For a photon in this toy ledger, energy is tied directly to momentum times the speed limit. With momentum 20 kg m per s and toy speed limit 5 m/s, the checked product is 100 J. Real photon energy is measured through frequency, wavelength, detector calibration, and uncertainty, so this exact multiplication is a training relation, not a full measurement claim.
A massless photon uses the exact product of momentum and speed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Photon energy is tied to momentum
With toy speed limit 5 m/s and momentum 20 kg m per s, the photon energy is checked.