Solve the Equations of Motion in a Gravitational Field?

    I want to find out the general equations of motion for a particle with an initial velocity v_0 in a gravitational field by a point/spherical mass (assuming this is a large mass which doesn't move). Assume that the origin of the coordinate system is the point mass. If the vector equation of the...
  31. A

    Calculating the gravitational field due to a horizontal uniform thin disk

    Homework Statement Show that the gravitational field due to a horizontal uniform thin disc (thickness D, radius R and density r) at a distance h vertically above the centre of the disc has magnitude 2πGρd(1-h/(R2+h2)1/2) A pendulum clock in the centre of a large room is observed to keep...
  32. E

    Gravitational field from earth/ center of mass

    When calculating the gravitational field from the earth, why can we make the assumption that all of the mass of the Earth is 'averaged' at the the geometrical center? If we imagine the Earth as a bunch of pieces, and then calculate the sum of forces from each of these pieces, would it not be...
  33. K

    Motion of light in a gravitational field

    Would it be correct to say that light follows a cycloidal path in the presence of a gravitaitonal field, in accordance with the principle of least time?
  34. C

    Varying Intensity of Gravitational Field and Equivalence

    Hi all, I was just looking for some assistance in reconciling the equivalence principle and the varying intensity of the gravitational field. (I'm in high school so go easy on me, I'm just studying Einstein's Relativity for the general reader). For convenience let's keep with Einstein's example...
  35. tom.stoer

    Radiation of free falling charges and charges at rest in a static gravitational field

    The question is rather old but I am not sure if we have a final conclusive answer (I was not able to figure it out for myself, nor have I found consensus in the literature) 1) do free-falling charges in a static gravitational field radiate? 2) do charges at rest in a static gravitational...
  36. J

    Calculate net gravitational field on the moon, without knowing its mass?

    Homework Statement The Moon is 3.9*10^5 km from Earth's center and 1.5*10^8 km from the Sun's center. The masses of Earth and the Sun are 6*10^24 and 2*10^30 kg, respectively. a. The ratio of the gravitational fields due to the Sun and Earth at the center of the moon is: 1) 2300 2) 230...
  37. mrspeedybob

    Traveling parallel to a ray of light through a gravitational field

    I was thinking yesterday about a scenario where a ray of light passes a massive body and is deflected. If I were in a rocket moving at the speed of light along the same trajectory, I should follow the same path through the gravitational field and so should observe the ray of light to be...
  38. ShayanJ

    Rotation of a sphere because of its gravitational field

    I want to know how can I prove that a massive sphere rotates becuase of its gravitational field.I thought about it and did the following: The gravitational force applied to a mass m which is on the axis prependicular to the plane of a hollow ring and passing through its center is as follows...
  39. U

    Finding the (Newtonian) movement equation of an object in a gravitational field

    This is a problem I've been looking to solve for some time. Homework Statement You must find a movement equation for an object in a gravitational field knowing traditional formulas of force and acceleration of gravity (see below).Homework Equations absolute value of the acceleration at a...
  40. M

    Gravitational field calculation question

    I do believe that somewhere I have read that an electromagnetic field does produce a gravitational field. So, considering the statement above, how can one calculate the strength of a gravitational field if the strength of an electromagnetic field is given?
  41. C

    A charged particle in constant gravitational field

    In classical electrodynamics a paradox arises when we compare the power radiated by a charge falling under gravity using larmor formula (proportional to square of the acceleration, hence g^2) but the radiation reaction , given by abraham lorentz formula gives zero.(since it depends on the time...
  42. P

    Gravitational field conceptual question

    Suppose a case where two masses are essentially far away form all other bodies so that we can assume the two masses are not affected by any other interactions going on beside them and they do not have the property of gravitation until a time( they are perfectly isolated{in theory everything is...
  43. E

    Gravitational Field From Ring Mass

    Most introductory physics textbooks derive the following formula for the gravitational field for a point mass on the on the axis of the ring: g_{x} = -\frac{Gmx}{(x^2 + a^2)^{1.5} } where, m = total mass of ring x = distance from point mass to ring center a = radius of ring Is...
  44. M

    Varying Gravitational Field - Invariant Tetrahedron?

    Varying Gravitational Field - Invariant Tetrahedron?? Classical Theory of Fields, Landau Lifgarbagez, page 246: "Strictly speaking, the number of particles should be greater than four. Since we can construct a tetrahedron from any six line segments, we can always, by a suitable definition of...
  45. C

    Gravitational field has energy, E field does not?

    Watched a physics lecture yesterday in which the teacher stated that a G field has energy, but an E filed (due to the Coulomb force does not). This does not compute. Both fields have potential energy, yes?
  46. J

    Could dark energy simply be gravitational field energy?

    Could dark energy simply be gravitational field energy? If we assume that the Universe is always spatially flat then we have the equation: H(t)^2 = 8 Pi G rho(t) / 3 where H(t) and rho(t) are the Hubble constant and total matter density at time t. Now we have the equation c = H(t)...
  47. F

    Mass distribution and gravitational field

    The field outside a body with spherical mass distribution is well known but for a non simmetrical body (or a almost simmetrical body like the earth) which is the approach?. Do you know any reference? thank you
  48. T

    Ideal gas inside a sphere under the influence of gravitational field

    Homework Statement We have an ideal gas consisted of N classical particles, each having mass m. The system is inside a spherical bottle of radius R and is inside the gravitational field of the earth. The system is also in thermal equilibrium with a heat reservoir which has temperature T. I...
  49. M

    Separation of Variables for a Rocket Launched in a Gravitational Field

    Homework Statement Taking the equation of motion for a rocket launched from rest in a gravitational field g, m\dot{v} = -\dot{m}v_{ex} - mg , and knowing that the rocket ejects mass (fuel) at a constant rate \dot{m} = -k (where k is a positive constant), so that m = m_{o} - kt . Solve...
  50. V

    Bending of light in a uniform gravitational field

    Homework Statement This is supposed to be a simple problem that shows that light bends or gets pulled in a gravitational field. A light pulse emitted in the positive x direction travels 10 s in a uniform gravitational field of 100 m/s^2. the gravitational field points in the negative y...
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