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

**Impulse****Conservation of momentum****Rotational motion**

### 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:

- The ball carrier has the same momentum as the tackler.
- The ball carrier has more momentum than the tackler.
- 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.

- 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.
- 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.
- 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.

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