Massive and photon rows show the two checked energy rules with small numbers. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Massive rest energy uses mass times speed squared
The massive row uses toy mass 4 kg and speed limit 5 m/s, giving rest and total energy 100 J when momentum is zero.
4⋅5⋅5=100
Photon momentum two gives ten joules
The photon row has no rest energy. Momentum 2 kg m/s times speed limit 5 m/s gives total energy 10 J.
2⋅5=10
Photon momentum four gives twenty joules
Doubling photon momentum to 4 kg m/s doubles the energy to 20 J at the same toy speed limit.