What is Center of mass: Definition and 914 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.

View More On Wikipedia.org
  1. D

    What is the center of mass in a triangular beam?

    Hello,, I'm reading about beams in mechanics of matrials. But I'm a bit stuck because of one thing that I either don't understand or just don't comply with! So, here's a picture from the book: picture 003 It says that the center of mass should be at 2/3 of the length.. I tried to...
  2. B

    Find Center of Mass for Uniform Log: 2.33m, 94.9kg, 72.7kg, 26.1kg

    A uniform log of length 2.33 m has a mass of 94.9 kg and is floating in water. Standing on this log is a 72.7-kg man, located 18.9 cm from one end. On the other end is his daughter (m = 26.1 kg), standing 1.15 m from the end. Find center of mass [b]2. m1r1+m2r2/(m1+m2)=R [b]3. I used...
  3. V

    ChetFinding the Center of Mass of a Figure with a Circular Cutout

    Homework Statement http://img9.imageshack.us/img9/2866/siyx.png Uploaded with ImageShack.us Homework Equations Where is the COM of the figure? (width = 8, height = 8) The Attempt at a Solution area of triangle: 0.5(8*8) = 32 32 - (pi(0.52) - (2.5*1) - (2.5*0.5) = 27.4646 I...
  4. S

    Torque as measured from center of mass.

    The angular momentum of a system of particles α = 1,2,3,...n can be written as the sum of the two angular momenta: \vec{L} = \vec{R} \times \vec{P} + \sum_{\alpha} \vec{r_{\alpha}}' x \vec{\rho_{\alpha}}' where the first term is the angular momentum of the center of mass with all mass M = Ʃ mα...
  5. B

    Center of Mass (Boxcar water example)

    A boxcar of length 8.00 m and height 2.40 m is at rest on frictionless rails. The empty boxcar has a mass of 3750 kg. Inside the boxcar, located at the left end, is a tank containing 2450 kg of water. The tank is 2.00 m long and 2.40 m tall. The tank starts to leak, and the water fills the...
  6. N

    Physics Center of Mass, driving me nuts

    Homework Statement A stone is dropped at t = 0 s. A second stone, with a mass 2.00 times that of the first, is dropped from the same point at t = 0.220 s. Assuming that neither stone has yet reached the ground, how far from the release point is the centre-of-mass of the two stones at t...
  7. E

    Center of Mass Formula: Understanding the Intricacies

    Hello, I'm reading Mathematical Methods in the physical sciences by Mary Boas and in it, she defines the center of mass of a body in 3 dimensions \int \overline {x}dM=\int xdM \int \overline {y}dM=\int ydM \int \overline {z}dM=\int zdM In standard undergraduate textbooks...
  8. nearc

    Center of magnetic potential vs center of mass

    first i might be using magnetic potential incorrectly so please be nice when correcting me. i'm attempting to model the behavior of a magnetic compass when it is placed next to a ferrous piece of metal. basically the compass will point to the metal rather than north. my first simplification...
  9. P

    Center of Mass of particles

    Homework Statement 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...
  10. P

    Binary star revolving arround center of mass

    why the binary star revolving around center of mass ? could type 2 supernova ever had a stellar wind after explosion ?
  11. A

    Finding Center of Mass of 800g Steel Plate Triangle

    Homework Statement An 800 g steel plate has the shape of the isosceles triangle shown in the figure. What are the x and y coordinates of the center of mass? https://www.physicsforums.com/attachment.php?attachmentid=13559&d=1208292919 Homework Equations x=1/M ∫ x dm [b]3. The...
  12. E

    Moment of Inertia through center of mass

    Homework Statement A rod is 10 ft long and has a density that goes from 4 to 24 lb/ft. a) Find the mass. b) Find the center of mass. c) Find the moment of inertia through the center of mass. Homework Equations I = Ml^2 I = I(cm) + Md^2 (parallel axis theorem) The Attempt at...
  13. PsychonautQQ

    Why Is Total Mass Used as the Denominator in the Center of Mass Equation?

    so for the center of mass of an object.. x = (m1x1 + m2x2 + ...) / M total y = (m1y1 + m2y2 + ...) / M total how come in this equation the denominator is the total mass of the system? It seems like it would make more sense to have the denominator the total mass in whatever component you...
  14. B

    What is the definition of center of mass?

