What principle states that forces are always equal and opposite?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This principle highlights the relationship between two interacting bodies: when one body exerts a force on another, the second body exerts a force of equal magnitude but in the opposite direction back onto the first body. This is a fundamental concept in physics that helps explain a wide range of phenomena, from simple interactions like pushing a swing to complex systems in motion.

For instance, when you jump off a small boat onto the dock, you push down on the boat (action), causing it to move backward (reaction). Both forces are equal in strength but opposite in direction, illustrating Newton's third law in action. The law helps to understand various physical interactions and is crucial for analyzing dynamics, making it essential for fields ranging from engineering to biomechanics.