What is Center: Definition and 1000 Discussions

In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may be applied to cause a linear acceleration without an angular acceleration. Calculations in mechanics are often simplified when formulated with respect to the center of mass. It is a hypothetical point where the entire mass of an object may be assumed to be concentrated to visualise its motion. In other words, the center of mass is the particle equivalent of a given object for application of Newton's laws of motion.
In the case of a single rigid body, the center of mass is fixed in relation to the body, and if the body has uniform density, it will be located at the centroid. The center of mass may be located outside the physical body, as is sometimes the case for hollow or open-shaped objects, such as a horseshoe. In the case of a distribution of separate bodies, such as the planets of the Solar System, the center of mass may not correspond to the position of any individual member of the system.
The center of mass is a useful reference point for calculations in mechanics that involve masses distributed in space, such as the linear and angular momentum of planetary bodies and rigid body dynamics. In orbital mechanics, the equations of motion of planets are formulated as point masses located at the centers of mass. The center of mass frame is an inertial frame in which the center of mass of a system is at rest with respect to the origin of the coordinate system.

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  1. B

    Center on mass trigonometric trick

    hey there I am doing a lot of center of mass problem most of them with collisions , and most of the solutions I see, have a big part of "trigonometric trciks", like the way u watch the vectors in cm frame and the way in the lab frame. does anyone met this kind of solutions and have some tricks...
  2. O

    A rock falling towards the center of earth according to GR

    On youtube, I came across to one of many videos trying to explain General Relativity. One of the comments was interesting to me: "I understand how gravity causes an object with velocity to go in curves or to orbit, due to the curvature of space-time: the object thinks that it's traveling in a...
  3. A

    Engineering Instantaneous Center of Zero Acceleration of Rigid Body

    hi i am araz a mechanic master engineer and i am looking for any article or information about" Instantaneous Center of Zero Acceleration of Rigid Body" because i couldn't find any but "Instantaneous Center of Zero velocity":confused: i hope here i can find some help?!
  4. P

    Center of Mass Velocity in Elastic Collision

    Homework Statement A cart of mass m moving at 12 m/s to the right collides elastically with a cart of mass 4.0 kg that is originally at rest. After the collision, the cart of mass m moves to the left with a velocity of 6.0 m/s. Assuming an elastic collision in one dimension only, what is the...
  5. lqgfan

    Which is the best MSc Physics Entrance Coaching Center in Delhi?

    I am sure this topic has not been posted this forum before since it didn't come up in my search. I am preparing for MSc and Integrated PhD (in physics) entrance examinations in India using self study materials. However, I wish to join a good entrance coaching center since there is huge...
  6. W

    What happens in the center of mass of binary star system ?

    Good night! Please, what happens to an object (eg a spaceship) placed exactly in the center of mass of a binary star system (composed by two similar stars)? Would it be compressed by a huge gravitational mass (something like the mass of both stars)? Would it be stretched and torn? Or would it...
  7. M

    Understanding Center of Gravity: Exploring the Gravitational Pull of Objects

    The gravitational pull of an object always points toward the center of that, but what makes something an object? If you cut an object in half would each half of the object have a gravitational force toward its center of mass, or would the force point toward the center of mass of the original object?
  8. oreo

    Understanding Center of Mass: Is it a Vector Quantity & Its Direction?

    Is center of mass a vector quantity. If so then how? Is it directed towards Earth's center?
  9. Symmetry777

    Concerning Quantum Phenomenon the Center of the Torus

    Concerning Quantum Phenomenon: I understand the effects of liquid nitrogen on the magnetic fields. However If you ran a tube of liquid nitrogen in the center of a torus magnet or toroidal inductor - would the magnetic field increase, toroidal magnetic vector potential increase. (Zero-point...
  10. Z

    Center of mass and center of gravity

    what is the difference between center of mass and center of gravity ? please explain it in an easy way ...
  11. C

    Exploring the Concept of Center of Inertia: A Scientific Perspective

    What is the center of inertia? Is it the same as the center of mass?
  12. C

    The weight with a fluid: center of mass or force at bottom?