    Hello, I am trying to describe the concept of center of mass, and need some help with particulars: The center of mass can be thought of the balancing point of a body; it is the point where mass is "evenly distributed" with respect to this point, such that if you were to place the body in a...
  15. H

    How does a motocylist move his center of mass?

    Hi all:we all know that a motorcyclist need to tilt his/her center of mass with the motorcycle when he is turning in order to balance the torque exerted on him.When he is tilted, gravity exert a torque on him that allows him to balance torque by centrifugal force about the point of contact with...
  16. andyrk

    Center of Mass of Uniform Symmetric Bodies

    Why are centre of mass of all uniform symmetric bodies at there geometric centre? We know the following result: "Consider a system of point masses m1,m2,m3... located at co-ordinates (x1,y1,z1), (x2,y2,z2)...respectively. The centre of mass of this system of masses is a point whose co-ordinates...
  17. L

    Compare Velocity of Center of Mass - Two Hollow Cylinders

    Homework Statement Homework Equations mgh=\frac{1}{2}mv^2+\frac{1}{2}Iω^2 The Attempt at a Solution I used conservation of energy, and found that the final velocity of the center of mass is independent of the object's mass, so in both cases: v=\sqrt{gh} . Therefore, the ratio is 1. However...
  18. D

    Center of Mass: Definition, 3D Calculation & Frame

    What exactly is center of mass, and how do I calculate in three dimensions? Also, I need to know how to calculate it in three dimensions at relativistically high speeds? Also, what do they mean by 'center of mass frame', as in the question "What are the energy and momentum in the center of...
  19. E

    Rotation induced by moving center of mass

    Two identical uniform beams are connected together by a set of servo motors. When the motors activate the two beams will come together uniformly and the center of mass of the system will move outward along the arrow shown. I would expect that the torque exerted by the motors would only cause the...
  20. J

    Finding the center of mass of an incomplete circle

    Homework Statement Locate the x coordinate of the center of mass of the homogeneous rod bent into the shape of a circular arc. Take r = 170 . The arc goes from (-5/6) to (5/6)pi (counterclockwise). It has a radius of 170mm.Homework Equations x=rcosθ, y=rsinθ, dL=r*dθThe Attempt at a Solution...
  21. J

    Fidning the center of mass of an incomplete circle

    Homework Statement Locate the x coordinate of the center of mass of the homogeneous rod bent into the shape of a circular arc. Take r = 170 . The arc goes from (-5/6) to (5/6)pi (counterclockwise). It has a radius of 170mm.Homework Equations x=rcosθ, y=rsinθ, dL=r*dθThe Attempt at a Solution...
  22. J

    Center of mass of a slender rod with variable density

    Hi to everybody Homework Statement I´ll show the problem with a picture: Homework Equations L\overline{x}=∫xc ρ dl The Attempt at a Solution Well the total length of the rod is 1 feet, I only need to calculate the integral. The moment xc of a differential element of mass of the rod is...
  23. M

    Finding the Center of Mass of a Lamina with Proportional Density

    Homework Statement The boundary of a lamina consists of the semicircles y=\sqrt{1-x^{2}} and y=\sqrt{4-x^{2}} together with the portions of the x-axis that join them. Find the centre of mass of the lamina if the density at any point is proportion to its distance from the origin. Homework...
  24. J

    How to find the center of mass and unknown weight

    Homework Statement A canoe of mass 38 kg lies at rest in still water. A man and a woman are at opposite ends of the canoe 4.0 m apart and symmetrically located with respect to the canoe’s centre (which is also its centre of mass). The mass of the man is 65 kg and the woman’s mass is smaller...
  25. R

    Center of Mass (Triple Integral)

    Homework Statement T is the solid bounded by the cylinder y^2+z^2=4 and the planes x=0 and x=3. The mass density at a point P of T is directly proportional to the distance between P and the yz-plane. Find the center of mass of the solid T. Homework Equations y^2+z^2=4 x=0 x=3 The...
  26. B

