Blocking and Tackling

Tackling and blocking runners relies on three important principles of physics:

• Impulse
• Conservation of momentum
• Rotational motion

Photo courtesy North Carolina State University
Players use physics to stop each other on the football field.

When Runner and Tackler Meet
When our running back is moving in the open field, he has a momentum of 960 kg-m/s. To stop him -- change his momentum -- a tackler must apply an impulse in the opposite direction. Impulse is the product of the applied force and the time over which that force is applied. Because impulse is a product like momentum, the same impulse can be applied if one varies either the force of impact or the time of contact. If a defensive back wanted to tackle our running back, he would have to apply an impulse of 960 kg-m/s. If the tackle occurred in 0.5 s, the force applied would be:

• F = impulse/t = (960 kg-m/s)/(0.5 s) = 1921 N = 423 lb

Alternatively, if the defensive back increased the time in contact with the running back, he could use less force to stop him.

In any collision or tackle in which there is no force other than that created by the collision itself, the total momentum of those involved must be the same before and after the collision -- this is the conservation of momentum. Let's look at three cases:

1. The ball carrier has the same momentum as the tackler.
2. The ball carrier has more momentum than the tackler.
3. The ball carrier has less momentum than the tackler.

For the discussion, we will consider an elastic collision, in which the players do not remain in contact after they collide.

1. If the ball carrier and tackler have equal momentum, the forward momentum of the ball carrier is exactly matched by the backward momentum of the tackler. The motion of the two will stop at the point of contact.

2. If the ball carrier has more momentum than the tackler, he will knock the tackler back with a momentum that is equal to the difference between the two players, and will likely break the tackle. After breaking the tackle, the ball carrier will accelerate.

3. If the ball carrier has less momentum than the tackler, he will be knocked backwards with a momentum equal to the difference between the two players.

In many instances, tacklers try to hold on to the ball carrier, and the two may travel together. In these inelastic collisions, the general reactions would be the same as those above; however, in cases 2 and 3, the speeds at which the combined players would move forward or backward would be reduced. This reduction in speed is due to the fact that the difference in momentum is now distributed over the combined mass of the two players, instead of the mass of the one player with the lesser momentum.

The Tackling Process
Coaches often tell their players to tackle a runner low. In this way, the runner's feet will be rotated in the air in the direction of the tackle. Let's look at this closely:

Tackling a runner low requires less force because the tackler is farther away from the runner's center of mass.

Talking Physics Center of mass - The point in a body's distribution of mass at which all of the mass can be considered to be concentrated. Torque - A force that tends to produce rotation or twisting

Imagine that the runner's mass is concentrated in a point called the center of mass. In men, the center of mass is located at or slightly above the navel; women tend to have their center of mass below their navels, closer to their hips. All bodies will rotate easiest about their center of mass. So, if a force is applied on either side of the center of mass, the object will rotate. This rotational force is called torque, and is the product of the amount of force applied and the distance from the center of mass at which the force applied. Because torque is a product, the same torque can be applied to an object at different distances from the center of mass by changing the amount of force applied: Less force is required farther out from the center of mass than closer in. So, by tackling a runner low -- far from the center of mass -- it takes less force to tackle him than if he were tackled high. Furthermore, if a runner is hit exactly at his center of mass, he will not rotate, but instead will be driven in the direction of the tackle.

A lineman crouches low so that his center of mass is closer to the ground. This makes it hard for an opposing player to move him.

Similarly, coaches often advise linemen to stay low. This brings their center of mass closer to the ground, so an opposing player, no matter how low he goes, can only contact them near their center of mass. This makes it difficult for an opposing player to move them, as they will not rotate upon contact. This technique is critical for a defensive lineman in defending his own goal in the "red" zone, the last 10 yards before the goal line.

Thanks! Special thanks to Dr. David Haase, professor of physics and director of The Science House at North Carolina State University in Raleigh, NC.

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We have only touched on some of the applications of physics as they relate to football. Remember, this knowledge appears to be instinctive; Most often, players and coaches don't consciously translate the mechanics of physics into their playing of the sport. But by making that translation, we can understand and appreciate even more just how amazing some of the physical feats on the football field really are. Also, applying physics to football leads to better and safer equipment, affects the rules of the sport, improves athletic performance, and enhances our connection to the game.

For more information on football physics and related topics, check out the links on the next page.

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