Working out is the act of building mucsle and exercising your body. In preforming acts of runnig lifting or endurance, you engage in a variety of physics topics including friction, resistence, energy, forces and momentum.
For working out, the act of building mucsle demonstrates an example of resistance and or friction. When lifting heavy objects or moving in a forceful manor, it requires you to condemn in motion that essentialy tears the musles cause the force required is working out your mucsles and you gain strength when they grow back allowing you to deliver a greater force.
In terms of energy, chemical energy is converted, therefor conserved and then transfered to the body in a new form of mechanical energy which allows you to move things and or run and exercise to get in shape. Continueing to expell energy requires more energy to keep up your endurance and allowing maximum potential to work out. By having stored energy or potential enrgy, you have the ability to move and then its transfered to kinetic energy in your work out secssion.
In terms of momentum, bigger and more heavy objects when being lifted contribute a greater momentum against your body inhibiting a greater level or degree of diffuculty for bigger objects and will make more of an impact for you. Because of the more intensive strain it provides. Lesser momentum makes it easier to life and "no pain no gain" implies you will not see great results.
Working out is an action that delivers wide diverse physics topics which are good to understand so you know what is happennning to yourself durring work outs.