Motion is a fundamental concept in physics that describes the change in position of an object over time. In our daily life, we observe various types of motion everywhere. A car moving along the road demonstrates translational motion, birds flying through the air show complex three-dimensional movement, and celestial bodies like Earth orbiting the Sun represent rotational motion. Understanding motion requires us to carefully observe and describe how objects change their positions relative to other objects.
A reference frame is a coordinate system from which we observe and measure motion. The choice of reference frame is crucial because the same motion can appear different depending on our viewpoint. For example, consider a person sitting inside a moving train. From the perspective of an observer standing on the ground, both the train and the person inside are moving. However, from the perspective of another passenger in the same train, the person appears to be at rest. This demonstrates that motion is relative and depends entirely on our chosen reference frame.
Relative motion demonstrates how the velocity of an object depends on the chosen reference frame. Consider a boat crossing a river. The boat has its own velocity relative to the water, while the water flows with its own velocity relative to the shore. From the perspective of an observer on the shore, the boat's actual motion is the vector sum of these two velocities. This resultant velocity determines the boat's path as seen from the shore. The concept of relative motion is fundamental in understanding how different observers perceive the same physical phenomenon differently.
The distinction between absolute and relative motion is crucial in physics. Absolute motion refers to motion measured relative to a fixed reference frame, such as the ground. For example, if one car travels at 60 kilometers per hour eastward and another car travels at 40 kilometers per hour westward, their absolute velocities are measured relative to the ground. However, relative motion describes how these objects move with respect to each other. From the perspective of the first car, the second car appears to approach at 100 kilometers per hour, which is the sum of their absolute speeds since they move in opposite directions.