One key to understanding brick breaking is a basic principle of motion: The more momentum an object has, the more force it can generate. When it hit the brick, Karate fighter's hand had reached a speed of 11 meters per second (24 miles per hour). At this speed, his hand exerted a whopping force of 3,000 newtowns--or 675 pounds--on the concrete. A slab of concrete could likely support the weight of a few people weighing a total of 675 pounds (306 kilograms). But apply that amount of force concentrated into an area as small as a fist and the concrete slab will break. In other words we can understand it interms of kicking a football ,See when u kick a ball u apply a lot of force by moving your leg with much great velocity and there is a contact of ur foot on the ball over a small area, hence, the area is less and the force is very much so the pressure will be a lot and there will be a lot of impulse(momentum attained by the ball) and so the effect will be more.
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