# regarding PS passage V in TBR CBT #7

#### RuSkaya

10+ Year Member
5+ Year Member
I am confused with the following idea (it was in an answer explanation on this TBR test):

given a reaction and conservation of energy,

(energy of reactant 1) + (energy of reactant 2) = (energy of product 1) + (energy of product 2) - mc^2

I don't understand why mc^2 is being subtracted from the product side. I understand mc^2 to be ADDITIONAL energy as a result of mass being converted to it... so shouldn't it be added to the final energy?

#### dingyibvs

##### Psych!
10+ Year Member
I am confused with the following idea (it was in an answer explanation on this TBR test):

given a reaction and conservation of energy,

(energy of reactant 1) + (energy of reactant 2) = (energy of product 1) + (energy of product 2) - mc^2

I don't understand why mc^2 is being subtracted from the product side. I understand mc^2 to be ADDITIONAL energy as a result of mass being converted to it... so shouldn't it be added to the final energy?
Right, mc^2 is the additional energy, so if you don't subtract it, the two sides will no longer be equal as the products will have an extra mc^2 joules of energy.