    Homework Statement It is a question I asked to myself because I need to reply to another one. I imagined this case: http://imageshack.com/a/img912/7730/R964Vq.png The step2 needs the energy E1, the step3 needs nothing because the gravity is 0 (I imagine the device between the Earth and the...
  13. Firedog89

    The Center of the Earth and Black holes and Gravity

    I was thinking of the conditions exerted at the center of our Earth. If a man let's say in America falls toward the Center of the Earth and a man on the opposite side of the Earth falls, where would they meet? I read that the gravitation force at the center of the Earth is at 0. If they both...
  14. A

    Question about torque and center of mass

    I am a beginner and this is my first post. We know torque ## \vec T = \vec r × \vec F ## Now, should r be fixed in magnitude? I mean can I calculate torque only if the particle is rotating at a fixed distance around a fixed axis? I am having this idea because torque is explained (from where I...
  15. R

    Time for Revolution (Rotating about the Center of Mass)

    Homework Statement Two men with masses 70 kg and 120 kg rotate at 1 rpm on a frictionless surface and are attached by a 15 m rope. If they pull the rope so that only 10 m is between them when they rotate, how long does it take to make 1 revolution? Homework Equations Angular Momentum and...
  16. C

    Center of gravity of a 3d triangle using weight

    Hi all, I am after some help to find out how to do the following: I have a triangular object (weighing around 40,000kg) which I can measure and weigh. I have 4 load indicators I can position anywhere beneath the object, these will give me a very accurate reading of the weight of the object at...
  17. G

    Finding the Center of Mass of a System Using Particle Concentration

    Homework Statement Suppose that a system consists of several bodies, and that the position of the center of mass of each body is known. Prove that the center of mass of the system can be found by treating each body as a particle concentrated at its center of mass. Homework Equations ## \vec R...
  18. B

    Torque problem -- designating center of mass as pivot point

    Homework Statement A uniform horizontal beam of length 8.00 m and weight 200 N is attached to a wall by a pin connection. Its far end is supported by a cable that makes an angle of 53.0 degrees with the horizontal. If a 600-N man stands 2.00 m from the wall, find the tension in the cable and...
  19. G

    Center of mass of an equilateral triangle (Kleppner)

    Homework Statement Find the center of mass of an equilateral triangle with side ##a## Homework Equations ## \vec R = \frac{1}{M} \int \vec r \ dm ## ## dm = \frac{M}{A} dx dy ## ## A = \frac{\sqrt{3}}{4}a^2 ## The Attempt at a Solution I set a pair of orthogonal axis ##(\vec x,\vec y)##...
  20. TheExibo

    Is this known as the center of mass, or something else?

    If an object that has more mass at one end of it was dropped, how come the heavier end would be facing closer to the ground than the lighter end? For instance, a toliet plunger is dropped, and the heavier, rubber end of it positions itself at the bottom during the fall. What is this called?
  21. R

    Moment of Inertia - center of a rod w/ unequal distribution

    Hello everyone! New member here, as an ME major I always seem to come across very valuable information here, so I figured I would see if possibly someone here could help me. The problem I am dealing with involves modeling a car through certain motions such as hitting a speed bump, and part...
  22. J

    Find the Center of Mass of a Lamina

    Homework Statement Find the x-coordinate of the center of mass of the lamina that occupies the region cut from the first quadrant by the lines x = 6 and y = 1 if the density function is p(x,y) = x + y + 1 Homework Equations Mass = ∫∫spdσ X Coordinate of center of Mass = Myz/M Myz = ∫∫sxpdσ...
  23. W

    Pendulum with a bob that changes its center of mass

    Hello all, this is my first post here. I will try to phrase it as best I can, my physics background is weak, so please be patient, and thanks in advance for your help! I tried researching simple pendulums but couldn't quite find an answer. Suppose a person is suspended, prone, from a pendulum...
  24. A

    A question related to Center of Mass and Linear Momentum

    A uniform rod of length 2L and mass 2m is bent on its center at a right angle. The rod hangs down from the ceiling using a thread, so that one part, AB, is horizontal, and the other part, BC, is vertical. The question is what's the distance (x) between the suspension point D to the point A (the...
  25. M

    Center of a group with finite index

    Homework Statement Let ##G## be a group such that its center ##Z(G)## has finite index. Prove that every conjugacy class has finite elements. Homework EquationsThe Attempt at a Solution I know that ##[G:Z(G)]<\infty##. If I consider the action on ##G## on itself by conjugation, each...
  26. Y

    How to solve the instantaneous center problem for rotating tires?

