Question:

When the cooking oil is added, will the golf ball float higher, lower, or same in the brine (salt solution)?

Answer:

When there is only the brine (see Figure 1), the weight of the brine displaced (labelled A) is equal to the weight of the golf ball.

Let’s imagine the golf ball floating at the same level as before after oil has been added on top (see Figure 2). Since the golf ball now displaces oil as well, it must receive an additional upthrust that is equal to the weight of the displaced oil (labelled B). This means that at this level, there is a net upward force acting on the the golf ball. So surely the golf ball will float higher until the weight of the displaced brine plus oil equals the weight of the golf ball again (see Figure 3).

Many students wrongly predicted that the addition of oil would cause the oil to exert a downward pressure force on the golf ball and push it further into the brine. These students overlook the fact the layer of oil also increases the fluid pressure in the brine. The increase in the upward pressure force by the brine actually pushes golf ball further out of the brine.

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