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When we engage with objects around us, whether it's opening a door or kicking a ball, we're using something called applied force. It's the force that a person or another object uses to move or try to move something. In the context of our exercise, when you push on an object with your fingers, the muscles in your hand and arm exert an applied force upon the object. This force is intentional, directed, and has both magnitude and direction, following the principles of vector quantities in physics.

Understanding the applied force is important, not just in identifying how objects move, but also in determining the subsequent forces that may come into play. In this case, when your fingers apply a force to an object, it could respond in various ways depending on its physical properties like mass, friction, and rigidity. The key here is that the applied force is your direct interaction with the object's state of motion or stillness.

Now, Newton's third law of motion famously states, 'For every action, there is an equal and opposite reaction.' This brings us to the concept of reaction force. The moment your fingers exert an applied force on the object, the object isn't merely a passive recipient of this force. It actively exerts a force back upon your fingers, which is the reaction force.

It's a fundamental concept that helps explain why you feel resistance when pushing against objects. This reaction force is of the same magnitude but opposite in direction to the applied force. It's a crucial component of interactions as it ensures that forces are always interactions between two objects, not an isolated phenomenon. For example, when you push against a wall, the wall applies a force right back onto your hand, and you're able to feel this reaction.

Exploring the dynamics between applied and reaction forces leads us directly to the understanding of a pair of forces in physics. When you exert a force on an object, and it simultaneously exerts a force back on you, you're experiencing a pair of forces at work. These forces are a foundational concept in physics, embodying Newton's third law.

In our earlier context, the force your fingers apply to the object and the force that the object applies back are a pair of forces. What's compelling about this is that the interaction gives rise to a detailed understanding of not just movement, but also force sequences and reactions in any two interacting entities. This concept extends beyond simple pushing and pulling to complex systems involving gravitational forces, magnetic forces, and even electrical forces where pairs of forces are continuously at play.