Physics of Buoyancy and Forces

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Buoyancy

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Buoyancy is the upward force that a fluid (like water) exerts on an object that is placed in it. This force makes objects feel lighter in water and can make them float. Example: A rubber duck floats in a bathtub because the water pushes up against it with a buoyant force that is stronger than the duck’s weight.

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Displacement

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Displacement happens when an object is placed in a fluid and pushes some of that fluid out of the way. The amount of fluid pushed out is equal to the volume of the object that’s underwater. Example: When you step into a full bathtub, the water spills over the edge. That spilled water is the amount you displaced with your body.

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Gravitational Force

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Gravitational force is the pull that Earth (or any object with mass) exerts on another object. This force gives objects weight and pulls them toward the ground. Example: When you drop a pencil, it falls to the floor because Earth's gravity pulls it down.

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Archimedes’ Principle

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Archimedes’ Principle states that the buoyant force on an object in a fluid is equal to the weight of the fluid the object displaces. Example: If a rock pushes 1 liter of water out of a container when dropped in, the water pushes back with a force equal to the weight of that 1 liter of water.

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Plimsoll Line

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The Plimsoll line is a line marked on the side of a ship that shows how much cargo it can safely carry without sinking too low in the water.

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Hydraulic Device

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A hydraulic device uses a liquid (usually oil or water) to transmit force from one place to another. Example: A car jack uses hydraulics to lift a car off the ground when you’re changing a tire.

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Pneumatic Device

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A pneumatic device uses compressed air or gas to transfer force. Example: A dentist’s drill uses air pressure to spin at high speeds and clean teeth.

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Pressure

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Pressure is the amount of force applied to a certain area. It’s measured as force divided by area. Example: When you push a thumbtack into a wall, the small point puts a lot of pressure on a small area, making it easier to pierce the wall.

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Pascal (Pa)

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A pascal is the unit used to measure pressure. One pascal is equal to one newton of force applied to one square meter of area. Example: A tire might be filled to 200,000 pascals (or 200 kilopascals) of air pressure to work properly.

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Pascal’s Law

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Pascal’s Law says that when pressure is applied to a fluid in a closed system, the pressure is transmitted equally in all directions. Example: When you squeeze one end of a sealed plastic ketchup packet, the ketchup shoots out the other end—because the pressure moved through the entire packet.