Kinetics

For every action, there is an equal and opposite reaction.

By action and reaction it is meant action and reaction forces. We know that forces arise in interactions of two bodies⁴¹. When one body exerts a force on another body, the second body exerts a force on the first body that is equal in magnitude but opposite in direction.

When a journalist interviewed the hockey player Jaromír Jágr, he paid him a compliment: „being so big gives you an advantage when somebody tries to board you“. However, Jaromír Jágr smartly replied: „but being big also means that I have to cope with greater weight when I am boarded“. Let us have a look at a situation when a big ice hockey defenseman rams into a smaller forward who is moving with the same velocity but in the opposite direction. Which of them exerts a greater force? They both exert forces of the same magnitude but opposite direction (Newton's Third Law)! What, however, is the effect of these forces? From Newton's Second Law we know that acceleration of a body depends not only on force but also on mass. Because the smaller forward has less mass, after the collision he will accelerate in the direction of the force exerted by the defenseman (assuming their foothold is the same). In such a collision we can see that ramming into an opponent with less mass has more kinetic effect. The bigger the mass of a player, the less kinetic effect (acceleration) we can see after a collision with a player of lesser mass. Newton's Third Law helps us to explain how forces act and what they act on. But it does not directly state what motions of bodies forces cause. It only states that forces arise in pairs and that each of them act on a different body. Therefore we do not add vectors of the forces of action and reaction.

⁴¹ With the exception of inertia forces in mechanics.Zpět