In my years of teaching Karate to young children I have noticed several physics laws that become perfectly obvious. Among the most obvious is the law of conservation of momentum. Take for example when we practice flying kicks. We have it set up where the sensei, usually me, holds a bag at one end of the room and the student is at the opposite and runs, jumps, and then kicks the bag. What I have noticed is that the force that hits me depends on 1) the speed at which the kid runs and 2) the size of the kid. As the velocity goes up, the force goes up and the same with the mass.