• Pixel-art sticker of Mometon, personifying Energy–Momentum Relation, with the caption “Mass And Motion, One Energy.”

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Mometon

Energy–Momentum Relation Physics

\[E^2 = (p c)^2 + (m c^2)^2\]
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Meet Mometon, a right-angled triangle who shows how mass and motion add up to energy. Energy squared equals (pc)² plus (mc²)², like Pythagoras: an electron with 0.511 MeV rest energy and 1.00 MeV of pc has about 1.12 MeV. At rest it gives E = mc². Pixel-art kiss-cut sticker in 2, 3, 4 and 6 inches; equation, symbols and sources in the physics panel below.

Size
Surface

Flat $6 shipping worldwideMade to order, ships in 2–5 business days

Details

Size3″ × 3″

3″ × 3″ kiss-cut sticker.

Shipping

Flat $6 shipping on every order, worldwide, whatever's in it.

Delivery times & where we ship

Returns & replacements21 days

Every item is printed to order, so we can’t accept returns or exchanges for change of mind, or if you ordered the wrong size or colour.

Print defect, misprint, damage or the wrong item? We’ll send a free replacement, or a refund if you prefer. Report it within 21 days of delivery with a photo; no need to send it back.

Full refund & replacement policy

The physics

Meet Mometon, a right-angled triangle who shows how mass and motion add up to energy.

Einstein's relativity links a particle's total energy E, its momentum p and its mass m. The rule is E² = (pc)² + (mc²)², which looks just like Pythagoras. The rest energy mc² and the momentum term pc are the two short sides, and E is the long side.

An electron has mc² ≈ 0.511 MeV. If it also has pc = 1.00 MeV, then E = √(1.00² + 0.511²) ≈ 1.12 MeV. The energies do not simply add, because the two parts meet at a right angle.

For a particle at rest, p = 0 and the rule gives the famous E = mc². For light, m = 0, so E = pc.

Particle physicists use this every day. Measuring a particle's energy and momentum lets them work out its mass, which is how new particles are identified. Even sunlight presses gently on everything it falls on, which is how a solar sail can be pushed.

Equations

\[E^2 = (p c)^2 + (m c^2)^2\]
SymbolMeaningUnit
\(E\) total energy \(\mathrm{J}\)
\(p\) momentum \(\mathrm{kg·m/s}\)
\(c\) speed of light \(\mathrm{m/s}\)
\(m\) rest mass \(\mathrm{kg}\)

Sources

  1. OpenStax University Physics Vol. 3 — Relativistic Energy
  2. NIST CODATA — electron mass energy equivalent in MeV

Field notes

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    How Does a Foucault Pendulum Prove Earth Rotates?

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