At least two forces are acting on you right now. What are these forces? On what is the equal and opposite force to each of these acting?

Understanding gravitational force is essential when you're studying the fundamental forces that govern our universe. It's the force that attracts two bodies towards one another. On Earth, the most noticeable effect of gravity is that it pulls everything towards the center of the planet. This is why we don't float off into space.

In a practical sense, this force can be calculated by the formula: \[ F = G \frac{m_{1}m_{2}}{r^2} \] where \( F \) is the magnitude of the gravitational force, \( G \) is the gravitational constant, \( m_{1} \) and \( m_{2} \) are the masses of the objects, and \( r \) is the distance between the centers of their masses.

Applied to our daily lives, the Earth's gravitational pull on us creates what we consider our weight. And interestingly, while we often overlook it, we're also pulling on the Earth with the same force in return, according to Newton's third law!

The normal force is another important concept, often acting as the unsung hero keeping us grounded. It's the support force exerted upon an object that is in contact with another stable object. For example, when you're sitting on a chair, the chair pushes up against you; that push is the normal force. You don't fall through the chair because it's providing a force equal to your weight (assuming the force doesn't exceed the chair's strength), directly counteracting gravity's pull.

The normal force is usually denoted by \( N \) and, in the simplest case of an object resting on a horizontal surface, it is equal in magnitude and opposite in direction to the force due to gravity, thus: \[ N = mg \] where \( m \) is the mass of the object and \( g \) is the acceleration due to gravity.

Newton's Third Law of Motion is a cornerstone in understanding mechanics. It's elegantly simple yet universally applicable: 'For every action, there is an equal and opposite reaction.' This law tells us that forces always come in pairs - the action force and the reaction force.

A classic example to visualize this is when you push on a wall. The force of your push (action force) is met with an equal and opposite force from the wall (reaction force), which you can feel on your hand. It's the same when you're walking: your feet push back against the ground, and the ground pushes your feet forward, propelling you ahead.

Understanding these action-reaction force pairs is crucial in solving many physics problems and is key to seeing how forces interact in pairs to create motion - or the lack thereof. So whenever you identify a force in a problem, always remember to look for its counterpart!