Buoyancy, Density and Upthrust

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Part 1: Key Ideas and Multiple Choice

Density describes how much mass is packed into a certain volume. It can be calculated using:

density = mass ÷ volume

Buoyancy is the ability of an object to float. Upthrust is the upward force exerted by a liquid on an object. The upthrust depends on the density of the liquid and the volume of liquid displaced.

An object usually floats when its average density is less than the fluid's density. It sinks when its average density is greater than the fluid's density. If the forces are balanced, a floating object remains at rest.

1. Which statement best describes density?

The size of an object

The mass packed into a certain volume

The upward force on an object

The amount of liquid displaced

2. Upthrust is a force that acts:

Downwards

Sideways

Upwards

Only on sinking objects

3. An object has a density of 0.8 g/cm³. Fresh water has a density of about 1.0 g/cm³. What will happen?

It will float

It will sink

It will dissolve

There is not enough information

4. Which change would increase the upthrust on an object in the same liquid?

Using a less dense liquid

Displacing a greater volume of liquid

Removing the object's weight

Making gravity disappear

Part 2: Apply and Explain

5. Study the labelled force diagrams. Complete the sentences.

Floating object: Upthrust ↑ Weight ↓

Sinking object: Upthrust ↑ Weight ↓

For a floating object, the upthrust is __________ the weight. For a sinking object, the weight is __________ the upthrust.

6. A metal block has a mass of 240 g and a volume of 30 cm³. Calculate its density and decide whether it will float or sink in water. Show your working.

7. Three objects are placed in water. Use the data to predict what each will do.

Object A: mass = 50 g, volume = 100 cm³

Object B: mass = 180 g, volume = 100 cm³

Object C: mass = 100 g, volume = 100 cm³

Calculate the density of each object. Water has a density of 1.0 g/cm³. Which object is most likely to float partially submerged? Explain.

8. Ships are made mainly from steel, which is denser than water. Explain how a steel ship can float. Use the words mass, volume, average density, upthrust and weight.

9. Challenge: A submarine dives by allowing water into its ballast tanks. Explain how this changes its average density and the balance between weight and upthrust.

Self-check

Tick each statement when you can do it.

I can calculate density using mass ÷ volume.

I can identify weight and upthrust on a force diagram.

I can explain why an object floats or sinks.

I used scientific vocabulary in my explanations.

Teacher Answer Key

1. The mass packed into a certain volume.

2. Upwards.

3. It will float, because 0.8 g/cm³ is less than water's density of 1.0 g/cm³.

4. Displacing a greater volume of liquid.

5. For a floating object, upthrust is equal to the weight. For a sinking object, the weight is greater than the upthrust. In the diagrams, weight acts downwards and upthrust acts upwards.

6. Worked calculation: density = mass ÷ volume = 240 g ÷ 30 cm³ = 8 g/cm³. This is greater than water's density, so the block will sink.

7. Worked calculations:

Object A: 50 g ÷ 100 cm³ = 0.5 g/cm³; floats.

Object B: 180 g ÷ 100 cm³ = 1.8 g/cm³; sinks.

Object C: 100 g ÷ 100 cm³ = 1.0 g/cm³; neutrally buoyant or able to remain fully submerged. Object A is most likely to float partially submerged because its density is less than water's density.

8. Sample answer: Although steel is denser than water, a ship is hollow and contains air. The ship's mass is spread over a large volume, so its average density is less than water. It displaces enough water to produce an upthrust equal to its weight, so it floats.

9. Sample answer: Water entering the ballast tanks increases the submarine's mass without increasing its outside volume by the same amount. Its average density increases. Its weight becomes greater than the upthrust, so it sinks. To rise, water is forced out, decreasing its average density and allowing upthrust to exceed weight.