    Homework Statement Homework Equations v = rw The Attempt at a Solution I am not sure where to begin, honestly. For part (a), the angular velocity w will be v/r for the lower tire and upper tire both so all I need is the right values of v and r. Now for r, what is the reference point...
  27. S

    Balls elastic collision off center

    Problem Statement: Moe Mentum and Ken Ettik are playing pool. Moe hits the cue ball, sending it towards the 6 ball at 2 m/s. It strikes the stationary 6 ball off-center, moving off at a 60 degree angle from the original direction after a perfectly elastic collision. Both balls have a mass of...
  28. W

    Ampere's Law - Showing that B at center is due only to wire

    Homework Statement In Fig. 29-71, a long circular pipe with outside radius R = 2.6 cm carries a (uniformly distributed) current i = 8.00 mA into the page.A wire runs parallel to the pipe at a distance of 3.00R from center to center. Find the (a) magnitude and (b) direction (into or out of the...
  29. H

    The center of a 1.00 km diameter spherical pocket of oil

    Homework Statement The center of a 1.00 km diameter spherical pocket of oil is 1.00 km beneath the Earth's surface[/B]. Estimate by what percentage g directly above the pocket of oil would differ from the expected value of g for a uniform Earth? Assume the density of oil is 8.0*102 kg/m3...
  30. L

    Magnetic field at the center of a star

    Homework Statement Imagine a giant star, lit with twinkly lights for the holiday season. Are you interested in the magnetic field at the center of the star? Sure you are. The star has “arm” length (AF) equal to 1 m and 1 amp flows clockwise around the star. 1. Find the following lengths: BG =...
  31. TheSodesa

    Problem related to the center of mass of a binary star.

    So here's my problem: The distance between two stars is constant(d = 4,3 * 1010m), and they have a common center of mass. Ms = mass of our star, ma = 0,82 * Ms and mb = 2,2 * Ms. What I'm supposed to do is calculate the period of orbit of both stars, which is the same for them both, since the...
  32. moriheru

    Quantum scattering theory (lab frame, center of mass frame)

    Greetings, Here is my question: What is the difference between the labframe and the center of mass frame in QM scattering theory? Is it that in the labframe both particles are moving towards each other and in the center of mass frame one particle is static and a second one is moving towards the...
  33. M

    Center of Mass Calculation for a Club-Axe: How Far from the Handle is the COM?

    Homework Statement A club-axe consists of a symmetrical 23.0 kg stone attached to the end of a uniform 2.8 kg stick. The length of the handle is L1=91.0m and the length of the stone is L2=13.0cm. How far is the center of mass from the handle end of the club? Homework Equations Center of mass...
  34. P

    Zero gravity in spiral galaxy center?

    In many vortex's, the center has low pressure, from the center of a stirring cup of tea to the centere of a hurricane or storm system. I'm trying to crudely model how this might apply to the center of a spiral galaxy. If the behavior is similar, and the center has a low pressure void...
  35. C

    Finding center of mass of three disks

    Homework Statement All three disks are made of sheet metal of the same material, and the diameters are 1.2m ,2.4m , and3.6m . Assume that the x-axis has its origin at the left-most point of the left-most object and it points to the right. A) Determine the location of the center of mass of the...
  36. S

    Center of Mass of Three Beads Equation

    Homework Statement Three beads are placed on the vertices of an equilateral triangle of side d = 1.8 cm. The first bead of mass m1 = 150 g is placed on the top vertex. The second bead of mass m2 = 55 g is placed on the left vertex. The third bead of mass m3 = 85 g is placed on the right...
  37. X

    Recalculation of center of mass on a grid

    Hi, I checked the forum rules and homework / coursework is not allowed so I am not sure if this is appropriate or not. Feel free to guide me through appropriate forum and/or lock this thread if such questions are not allowed. The question I have is mostly related with my research and since I am...
  38. C

    Does the Center of Mass Have Horizontal Velocity in a Frictionless System?

