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. M

    Application of force not acting through center of mass.

    Hello PF! I've got a lab on rigid body motion tomorrow, and I need help completing one of the prep questions: A rigid body is acted on by a force F through its center of mass, and also experiences a torque caused by a similar force F at radius R. At time t, what are the linear and angular...
  2. H

    Center of Mass of a Triangle (uniform)

    Homework Statement Where exactly is the center of mass of a triangle ? the file attached shows a diagram of a triangle which is equilateral, and the blue spots are the mid-points of each side. Homework Equations a distance = 2/3 from the vertex (along the median) The Attempt...
  3. Z

    Finding work to move a point charge to the center of a thin ring.

    1. Find the work required to move a point charge from infinitely far away to the center of a thin ring. The point charge is q= 1nanocoulomb. The rings charge is Q= 2 nanoC. The ring has a radius r=2m.Homework Equations U= qV W= -U Thoughts I think the first thing to consider is the field E...
  4. S

    Force acting on the particle is always directed towards the center

    r=acos(wt)i+bsin(wt)j is the equation(it is an ellipse) I need to somehow show that the force will always act towards the center. Is there anyone who can possibly help?
  5. T

    Instantaneous Center of Zero Velocity

    Homework Statement Why is the angular velocity of link BD the same as the angular velocity of ICD? Shouldn't it be ωICD=vD/rD/IC?
  6. R

    Center of mass velocity greater than 1c?

    center of mass velocity greater than 1c? Basically I have two electrons moving in opposite directions towards each other. One is moving 2.5x10^8 m/s and the other is 2.0x10^8 m/s and I'm trying to use the following equation for finding the center of mass velocity: V_cm = ((p1 +...
  7. D

    Center of Mass involving Dumbbell With Uneven Weigths

    Homework Statement You work in a plant that manufactures heavy dumbbells. Due to a manufacturing error, one side of a 100 kg dumbbell was found to be 60 kg (M1) while the other was only 40 kg (M2). The mass of the bar itself is negligible. The factory has provided you and your colleague...
  8. H

    Center of mass and center of area

    what is the difference between center of mass and center of area ?? i know that they are the same for an constant density body but why ?and why they are different for nonconst density body?? thank u ,,
  9. mnb96

    Question on center of mass (centroid)

    Hello, I was thinking of the following question: suppose we have a body in two-dimensional space and its density is described by the function f(x,y) (Note! in Cartesian coordinates!). The x-coordinate of the center of mass is given by C_x= \frac{\int\int xf(x,y)dxdy}{\int\int f(x,y)dxdy}...
  10. A

    Center of mass of two extended bodies

    Homework Statement Consider a system comprising two extended bodies, which have masses M1 and M2 and centers of mass at R1 and R2. Prove that the CM of the whole system is at [M1R1+M2R2] / [M1+M2 ] Homework Equations Definition of CM The Attempt at a Solution First I...
  11. A

    Electric potential at the center of meter stick

    Two 3.00-μC charges are at the ends of a meter stick. Find the electrical potential for the center of the meter stick. V=kq/r I just want to confirm my answer here. What I did is solve this for both charges in the equation above using r = 0.5m q= 3.00 uC. So I factored out k/r...
  12. H

    Center of mass of fluid in rotating cylinder

    Homework Statement A closed cylindrical canister with central axis coincident with the Z axis has a height H and a radius R. It is suspended by a rod coincident with the Y axis that passes through the canister, transecting its central axis at a height h above the bottom surface of the canister...
  13. E

    Determining angular velocity radius and velocity of the center of rotation

    Hey, I am trying to solve the following problem: va=vr+ ω × rra vb=vr+ ω ×(rra+rab) vc=vr+ ω × (rra+rac) I know the vectors va, vb, vc, rab and rac and I want to know the angular velocity vector, the radius rra and the velocity vr. Physically it means that I know 3 points on a rigid...
  14. M

    Center of mass of right triangle

    I've been reading that the center of mass of a right triangle - the coordinates of the COM, is (1/3b,1/3h)- I can't for the life of me figure out why this is. Is there some sort of clear proof I can take a look at? I don't really know what to integrate..
  15. A

    Rotation around center of mass question

    I had quite a few posts about this some weeks ago, but I am still not sure about it. My question is about why you can always view the motion of an object as a traslation of the cm and a rotation around it. It makes sense in the light of the cm being the point which moves as a point particle only...
  16. Pruddy

    Dropping a mass in a hole through the Center of Earth

    Homework Statement Assume that the Earth is a uniform density spherical mass. (This assumption is not correct but we will use it for simplicity in working the problem.) The deepest hole drilled into the Earth's surface went to a depth of 40,230 ft (Wikipedia.org). Imagine that this hole was...
  17. A

    Understanding the Center of Mass of a Spring: A Conceptual Explanation

    A week ago I posted a thread about my conceptual understanding of the center of mass of a body. I have however not yet gained the intuition that I want, so let me ask a question about the center of mass of a spring. Consider a spring which is elongated in outer space and left to oscillate. As...
  18. R

    Finding the Center of Mass for a Hemisphere and Right Cone

    Suppose there's a hemisphere of radius R (say) and a right cone of same radius R but ht. R/2 is scooped out of it then i have to find the center of mass of the remaining part. Here's how i approached... clearly by symmetry, Xcm = 0 Now, Let M be the mass of the hemisphere so...
  19. R

    Center of mass of a solid hemisphere (where's the error)?

