Here's why:

* Force and Mass: Force is directly proportional to mass. This means that the more massive an object is (the heavier it is), the more force you need to move it.

* Newton's Second Law: This law of motion states that force (F) equals mass (m) multiplied by acceleration (a): F = m * a. To move something, you need to accelerate it, and a heavier object requires more force to achieve the same acceleration.

Example:

Imagine pushing a small, empty box versus pushing a large, full box. The full box has much more mass, so you'd need to apply significantly more force to get it moving.