A football player makes a tackle to stop a touchdown.What will happen to the offensive player?What law of motion applies and why?

Well, lots of things can happen but what you can count on is conservation of momentum of the overall runner, tackler, ground system. Any forces from the ground on the people can change the momentum of the merged runner-tackler system.

If they were both on ice and ran into each other with the same masses and opposite velocities, and stuck, they would both stop.
If one had greater speed or mass or both, that one would push the other back.
If one has bigger cleats, then as the situation evolves, that one wins, but that requires time :)

When a football player makes a tackle to stop a touchdown, the offensive player will be stopped in their forward motion, potentially brought to the ground, and prevented from reaching the end zone. The specific consequences will depend on the force of the tackle and the defensive player's technique.

The law of motion that applies to this scenario is Newton's First Law of Motion, also known as the law of inertia. This law states that an object at rest will remain at rest, and an object in motion will continue to move in a straight line at a constant velocity, unless acted upon by an external force.

In the case of the offensive player, they have a forward motion towards the end zone, aiming to score a touchdown. However, when the defensive player tackles them, they apply an external force that opposes the offensive player's motion. According to Newton's First Law, this force will cause the offensive player to stop or change their motion.

The amount of force required to stop the offensive player will depend on various factors including the mass and speed of the players involved, as well as the angle and technique of the tackle. To precisely calculate the effects of the tackle, you would need to consider these factors and apply Newton's laws of motion, particularly the second law (F = ma) to calculate the force applied during the collision.