    On finding the center of mass of a solid hemisphere i came up with some different result. Here's what i did... consider a small ring at a distance r from the center of the hemisphere and one more ring at a distance of r+dr from center of the ring. let, mass of the small element formed...
  20. R

    Solving Center of Mass Doubt: What is 'y' in Integration?

    A silly doubt regarding center of mass... As we know for bodies having continuous distribution of mass we can know their center of mass by the method of integration... like, Xcm = 1/M∫x.dm but what is x here? in many cases... like in finding the COM of a ring Xcm = 0 and Ycm = 2r/∏...
  21. K

    Rotational Dynamics about an axis parellel to the axis at the center of mass

    Homework Statement A uniform sign is nailed into a wall at its top two corners. The nail on the upper-right corner breaks, and the sign begins to rotate about the remaining nail in the upper-left corner. The mass of the sign is 69 kg, L (length) = 1.6 m, H (height) = 2.2 m, and the moment of...
  22. A

    Understanding Center of Mass: Properties & Physically Representing

    I don't feel I have a good understanding of what the center of mass of an object it, and what its properties are. I know it's the position of all mass elements weighted by their mass and divided by the total mass. I have learned that the center of mass moves as if it was only subject to...
  23. M

    How is it possible for a car to skid away from the center at a curve?

    When a car turns too fast, it skids away from the center and I don't understand how that's possible in terms of forces. Background idea: The confusion came about when I was approaching a question where the net force of a car on a curve was towards the center of the curve as static friction. The...
  24. N

    Instant Center and Relative Velocity/Acceleration Questions

    Hey everyone, first time poster, long time reader. I have two questions about a Dynamics I course I am currently taking. Because there is so much information, I have included a zipped file that contains the questions, FBD's I used, and typed up solutions (in both .doc and .docx formats) to make...
  25. S

    Finding magnitude of electric field at center of square

    In each situation below, electric charges are arranged at the corner of a square. Each charge Q has the same magnitude with the signs indicated in the diagrams. Rank the electric potential from most positive to most negative, and the magnitude of the electric field at the center of the square...
  26. T

    How to find magnetic flux density at center and ends of solenoid

    Homework Statement A solenoid has a radius of 2mm and a length of 1.2cm. If the # of turns per unit length is 200 and the current is 12A, calculate the magnetic flux density at a) the center and b) the ends of the solenoid Homework Equations The biot-savart law: \vec{B} = \frac{\mu_0}{4...
  27. C

    How can EVERY point in the universe be the center?

    It has been stated time and time again that the fact that galaxies are moving away from us makes us appear to be the center of the universe. Of course, someone making this observation from a different galaxy would come to the same conclusion. The implication is that any given observer in any...
  28. X

    Equation of a Circle with a Center and Tangent Point

    What is the equation of the circle with a center point of (10, -14) when the circle is tangent to x=13? D = √(13-10)^2 + (0-(14))^2 D = √(3)^2 + (14))^2 D = √9+196 D = √205 Radius = √205 (x-10)^2 + (y-(-14))^2 = √205^2 (x-10)^2 + (y+14)^2 = 205 But how am I suppose to graph this?
  29. J

    Center of Mass constant with near-correct attempt

    Homework Statement A wagon wheel is made entirely of wood. Its components consist of a rim, 16 spokes, and a hub. The rim has mass 5.1 kg, outer radius 0.90 m, and inner radius 0.86 m. The hub is a solid cylinder with mass 3.1 kg and radius 0.12 m. The spokes are thin rods of mass 1.1 kg...
  30. G

    Is time still affected (slowed down) at the center of the earth?

    Assumptions: 1. Gravity at the center of the Earth is zero. 2. Time is slower on the surface of the Earth with respect to time in empty space (far away) Is time still affected (slowed down) at the center of the earth?
  31. phosgene

    Center of mass of infinite cylinder of air

    Homework Statement The density of air at height z above the Earth’s surface is proportional to e^(−az) , where a is a constant > 0. Find the centre of mass of an infinite cylinder of air above a small flat area on the Earth’s surface. Hint : Consider line density and the identities...
  32. A

    Understanding Why a Rod Doesn't Rotate Around Its Center of Mass

    Consider a free rod lying horizontically in the air. Gravity produces no torque around the center of mass. Now let the rod be attached to a hinge like of that one the picture. Now the hinge provides an upwards force on the left end of the rod, whilst there is a torque effectively acting at the...
  33. M

    Wing and wind turbine blade aerodynamic center?

    Hello everyone, Now i am trying to model wind turbine blade using solidworks. I have the airfoil coordinates at each section of the blade, but the coordinates are distance per chord (x/c,y/c) so i have to scale it using the calculated chord. When i scale it with respect to the origin -...
  34. I

    What is the Center of Mass for a Donut and an Empty Box within a Box?

