An air bubble rises through a liquid. Apart from the buoyant force, which other forces act on the bubble during its upward motion?

Only the force of gravity.
The force due to gravity and the force due to the pressure of the liquid.
The force due to gravity, the force due to liquid pressure, and the viscous force of the liquid.
The force due to gravity and the viscous force of the liquid.

Correct answer: The force due to gravity, the force due to liquid pressure, and the viscous force of the liquid.

Solution

Buoyancy itself comes from the pressure of the liquid; but apart from the (named) buoyant force, the bubble still has its own weight (gravity), and since it moves through a viscous medium it experiences a viscous drag opposing motion. The liquid pressure also acts on it. Hence gravity, liquid pressure, and viscosity all act, in addition to buoyancy.

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