If you held a piece of tissue paper at arms length and asked the heavyweight champion of the world to hit the paper with 50 pounds of force, what would happen? explain using your understanding of force.

Force is equal to mass x acceleration. The heavyweight is packing both greater acceleration and mass than the tissue paper. 
When his fist comes in contact with the paper, the tissue reacts by exerting its own opposite force, providing a tiny bit of resistance until the fist breaks it apart and passes through. 
The fist, although by a a very tiny amount, is decelerated (slowed down) by the paper before it breaks through.