    QFT: differential cross section from center of mass to lab frame

    I have the following process: two ingoing particles, a photon hitting a nucleus, and two outgoing particles, the nucleus and a pion. I have computed |M|2 and the differential cross section in the center of mass frame dσ/dΩCM; I now have to go into the lab frame, where the nucleus is initially at...
  27. J

    Finding mass and center of mass of solid

    Homework Statement Find the mass and the center of mass of the solid E with the given density function ρ(x,y,z). E is bounded by the parabolic cylinder z = 1 – y2 and the planes x + 4z = 4, x=0, and z=0; ρ(x,y,z) =6. m=? x=? y=? z=? Homework Equations z = 1 – y2 and the planes x...
  28. Sneakatone

    Find kinetic energy from center of mass.

    two automobiles each of mass 1200kg traveling a the same direction. Speed of one automobile is 29.0 m/s and the other is 19 m/s. Regard these automobiles as a system of two particles. a)what is translational energy of center of mass? I found velocity at center of mass which is 24 m/s and...
  29. Sneakatone

    What is the velocity of the center of mass of the system?

    a(n) proton (mass 1.67x10^-27 kg) of kenetic energy 1.58x10^-12 J is moving toward a(n) proton (mass 1.67x10^-27) at rest. what is the velocity of the center of mass of the system I used the equation k=1/2mv^2 -> sqrt(2k/m)=v sqrt(2*1.58x10^-12 /1.67x10^-27)=4.34x10^7
  30. Sneakatone

    Need help finding center of mass of a molecule.

    Figure below shows the shape of a nitric acid (HNO3)molecule and its dimensions. Treating the atoms as particles, findthe center of mass of this molecule. a)I know that xcm=m1x1+m2x1+.../Total mass. The total mass in this case is 63 but based on the picture I don't know which mass I should...
  31. L

    Finding the Center of Mass of a System of Rods

    Homework Statement Three uniform thin rods, each of length L = 22 cm, form an inverted U. The vertical cords each have a mass of 14 grams. The horizontal rod has a mass of 42 grams. What are the x coordinate and y coordinate of the system's center of mass? Homework Equations xcom=...
  32. C

    V=wr. Is v tangential or center of mass?

    So...I have two different homework problems. I'm not asking for answers to these problems! Just clarification. In problem one I am asked: A thin disk with diameter d rotates about an axis through its center with 0.260J of kinetic energy.What is the speed of a point on the rim? I use E = 1/2Iω2...
  33. M

    Center of Mass and Translational Motion

    Suppose two friends, with masses m2 = 1.3 m1, are on a perfectly smooth, frictionless, frozen lake. They are both holding the end of a rope of length Lo . a. Find the position of the center of mass, in terms of Lo , from the smaller person. b. If the two pull half of the rope in such...
  34. P

    Decide the Center of Mass for Rotational Body K

    Homework Statement Homework Equations X_{T} = \frac{1}{m}\int_{K}^{} x dmThe Attempt at a Solution I have decided a) and I got the volume to be ∏ volume units. I need help with b). I try to use the formula and I get this: X_{T} = \frac{1}{\pi}\int_{K}^{} 3\pi x sin^2xcosx dx But I need help...
  35. P

    How to Calculate the Center of Mass for a Rotational Body Using Integration?

    I have decided a) and I got the volume to be ∏ volume units. I need help with b). I would need to have the physics of the problem explained. Please help me explain the physics, so that I from it can understand how to solve b). :smile:
  36. D

    Find m1's coordinates using center of mass equation | Extended Object"

    1. The coordinates of the center of mass for the extended object shown in the figure are (L/4, −L/5). What are the coordinates of m1? (Assume m1 = 7 kg, m2 = 3 kg, and m3 = 5 kg. Use any variable or symbol stated above as necessary.) 2. 3. I too the centroid cordinates and...
  37. I

    Calculating Volume With Center of Mass

    I remember seeing a way to do this, can someone link me to some relevant material or post a proof and equation? Thanks
  38. I

    Center of Mass and Linear Momentum

    1.Two particles lie on an x axis, particle 1 at x = 1 m and particle 2 at x = 2 m. If you gradually increase the mass of particle 1, what happens to the center of mass of the two-particle system? 2.If an external force acts on a system of several objects, can the external force change the net...
  39. M

    Center of gravity and Center of mass different?

