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Year 9

Floating, mass and shape

I can explain how the density of an object determines how well it floats.

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New
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Year 9

Floating, mass and shape

I can explain how the density of an object determines how well it floats.

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Share activities with pupils
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Lesson details

Key learning points

  1. The whole of an object is floating on water if part of it is above the surface.
  2. How well an object floats in a liquid does not depend on the depth of the liquid.
  3. An object floats in water if its shape can push its own mass of water out of the way without going under.

Keywords

  • Upthrust - A force acting upwards due to pressure is an upthrust.

  • Displaced - Water is displaced (moved) when an object floats on it or sinks in it.

  • Displacement can - A displacement can is used to find the volume of water displaced by objects when they float or sink.

  • Density - If the density of an object is less than the density of water, it will float.

Common misconception

Pupils may think that all light objects float and that all heavy objects sink.

Explore how mass and volume (determined by shape) affect whether objects float or sink - before discussing density.

Using video footage of deep sea diving vessels will help demonstrate the extreme pressures. Similar videos of aeroplane decompression can be used but these are often exaggerated so use with care.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which is the best definition of the weight of an object?
the amount of material it contains
Correct answer: the size of force needed to lift or support it
the space the object takes up
the resultant force acting on the object
Q2.
Which is the best definition of the mass of an object?
the force pulling down on it
Correct answer: the amount of material it contains
the total number of atoms inside it
the amount of space it takes up
Q3.
The figure shows two forces acting on a toy boat on the surface of water. Which statements are correct?
An image in a quiz
Correct answer: the boat will be speeding up
the boat will be at a steady speed
the boat will be slowing down
Correct answer: not all of the forces are shown in the figure
Q4.
A balance is used to measure the mass of a sample of water. If the mass of the water is 320 g, what is this mass in kilograms?
An image in a quiz
320 kg
32.0 kg
3.2 kg
Correct answer: 0.32 kg
0.032 kg
Q5.
A sample of water has a mass of 0.5 kg. What is its approximate weight on Earth?
An image in a quiz
0.05 N
0.5 N
Correct answer: 5 N
50 N
500 N
Q6.
This cruise ship is stationary and floating on the surface of the water. Which statements are correct?
An image in a quiz
There are no vertical forces acting on the ship.
Correct answer: There is a downwards gravitational force acting on the ship.
Correct answer: There is an upwards force acting on the ship.
There is a resultant force acting on the ship.
The ship has no weight.

Exit quiz

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6 Questions

Q1.
Match the key words to the explanations
Correct Answer:upthrust,A force acting on a floating object due to water pressure.

A force acting on a floating object due to water pressure.

Correct Answer:displaced,Moved away from its normal position.

Moved away from its normal position.

Correct Answer:displacement can,Used to find the volume or mass of displaced water.

Used to find the volume or mass of displaced water.

Correct Answer:density,If this is high an object will sink.

If this is high an object will sink.

Q2.
A cargo ship is travelling on water. Which of these statements is correct?
An image in a quiz
Correct answer: All of the ship is floating on the water.
only the part of the ship above the water is floating
only the part of the ship below the water is floating
none of the ship is floating
Q3.
A wooden boat floats in shallow water. Which of these describes what will happen when it is placed in deeper water?
The boat will sink
The boat will float higher in the water
The boat will float lower in the water
Correct answer: The boat will float at the same height in the water.
Q4.
Which of these is the correct condition for something to be able to float?
there is no upthrust acting on it
its weight is greater than the uptrust
Correct answer: it can push its own mass of water out of the way.
it pushes less than its own mass of water out of the way.
Q5.
The table shows the mass of objects and the mass of water they displace. Which objects float?
An image in a quiz
Object A
Correct answer: Object B
Correct answer: Object C
Object D
Object E
Q6.
A pupil makes a boat from aluminium foil. It floats on water. When they scrunch it up into a ball it sinks. Why?
An image in a quiz
The foil boat floats because aluminium is less dense than water.
The foil boat floats because aluminium is more dense than water.
Correct answer: The foil boat floats because the boat's density is less than that of water.
The ball sinks because its density is lower than that of water.
Correct answer: The ball sinks because its density is higher than that of water.