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.

455=1004\cdot5\cdot5=100
Particle energy rowVisible mass, momentum, rest energy, and total energy are checked.electronm=4 kgp=0 kg*m/sE0=100 JE=100 J

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.

25=102\cdot5=10
Particle energy rowVisible mass, momentum, rest energy, and total energy are checked.photonm=0 kgp=2 kg*m/sE0=0 JE=10 J

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.

45=204\cdot5=20
Particle energy rowVisible mass, momentum, rest energy, and total energy are checked.photonm=0 kgp=4 kg*m/sE0=0 JE=20 J