Centre of mass

  1. The centre of mass of a body is a point where the whole mass of body is supposed to be concentrated.

  2. If all the forces acting on the body were acting on the body were applied on centre of mass, the nature of motion of body shall remain unaffected.

  3. The coordinates of CM for a two particle system can be written as:

    x = \frac{m_1x_1+m_2x_2}{m_1+m_2} cm and y = \frac{m_1y_1+m_2y_2}{m_1+m_2} cm

    (x_1, y_1) and (x_2, y_2) are the coordinates of m_1 and m_2


    Distance of CM from m_1 is a_1= \frac{am_2}{m_1+m_2}

    And from m_2 is a_2= \frac{am_1}{m_1+mM_2}

    Where a is distance between two particles having mass m_1 and m_2.

Moment of inertia (I)

  1. Moment of inertia of a body about a given axis is the property by virtue of which, the body opposes any change in its state of rest or state of uniform rotation about that axis.

    -It plays same role in rotational motion as mass does in translation motion.

    -It is scalar quantity.

    -It depends upon mass and size of body & distribution of mass about axis of rotation.

    MOI is formulated as,

    • For a single particle, I = mr^2 (m is mass of particle and r is its distance from axis of rotation)

  2. • For large number of particles in a body, I = m_1r_1^2 + m_2r_2^2 + …….= Σmr^2

  3. \textbf{Radius of Gyration}

    -It is the perpendicular distance of a point from axis of rotation at which whole mass of a body is concentrated. It can be given as ; I = Mk^2 => K = \sqrt{\frac{I}{m}}

    -K depends upon axis of rotation.

    Note: Numerically, \frac{K^2}{r^2} = coefficient of mr^2


  1. Parallel axis theorem
  2. According to this theorem, the MOI of any body about any axis parallel to axis through centre of mass is equal to sum of MOI about axis through centre of mass and product of mass and square of distance between two axis.

    i.e. I = I_{cm} + md^2

    Here, I_{cm} is MOI about axis through centre of mass and I is MOI about axis parallel to it.

  3. \textbf{Perpendicular axis theorem}

    According to this theorem, the MOI of plane lamina about an axis perpendicular to its plane is equal to sum of its MOI about any two mutually perpendicular axes chosen on lamina through same point.


    either, I_X = I_Y + I_Z

    or, I_Y = I_X + I_Z

    or, I_Z = I_X + I_Y

Moments of Inertia for Different Objects

  • For Rod

MOI of Rod

  • For Sphere

MOI of Sphere

  • For Hoop

MOI of Hoop

  • For Cylinder

MOI of Cylinder

  • For Ring/Disc

Kinetic Energy of a rotating body and rolling body

  1. For rotating body, KE_{rot} = \frac{1}{2}I \omega^2 = \frac{1}{2}mK^2 (\frac{v}{r})^2 = \frac{1}{2}mv^2 \frac{K^2}{r^2}

  2. For rolling body (without sliding),

    KE_{roll} = KE_{trans} + KE_{rot}

    = \frac{1}{2}mv^2 + \frac{1}{2}mv^2 \frac{K^2}{r^2}

    = \frac{1}{2}mv^2 (\frac{K^2}{r^2} + 1)