University · Ages 18+First-year university · Physics olympiadsExtended

 

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Pi From Collisions

Two blocks slide on a frictionless floor with a wall on one side. When the left block is heavier than the right by a factor of 100ᴺ, the total number of collisions equals the first N+1 digits of π. It is a startling link between elastic-collision conservation laws and geometry — the collisions trace out an angle on a circle in velocity space.

Key formulas

  • Ncollisions=⌊π⋅10N⌋N_{\text{collisions}} = \lfloor \pi \cdot 10^{N} \rfloor

    π from colliding blocks

    NcollisionsN_{\text{collisions}}
    Number of collisions
    NN
    Mass ratio exponent: the big block is 100ᴺ times heavier
    Open the formula card
  • m1u1+m2u2=m1v1+m2v2m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2

    Conservation of momentum

    m1m_1
    Mass of body 1kg
    u1u_1
    Velocity of body 1 beforem/s
    m2m_2
    Mass of body 2kg
    u2u_2
    Velocity of body 2 beforem/s
    v1v_1
    Velocity of body 1 afterm/s
    v2v_2
    Velocity of body 2 afterm/s
    Open the formula card
  • K=12mv2K = \tfrac{1}{2} m v^{2}

    Kinetic energy

    KK
    Kinetic energyJ
    mm
    Masskg
    vv
    Speedm/s
    Open the formula card

What you can change

  • Mass of small block
  • Mass of large block
  • Initial velocities
  • Block sizes

Key concepts

  • Perfectly elastic collisions in one dimension
  • Conservation of momentum and kinetic energy
  • Configuration/velocity space and rotations
  • Why the count is exact (no collision may be missed)