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An inertial reference frame is a frame of reference in which Newton's laws of motion are valid. This means that an object either remains at rest or continues to move at a constant velocity, except when acted upon by a force. For example, a stationary observer or one moving with constant velocity can be considered to be in an inertial reference frame.
In the original exercise, Observer A is in a fixed frame, which is an ideal example of an inertial reference frame. Because there are no accelerating forces acting on the reference frame itself, the principles of impulse and momentum can be directly applied to determine the motion of the block as seen by Observer A.
It's crucial to note that inertial frames are relatively moving with respect to each other and not accelerating. In our exercise, Observer B's frame of reference moves with a constant velocity, hence it is still considered an inertial frame.

Relative motion is a straightforward but important concept. It explains how the movement of one object is perceived differently from different reference points or frames. In essence, motion can appear different depending on where you are looking from.
In the context of our exercise, Observer B experiences relative motion. While the block accelerates due to an external force, Observer B is moving with a constant velocity. Because Observer B's frame is moving relative to Observer A, the block's speed seems to be different when viewed from B's perspective. This effect is calculated by subtracting the velocity of the moving frame (which is 2 m/s in this case) from the velocity observed in the fixed frame.
Understanding relative motion helps clarify why the same event or motion can give different results when viewed from different points of references. It underscores the importance of clearly specifying the frame of reference when discussing motion.

The equation of motion is fundamental in analyzing the movement of objects. The basic equation used here is: \[ v = u + at \] where \( v \) is the final velocity, \( u \) is the initial velocity, \( a \) is the acceleration, and \( t \) is the time period.
This formula results from integrating the acceleration over time and is necessary to find how fast something is moving after a period of acceleration. In our exercise, acceleration is derived from the force acting (using \( a = F/m \)), and we plug this into the equation along with given initial values to solve for the final speed of the block.
The equation of motion is powerful in determining future velocities in a precise manner and is one of the building blocks in classical mechanics. It shows how velocity changes in response to acceleration over a given span of time, showing its invaluable role in momentum and impulse calculations.