Which of Newton's Laws cannot be demonstrated on Earth due to friction and gravity?

The first law of motion, often referred to as the law of inertia, states that an object at rest will remain at rest, and an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This principle is fundamentally reliant on the absence of external forces to demonstrate the pure concept of inertia.

On Earth, friction and gravity are always present, affecting the motion of objects. For instance, an object in motion will eventually come to a stop due to the force of friction opposing its motion. Similarly, gravity continuously acts on objects, pulling them towards the Earth, which means they will not remain in motion indefinitely in a straight line as stated in the first law.

Thus, while the first law can illustrate inertia conceptually, its demonstration in a practical environment like Earth is severely hindered by these forces, making it difficult to observe the law in its purest form without accounting for these influences.