A football fullback is running down the field at constant speed until he encounters a defensive back.

See the explanation below.

Explanation:

To solve this problem we must complete the statement of the problem, we conducted an internet search, which shows the following physics problem with the same description and more complete. (see the attached image).

Looking at the dotted line, we will take the first point as the point from which it begins to move to the right.

Looking at the dotted line, we will take the first point as the point from which it begins to move to the right. We note that the distances between points 1 to 11 are approximately equal, this means that it is moving at constant speed. In the second attached image, we mark this point (11) where the collision happens and we can see that the distances after this point (11) are decreasing.

For the second question, the defensive back performs force to the left to stop the forward movement( to the right).

Physics and mathematically can be explained using the principle of conservation of energy and work.

This equation tells us that the initial kinetic energy, plus the sum of the work done by the force of the defender, in the opposite direction to the movement (to the left), is equal to the final kinetic energy. We know that the full back's velocity is zero at the end, that is, his kinetic energy at the end is zero.

For the third answer, the moment decreases already this is defined as the difference of the product of the mass with the speed, decreasing the initial speed of the full's back, decreases his momentum.

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