A pair ledger separates signed momentum conservation from additive positive energy. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The left particle carries fifteen momentum units
The left-moving ledger entry is momentum 15 kg m/s and energy 125 J. Momentum keeps a direction sign; energy is a positive budget.
15kgm/sEL=125J
The right particle carries negative fifteen momentum units
The other entry has momentum -15 kg m/s and energy 125 J. The sign records opposite direction; it does not make energy negative.
−15kgm/sER=125J
Momentum cancels while energy adds
Adding the two signed momenta gives 0 kg m/s. Adding the two positive energies gives 250 J. The ledger is conserved by signed momentum and additive energy.