notes- sinking & floating

Notes- Sinking & FloatingNotes- Sinking & Floating

Chapter 3, Lesson 3Chapter 3, Lesson 3

Sink or Float?Sink or Float?

Whether an object sinks or floats Whether an object sinks or floats depends on the relative size of the depends on the relative size of the buoyant force on the object and buoyant force on the object and the weight of the object.the weight of the object.


If the buoyant force If the buoyant force on an object is less on an object is less than the object’s than the object’s weightweight

The net force on The net force on the object is the object is downwarddownward

The object will sink. The object will sink. (like the metal (like the metal block)block)


When an object is When an object is floating, it is not floating, it is not moving verticallymoving vertically

The upward (or The upward (or buoyant) force and buoyant) force and the downward the downward force (weight) are force (weight) are the same and the same and cancel. (like the cancel. (like the boat)boat)

The Buoyant Force & The Buoyant Force & DensityDensity

An object will float if it is less An object will float if it is less dense than the fluid around it.dense than the fluid around it.

The Buoyant Force & The Buoyant Force & DensityDensity

Metal boats float Metal boats float because they because they contain air, which contain air, which makes their total makes their total density less than the density less than the water around them.water around them.

The boat and block The boat and block have the same have the same weight, but the boat weight, but the boat has more bouyant has more bouyant force.force.

The Buoyant Force and The Buoyant Force and DensityDensity

A hydrometer is A hydrometer is an instrument an instrument that measures the that measures the density of a fluid.density of a fluid. In a less dense In a less dense

fluid, it displaces fluid, it displaces more fluid and more fluid and floats lower.floats lower.

In a denser fluid, it In a denser fluid, it displaces less fluid displaces less fluid and floats higher.and floats higher.

Floating & Sinking in the Floating & Sinking in the AtmosphereAtmosphere

Like any fluid, air exerts pressure Like any fluid, air exerts pressure on things around it.on things around it.

If the buoyant force due to air If the buoyant force due to air pressure on an object is large pressure on an object is large enough, the object can float in air.enough, the object can float in air.


Helium gas is less Helium gas is less dense than air dense than air (nitrogen & (nitrogen & oxygen)oxygen)

When a balloon is When a balloon is filled with helium, filled with helium, it is less dense it is less dense than the air than the air around it, and around it, and therefore floats.therefore floats.


Whether a hot-air Whether a hot-air balloon rises or falls balloon rises or falls can be controlled by can be controlled by changing the density changing the density of the air inside the of the air inside the balloon.balloon. Warm air has particles Warm air has particles

that are farther apart that are farther apart and therefore, it is less and therefore, it is less dense. dense. (balloon rises)(balloon rises)

Cool air has particles Cool air has particles that are closer together that are closer together and therefore, it is more and therefore, it is more dense. dense. (balloon sinks)(balloon sinks)

Why does an object sink in water?

A The object is denser than water.

B The buoyant force is greater than the object’s weight.

C The buoyant force is greater than the object’s mass.

D The object is less dense than water.

Why does a hot-air balloon float?

A The surrounding air is denser than the balloon.

B The surrounding air is less dense than the balloon.

C The balloon is denser than the surrounding air.

D The balloon has less mass than the surrounding air.

What does a hydrometer measure?

A mass of a solid

B density of a solid

C mass of a liquid

D density of a liquid

notes- sinking & floating

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