    1. FIgures in this link: ( a donut, and a Box with and empty box inside) http://postimage.org/image/smdut3cxb/ 2.Find the CoM of the two figures 3. as i attempted for the circle: Homework Statement Homework Equations The Attempt at a Solution
  35. F

    Center of Mass of a Cardioid

    Homework Statement Find the center of mass of a solid of density \delta = 1 enclosed by the spherical coordinate surface \rho = 1-cos\phi.Homework Equations The Attempt at a Solution I'm a bit confused about how to start here, mainly because the surface is defined by spherical coordinates...
  36. A

    Rigid body kinematics problem: finding the velocity of the center of mass

    I have to deal with the problem of finding an expression for the kinetic energy of a rigid body. One of its point is pivoted to a point that moves arbitrarily. So in order to find an expression for the kinetic energy I use König's theorem, but I need the velocity of the center of mass. I use...
  37. R

    Center of mass and rigidity and torsions

    I've been researching about this for hours at internet reading dozens of pdfs.. but can't seem to understand the concept. What does it mean if the center of mass and center of rigidity are not coincident, torsions would be produced.. can you give an example of it in more intuitive or using...
  38. L

    Having trouble figuring center of mass between Sun and Jupiter

    I am taking an astronomy class because I am interested in it and wanted to know more. I love all I am learning unfortunately my math skills are holding me back from getting all that I can out of this class. I realize this might not be the toughest math problem but can someone explain help me out...
  39. J

    Center of the universe the origin of the big bang?

    can we calculate the orgin of the big bang based on the wmap and drift of galaxies?
  40. W

    Potential at Center of Insulating Spherical Shell

    Homework Statement The inner radius of a spherical insulating shell is c=14.6 cm, and the outer radius is d=15.7 cm. The shell carries a charge of q=1451 E−8 C, distributed uniformly through its volume. The goal of this problem is to determine the potential at the center of the shell (r=0)...
  41. A

    Tunnels through the center of planet, oscillations

    Hi all, I know that if you drill a hole from one side of the planet to the other, through the planet's center, that a particle dropped in this tunnel will oscillate back and forth forever, like a mass on a spring, with the restoring force given by gravity. What if the tunnel forms a chord...
  42. X

    Applying forces to a body, outside it's center of mass.

    Hello. I'm currently coding on a physics engine. To do so, I looked up some code from open-source engines. I noted that there is a function where you can apply a force to a rigid body, outside it's center of mass. It goes something like this: ApplyForceAPosition(vector force, vector...
  43. A

    Field at the center of a toroidal permanent magnet

    I'd like to make a strong (~0.5 T) solenoidal field with a permanent magnet for a Faraday rotation experiment. If I take an ordinary cylindrical permanent magnet (with an axial field) and drill a hole down the center of it. What's the field deep inside at the center of the hole? Some of the flux...
  44. S

    Center of mass of half square without a half circle

    Homework Statement Find the position of the center of mass for a thin sheet and homogeneous, with sides R and 2R ,from which has been subtracted a half circle of radius R. [Xcm=(2/3)*R*(4-pi)]Homework Equations Rcm=(1/M)*∫rdm The Attempt at a Solution By symmetry we know Ycm=0. For de...
  45. Z

    Spinning Spool Physics Q: Is Circumference Dist. = Center Dist.?

    I have a question about a spinning spool, there is a spool spinning and is the sistance traveled by the circumference the same as the center of the circle? You can refer to the diagram attached for a question I had trouble with from a physics olympiad. The answer was (B). Thanks in advance!
  46. Jalo

    A.: Finding Center of Mass for a Rectangular Surface with a Hole

    Homework Statement Given a homogenous rectangular surface, sides of length a and b=4*a, with a circular hole in x=a and y=a/2, find the center of mass. Homework Equations R=1/M*Ʃmi*ri , M= total mass, r= position vector area of a circle = pi*r2 , r being the radius The...
  47. R

    Mass Hanging in Center of Chain Problem

    Homework Statement A mass of 200 kg is hanging directly in the center of a chain; the chain makes a 20o angle from its horizontal. The chain will break if more than 2000 N of force are applied at any point on the chain. Will the chain break?Homework Equations F=ma Tfy=mg(sin\Theta) g=10m/s The...
  48. R

    Center of mass in Lagrangian mechanics

    We all know the proof, from Newtonian mechanics, that the motion of the center of mass of a system of particles can be found by treating the center of mass as a particle with all the external forces acting on it. I want to prove the same think, but within the framework of Lagrangian mechanics...
  49. D

    Problem with center of mass exercise

    Hello! I'm here again, now with a new exercise! :biggrin: Homework Statement A block of mass m is at rest over this wedge (it's a triangle) of mass M that is at rest over a table. There is no friction. The point P of the block has a height h from the table. Then the block starts to move. What's...
  50. F

    Center of mass of a tank (military vehicle)

    Homework Statement a toy which have a shape of tank (military vehicle) has a platform of 192mm long, 140mm wide, weight of 2075grams. The canon is 239mm long, 50mm wide, and weight of 400grams. the canon is on top of the platform and being assembled 58mm from the front side of platform. where...
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