    Center of gravity and Center of mass different?? I was developing a test for my students and always look for a difficult question as the only bonus question. I came across the question in the book, "When are center of gravity and center of mass different?". I thought about it before looking at...
  40. M

    Find center of mass and coordinates using double integrals?

    Homework Statement Find the mass and center of mass of the lamina that occupies the region D and has the given density function ρ. D is bounded by the parabolas y = x^2 and x = y^2; ρ(x, y) = 23√x Homework Equations m = \int\int_{D} ρ(x, y) dA x-bar = \int\int_{D} x*ρ(x, y) dA y-bar =...
  41. S

    Center of Mass with Circle that has a Cut Out

    Homework Statement Find the center of mass of a 5kg circle with a radius of 4m with a circle that has a radius of 2m cut out from it. Homework Equations The Attempt at a Solution To be completely honest, I do not know where to start. I do know how to find the center of mass when...
  42. W

    Find Center of Mass of Thin Plate in Region Bounded by y-Axis, x=y-y^3

    Homework Statement find the center of mass of a thin plate with constant density in the given region. region bounded by y-axis, x=y-y^3 ; 0<=y<=1Homework Equations x(bar) = (integral)(a to b) α(x) * x * (f(x) - g(x)) ---------------------------------- (integral)(a to...
  43. W

    Find the center of mass of a thin plate of constant density

    Homework Statement Find the center of mass of a thin plate of constant density (δ covering the triangular region in this first quadrant between the circle x^2 + y^2 = 4 and the lines x=2 ; y=2. Homework Equations x^2 + y^2 = 4 and the lines x=2 ; y=2. The Attempt at a Solution...
  44. D

    MHB Center of Mass Calculus 2 Integration

    Uniform semi circle of radius R whose diameter is on the x axis. Since it is uniform and symmetric on the x axis, x = 0. For y, we have $$ y_{cm} = \frac{\int y\sigma dA}{\frac{\pi R^2}{2}} $$ In polar, $dA = rdrd\phi$. So the integral becomes $$ \int_0^{\pi}\int_0^R y^2\sin\phi drd\phi $$ How...
  45. D

    MHB Center of mass of a two particle system

    How does one prove the center of mass of a two particle system lies on the line joining them? Would we do this by contradiction? Suppose on the contrary that the CM doesn't lie on the line joining the two particles. Where do I go from here though?
  46. S

    Exploring the Force on an Object's Center of Mass

    Why can the total force exerted on an object be taken as if a single force was applied on the center of mass? I think at most the total force must be the sum of tiny equal forces uniformly distributed. The mass must at most be uniformly distributed too. And this only matters when we start...
  47. T

    Investigating Energy Transfer in a Collision Between Two Cars

    A car of mass m moving East at speed v (v << c no need to use relativity) collides with another car of mass 2 m moving North with speed 3v. a) What are the x and y components of the center of mass velocity of the system just before the collision? b) If the cars stick together during the...
  48. C

    Center of mass after particles move

    Homework Statement a 90 kg block 1 and 360 kg block 2 are on opposite ends of a 300 kg slab of length 12.0 m which is on a frozen lake. assume no friction between slab and ice. block 2 moves 4.0 m towards block 1 while block 1 moves 2.50 m towards block 2. how far does the slab move? Homework...
  49. R

    Center of Mass in Line Following Robot

    Hi Guys, I just want to know the effect of varying center of mass in a vehicle, specifically in a line following robot, I am going to put load on it, thus center of mass will vary, then i will study how will the varying center of mass affect the response of the line following robot, can anyone...
  50. J

    Find the Y component of the Center of Mass

    Homework Statement Find the Y component of the Center of Mass http://img607.imageshack.us/img607/7122/83370796.png Homework Equations ∫(r*dm)/M The Attempt at a Solution I keep coming up with (2/3)B but i know since that there is more mass near the origin axis, it should be...
Back
Top