Lesson 4 - Upthrust and Flotation

density, weight, upthrust, float

density, weight, upthrust, float

• Start
• Formula and units recall questions
• Main
• Prior knowledge checks
• Explain upthrust in terms of forces acting
• Class practical - Newton meters and blocks in water.
• A bit of Maths - proof that upthrust = weight of water displaced
• Design an experiment - demo (boiling tubes in water)
• Textbook questions on slide (there are no exam questions available on exampro for this lesson)
• Plenary
• Check your notes
• Application questions

density, weight, upthrust, float

Practical - Investigating upthrust

Safety

• Per group:
• Newton meter, 100g mass, 2 elastic bands to hang mass from Newton meter, large beaker.
• Glass - breakages

Aim

• Investigate that the upthrust experienced by an object is affected by how much of the object is submerged in water.

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Top tips

• Use both elastic bands to support the block, as shown in the diagram.

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Method

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density, weight, upthrust, float

Demo - What will affect how much of the boiling tube sticks out of the water?

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Safety

• Equipment: Boiling tubes with bungs, marbles, gas jars.
• Glass gas jars and boiling tubes. C
• Take care when putting marble in boiling tube.
• Salt - take care not to get it in eyes

Aim

• Students use what they have learned about upthrust and flotation to design an investigation.

Top tips

• You will need to add lots of salt to the water to change the density enough to see a change in the amount of the boiling tube which sticks out the water

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Method

• Students are shown equipment and asked to design an experiment which will change how much of the boiling tube will stick out of the water.
• Gently put a marble in the boiling tube so it stays upright in the water - put a bung in the top.
• Fill a gas jar with water and float the boiling tube in it.
• Use a ruler to measure how much of the BT is above the water line.
• Remove the boiling tube from the water.
• Dissolve lots of salt in the water.
• Put the boiling tube back in the water and measure how much now is now above the water line.
• The boiling tube should float higher in the more dense (salty) water because the upthrust acting on it is greater.

density, weight, upthrust, float

Lesson 4 - Upthrust and Flotation

Give the units for:

• Density
• Pressure
• Specific heat capacity

Give the formula to calculate:

• Electrical Resistance
• Work done
• Moment of a force
• Weight

density, weight, upthrust, float

Lesson 4 - Upthrust and Flotation

Check your answers

Give the units for:

• Density - kg/m³
• Pressure - Pa
• Specific heat capacity - J/kg°C

Give the formula to calculate:

• Electrical Resistance = V/I
• Work done = Fs
• Moment of a force = Fd
• Weight = mg

density, weight, upthrust, float

Lesson 4 - Upthrust and Flotation

Learning objectives:

• To calculate the upthrust acting on a submerged object by using the pressure difference on the top and bottom surfaces.
• To plan an investigation into the relationship between the average density of an object and the distance it submerges.

density, weight, upthrust, float

Float or sink?

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Why?

density, weight, upthrust, float

Why do helium balloons float?

density, weight, upthrust, float

What about hot air balloons?

density, weight, upthrust, float

Upthrust

Why can we ignore the pressure acting on the sides?

Why the pressure difference?

What would this cause?

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density, weight, upthrust, float

Upthrust

Because pressure increases with depth, the pressure of the water at the bottom of the cylinder is greater than at the top.

So the upward force of the water on the bottom is greater than the downwards force of the water on the top.

The upthrust is the resultant of these 2 forces.

density, weight, upthrust, float

Sink or float? What will happen next?

density, weight, upthrust, float

Practical

density, weight, upthrust, float

A bit of maths:

Pressure difference between top and bottom of cylinder:

p = L_cylinderρ_waterg

Force = pressure x area

So: F_upthrust = (L_cylinderρ_waterg) x A_cylinder

Volume of cylinder = Area x length

So: F_upthrust = V_cylinderρ_waterg

V_cylinder X ρ_water = mass of water displaced

So: F_upthrust = m_{water displaced} x g = W_{water displaced}

density, weight, upthrust, float

So what?

Objects will float if:

Weight of water displaced ≥ weight of object

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Objects will sink if:

Weight of water displaced < weight of object

density, weight, upthrust, float

What will affect how much of the boiling tube sticks out of the water?

Design an experiment to test how mass added to the test tube affects the height the top of the test tube above the water.

What would you expect? Why?

What would the variables be?

• Independent
• Dependent
• Control

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density, weight, upthrust, float

density, weight, upthrust, float

Check your answers:

density, weight, upthrust, float

Check your notes

density, weight, upthrust, float

Both of these are made of steel so why the buoyancy difference?

It is average density that matters - ship is full of low density air

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density, weight, upthrust, float

Why wouldn’t this work in a swimming pool?

This only happens in very salty, dense water.

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density, weight, upthrust, float

Plimsoll line

International shipping regulations specify the amount of freeboard a ship must always have.

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If a ship is loaded in a cold salt water port what would happen to the freeboard if it sails to a warm fresh water port?

density, weight, upthrust, float

Plimsoll line

The ship will float higher in the cold salt water and lower in the warm fresh water.

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This is because salt water is denser, so less water needs to be displaced in order for it to float.

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In warmer, fresh water (lower density) more water needs to be displaced for it to float so the ship will sit lower in the water.

density, weight, upthrust, float

Logos / symbols

density, weight, upthrust, float