    Hi. There are two masses connected by a massless bar, and from the unstable equilibrium position shown in the figure is slightly inclined so it falls down, being the final state of the system that both masses are in contact with the surface. There is no friction between the floor and m2. The...
  39. Satvik Pandey

    Find Velocity of Rod Center of Mass When Making 30° Angle with Horizontal

    Homework Statement A thin rigid rod of length ##l## is placed perfectly vertical on a smooth ground. A slight disturbance on the upper end of rod causes lower end of rod to slip along the ground and rod starts falling down. Find velocity of center of mass of rod when the rod makes ##30## with...
  40. R

    Center E field of an equilateral triangle.

    y/SUB]Homework Statement Determine the electric field at the center of the triangle. All sides are 15.6 cm. Vetex is 2.0 uC (Q1) while the bottom points are both -4.0 uC (Q2 and Q3). Homework Equations E = kQ/r^2 E = F/q The Attempt at a Solution Ecenter = E1 + (2)E2 = ? First calculate...
  41. O

    Potential inside grounded, conducting sphere with dipole at center.

    Homework Statement Suppose a grounded spherical conducting shell of radius R surrounds a pointlike dipole at the center with \vec{p}=p\vec{k} Find the potential V(r,\theta) for r <= R. Hint: Use spherical harmonics regular at r=0 to satisfy the boundary condition. Homework Equations General...
  42. M

    Maximizing Height Above Circle Center with Energy Conservation

    Homework Statement Use energy conservation to answer the following question. A mass is attached to one end of a massless string, the other end of which is attached to a fixed support. The mass swings around in a vertical circle as shown. Assuming that the mass has the minimum speed necessary at...
  43. E

    Determine center point of offset circle

    I am working on an algorithm which requires the coordinates for the center point of an offset circle. Dimensions available to find this are shown in the image below and the dimension required is labeled as X: The point at the very left of the arc is a quadrant therefore, the circle center also...
  44. E

    Total moment of shear stress about the center line of a cylinder

    http://postimg.org/image/onxjqc26l/ I have found the shear stress of the inner cylinder to be 405 Pa like in the solution but I am having trouble visualizing the total moment of this stress about the center line. I was wondering if someone could explain where that d(theta) in the moment...
  45. D

    Instantanious center of rotation problems

    Homework Statement [PLAIN]http://i61.tinypic.com/10dwmtv. what is the instantaneous center of rotation for the rigid body. OA = 16 AB = 25 AD = 60 BC = 35 ωOA=2s-1 Homework Equations Vb=ωCB*BC The Attempt at a Solution I am assuming the values angles and vectors from red...
  46. P

    Center of Force, Generalized Potential

    The problem states that a particle moves in a plane under the influence of the following central force: F = \frac{1}{r^2}\Big(1 - \frac{\dot{r}^2 - 2\ddot{r}r}{c^2}\Big) and I am asked to find the generalized potential that results in such a force. Goldstein gives the following equation...
  47. M

    What Happens at the Center of a Black Hole?

    i was bored and my mind wandered. a question popped up and it got me interested in what others thought who are more informed than me: since black hole is a theoretical construct (and hence can be geometrically perfect), what happens at the exact center of it. with uniform (assumed) density...
  48. J

    Why would we weigh less at earth's center?

    I don't understand why we would weigh less at Earth's center(as opposed to sea level). Looking around the net, I have seen different approaches to this problem, each with different solutions. According to Newton's universal law of gravitation: F= Gm1m2/r2 If this is the case, then as...
  49. M

    Center of mass for two small particles

    1. Two small particles of mass m1 and mass m2 attract each other with a force that varies with the inverse cube of their separation. At time t0 , m1 has velocity v directed towards m2 , which is at rest a distance d away. At time t1 , the particles collide. How far does m1 travel in the time...
  50. Nathanael

    Regions; "Each point of the set is the center of a circle "

    "A set in the plane is called a region if it satisfies the following two conditions: 1. Each point of the set is the center of a circle whose entire enterior consists of points of the set. 2. Every two points of the set can be joined by a curve which consists entirely of points of the set."...
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