Introduction:
When a karate expert strikes a slab of ice, they suddenly reduce speed. This phenomenon can be explained by Newton's laws of motion, specifically the law of conservation of momentum and the law of action and reaction.

Law of Conservation of Momentum:
The law of conservation of momentum states that the total momentum of an isolated system remains constant if no external forces act upon it. Momentum is the product of an object's mass and velocity, and it is a vector quantity. In this scenario, the karate expert and the slab of ice form an isolated system.

Explanation:
1. Initial State:
In the initial state, the karate expert's hand is moving at a certain velocity, and the slab of ice is at rest. Let's assume the mass of the karate expert's hand is m1, the velocity is v1, and the mass of the slab of ice is m2 (which is greater than m1).

2. Action:
When the karate expert strikes the slab of ice, the hand exerts a force on the ice. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. Therefore, the ice exerts an equal and opposite force on the hand.

3. Reaction:
The reaction force exerted by the ice on the hand causes a change in momentum. As per the law of conservation of momentum, the total momentum before and after the interaction must be the same. Initially, the momentum of the karate expert's hand is m1 * v1, and the momentum of the ice is 0 since it is at rest.

4. Momentum Transfer:
During the interaction, some of the karate expert's momentum is transferred to the slab of ice. The force exerted by the hand reduces the velocity of the hand, resulting in a decrease in momentum. Simultaneously, the force exerted by the ice increases its velocity, leading to an increase in momentum.

5. Velocity Redistribution:
As a result of momentum transfer, the velocity of the karate expert's hand decreases, while the velocity of the slab of ice increases. This redistribution of velocities obeys the principle of conservation of momentum.

6. Reduced Speed:
Due to the transfer of momentum, the karate expert's hand experiences a decrease in speed while striking the slab of ice. This reduction in speed is a consequence of the change in momentum caused by the force exerted by the ice on the hand.

Conclusion:
The reduced speed experienced by a karate expert while striking a slab of ice can be explained by the law of conservation of momentum and the law of action and reaction. The transfer of momentum from the hand to the ice results in a redistribution of velocities, causing the karate expert's hand to slow down. This phenomenon demonstrates the fundamental principles of Newton's laws of motion.