Rest energy uses mass times the speed limit squared, then gamma scales that rest ledger into total energy. Here mass 4 kg and the toy speed limit 5 m/s give rest energy 100 J, and gamma raises the total to 125 J. The exact arithmetic is for the stated toy constants, while real energy claims require measured masses, an actual value of c, frame choices, and uncertainty.
The toy speed limit makes rest energy an exact hundred joules. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Rest energy uses the speed limit
Mass 4 kg and toy speed limit 5 m/s give rest energy 100 J.