1. Gravity: This is the force that pulls the object downwards towards the center of the Earth. The weight of the object is directly related to the force of gravity acting on it.

2. Buoyancy: This is the upward force exerted by the fluid (in this case, water) on the object. The buoyant force is equal to the weight of the fluid displaced by the object.

Why an object sinks:

An object sinks when the force of gravity acting on it (its weight) is greater than the buoyant force pushing it upwards. This means the object is denser than the fluid it's submerged in.