Final answer:
The NFL changes in 1974, including moving goal posts and kickoffs, emphasized special teams by changing field dynamics. Coaches instruct players to stay on the ground to maximize momentum, work, and energy during play, increasing effectiveness and safety.
Step-by-step explanation:
In 1974, the NFL made changes such as moving the goal posts, moving kickoffs to the 35-yard line, and reducing some penalty yards. These changes put more emphasis on special teams. This was due to the fact that moving the kickoffs to the 35-yard line would result in more touchbacks and moving the goal posts made field goals more challenging, thereby increasing the importance of the kicking game within special teams units.
Football coaches advise players to block, hit, and tackle with their feet on the ground rather than leaping through the air, as it optimizes the use of momentum, work, and energy. Staying grounded allows players to maintain better balance and leverage, thereby making them more effective during contact. This also reduces the risk of injury and increases the force that can be applied to an opponent. A small force can impart the same momentum to an object as a large force if it acts over a longer period of time; this is due to the principle that momentum is the product of mass and velocity.