Reasons for the observation:
- Inertia of the card: The card exhibits the property of inertia, which is the tendency of an object to resist changes in its state of motion. When the coin is placed on the card, it also acquires some of the inertia of the card. As a result, when the card is tilted, the coin resists moving forward due to its inertia and remains on the card.
- Friction between the card and the coin: There is a frictional force acting between the card and the coin. This friction opposes the relative motion between the two surfaces in contact. As a result, when the card is tilted, the frictional force prevents the coin from sliding forward and causes it to stay on the card.
- Center of mass: The center of mass of an object is the point at which its mass is concentrated. When the card is tilted, the center of mass of the coin-card system remains within the base of support, which is the area of contact between the card and the post-it. This ensures that the coin does not fall off the card and remains in place.
- Balance: The coin, being a small object, can be considered as a point mass. When the card is tilted, it rotates around a pivot point, which is the edge of the glass. The coin's position on the card is such that it helps maintain the balance of the system. If the coin were to move forward, it would disrupt the balance and potentially cause the entire system to topple over.
- Adhesion between the post-it and the glass: The post-it is attached to the glass through adhesion, which is the attractive force between molecules of different substances. This adhesion ensures that the card, along with the coin, cannot move forward and fall into the glass. The post-it provides a stable surface for the card to rest on and prevents it from sliding forward.
- Gravity: The force of gravity acts vertically downwards on the coin. As long as the card is not tilted beyond a certain angle, the gravitational force is not sufficient to overcome the frictional force and cause the coin to slide forward and fall into the glass.

In conclusion, the observation that the coin cannot move ahead while the card is tilted is due to the inertia of the card, friction between the card and the coin, the position of the coin maintaining balance, adhesion between the post-it and the glass, and the gravitational force not being strong enough to overcome the other forces.