Question

The field: There is a lake... In the lake, there is a boat... In the boat, there is a boy. The boy takes a coin out of his pocket and throws it in the water. Does the boat go up or down, and why?

a. The boat goes up because the coin adds buoyancy.
b. The boat goes down because the coin increases weight.
c. The boat goes up because the coin reduces weight.
d. The boat goes down because the coin reduces buoyancy.

Answer 1

The boat will go up slightly when the boy throws the coin out because the overall mass of the boat is reduced, decreasing the downward force it exerts on the water and thus increasing the net buoyant force relative to the boat's new weight.

Step-by-step explanation:

The boat goes up or down when the boy throws the coin in the water is: c. The boat goes up because the coin reduces weight.

Once the coin is thrown from the boat into the lake, it's no longer part of the boat's overall mass. The weight of the boat, which is now missing the coin's weight, decreases, thus reducing the downward force it exerts on the water. According to Archimedes' principle, the buoyant force is equal to the weight of the fluid that the boat displaces. Since the boat now weighs less, minus the coin, it will displace slightly less water, and this effect results in an increase in the net buoyant force relative to the new (slightly lower) weight of the boat.

Therefore, the boat experiences an upward force and rises slightly in the water. This is counterintuitive because we might expect the boat to go down due to losing counterbalancing weight, but the water's buoyancy plays a significant role in this scenario.