Question

why is it not easy to witness inertia on earth?
magnetism is much stronger.
our mass is too small.
gravity and friction are always acting on objects.
our mass is too large.

Explanation:

To determine why inertia is not easy to witness on Earth, we analyze each option:

• "Magnetism is much stronger": Magnetism is not the primary factor affecting inertia observation. Inertia relates to mass and resistance to motion change, while magnetism is a different force, so this is incorrect.
• "Our mass is too small": Inertia is directly proportional to mass ($I \propto m$). Smaller mass means less inertia, but the difficulty in witnessing inertia on Earth is due to external forces, not mass size, so this is incorrect.
• "Gravity and friction are always acting on objects": Inertia is the tendency of an object to resist changes in its state of motion. On Earth, gravity pulls objects down, and friction opposes motion (or motion change). These forces constantly act on objects, masking the effects of inertia (e.g., an object at rest stays at rest because friction and gravity balance, and an object in motion slows due to friction). This explains why inertia is hard to see clearly.
• "Our mass is too large": Larger mass means more inertia, which would make inertia more noticeable, not less, so this is incorrect.

Answer:

C. Gravity and friction are always acting on objects.