# Move that mass!

Hike! The center whips the ball to the quarterback. The crowd goes wild. The quarterback shifts swiftly, scanning his teammates for a receiver. He has to make his move fast! A stampede of defensive linemen lunge for him with a single aim -- to slam him to the ground before the football escapes his hand.

The only problem is, to tackle the quarterback, defensive linemen have to mow past his "bodyguards" -- big, brawny tackles who tip the scales at more than 300 pounds. Good luck!

It's not just brawn that helps offensive and defensive tackles plow each other down like Mack trucks. Believe it or not, they're also pros in physics. In the "Super Bowl of Size" the player with the most mass (the quantity of matter to be moved) has a heavy advantage. After all, the more massive you are, the more force it takes to ram you to the ground.

Plenty of muscles and a big gut help. "I'm a real physical player and my size helps me out," says 320-pounder Erik Williams, offensive tackle for the Dallas Cowboys. Williams has "moved mass" in three Super Bowls.

Scientists use this equation to describe the relationship between force and mass:

Force (F) = mass (m) x

acceleration (a)

Acceleration is the change in speed or direction as a result of force. The equation says bigger football players are harder to move than less massive football players. For one thing, it takes a lot more force. For another, for a given force -- or shove -- the effect is smaller.

Think of it as trying to move two grocery carts blocking your way in market aisle. The cart stuffed with more groceries has more mass so it will be harder to move. You'll have to use more force. Likewise, it takes more force to make those giant football tackles change direction or speed. So a less massive player will have a tough time shoving Erik Williams off balance.

BIG BOYS

Tackles aren't always born big. They have to work to get huge. Richmond Webb, a 320-pound offensive tackle for the Miami Dolphins, lifts weights for more than an hour two to three times a week. He can bench-press more than 400 pounds! As for diet, "some guys pack it in," admits Jerry Wunsch (335 lbs.), offensive tackle for the Tampa Bay Buccaneers. "I could eat one of those 12-inch frozen pizzas for dinner, but that's stuffing myself."

Mass isn't a tackle's only weapon. He needs good technique, too. "You can have size and not be able to move," Wunsch says. "You also need explosive power. A small guy with explosive power is going to get more done than a big, massive guy."

Say at the sound of "Hike!" a truck-size defensive lineman lunges at you. What's the best way to keep your balance? Strategy No. 1: "Keep a good base -- a medium stance with your feet about shoulder-width apart," says Miami's Webb.

Strategy No. 2: "Crouch real low. The low man wins," Webb says. Crouching lowers a player's center of gravity -- the point around which the body's mass is evenly balanced. The lower your center of gravity, the more stable your stance. The higher your center of gravity, the more likely a powerful shove can topple you (see Pushy Science, right).

Finally, if you want to walk off the field in one piece, be careful! "At any moment you can get hurt," Wunsch says. Nobody likes to get clobbered, but these tackles might feel better knowing that smart physics will keep them standing.

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Pushy Science

You don't have to wear a helmet, shoulder pads, and black smudges under your eyes to learn a tackle's key strategy: Keep your balance by controlling your center of gravity. All you need is a partner and somebody to say "Hike!"

WHAT TO DO

1. Stand straight about an arm's length from your partner, and place your hands palm to palm.

2. When you hear "Hike," try to push your partner off balance. You can only touch palm to palm -- no rough stuff allowed. Stop when either of you moves a foot.

3. Try again, this time changing your stance (for example, crouch, lean back, or spread your feet, etc.).

CONCLUSIONS

Which stance gave you the best balance? The worst? What's the best strategy for throwing your partner off balance?