Introduction
When a plastic object is immersed in water, it experiences a buoyant force due to the displacement of water. This buoyant force is a result of Archimedes' principle, which states that the buoyant force acting on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.

Explanation
The buoyant force acting on the plastic object can be calculated using the following formula:

F_b = ρ_water * V * g

Where:
- F_b is the buoyant force
- ρ_water is the density of water
- V is the volume of water displaced by the object
- g is the acceleration due to gravity

In this case, the plastic object displaces 0.5g of water. To calculate the volume of water displaced, we need to use the formula:

V = m_water / ρ_water

Where:
- V is the volume of water displaced
- m_water is the mass of water displaced
- ρ_water is the density of water

Since the mass of water displaced is given as 0.5g, we can substitute these values into the equation to find the volume of water displaced.

V = 0.5g / ρ_water

Now, we can substitute the value of the volume of water displaced into the buoyant force formula to calculate the buoyant force acting on the plastic object.

F_b = ρ_water * (0.5g / ρ_water) * g

Simplifying the equation, we find that the density of water cancels out, leaving us with:

F_b = 0.5g

Therefore, the buoyant force acting on the plastic object is equal to 0.5g, where g is the acceleration due to gravity (approximately 9.8 m/s²).

Conclusion
When a plastic object displaces 0.5g of water, the buoyant force acting on the object is equal to 0.5g. This buoyant force is a result of Archimedes' principle and is proportional to the weight of the fluid displaced by the object.