11—14 science lesson 2 - british dam s

Lesson plan: 11—14 ScienceThe British Dam SocietyThe British Dam SocietyThe British Dam Society

About this lessonThis double lesson helps students consider, in very simple terms, four forces acting on a dam and develops their understanding of balanced and unbalanced forces. Students create force diagrams for a simple dam and explore what might happen to the dam if any pair of forces becomes unbalanced. Students then use their understanding to design a dam for one of three simple scenarios. Students use their understanding of forces to justify their design choices. Students develop their ideas by exploring and explaining why some dams are curved and how the water pressure on the dam helps secure the curved wall in place.

The lesson builds on students’ learning from lesson 1, but can be delivered independently.

Learning objectivesAt the end of the lesson students should be able to:

• Recall that forces act in pairs and can be balanced or unbalanced.

• Create a simple force diagram to show the main forces acting on a dam.

• Design a simple gravity dam, using their understanding of forces to justify their choices.

Background informationA dam is a structure built to form a reservoir to collect, retain and store water. Man has used dams since prehistoric times; the earliest known dams were built in the part of the Middle East and beyond known as the ‘fertile crescent’. These early dams managed scarce and unpredictable water supplies during the dawn of agriculture and the earliest civilisations.

There are nearly 500 large or major dams in the UK today. Many were built in the 19th century in response to the needs of growing industrial towns and their populations — as the UK’s population grew, dam construction continued apace.

Most UK dams store water to secure its supply and manage variations in availability across the seasons, or to provide large or small–scale fl ood management. While some include hydroelectric power, this currently only contributes to around 1% of UK electricity consumption.

Most new dams currently being built in the UK are to manage water at a local level for fl ood alleviation. While many are small, some can reach over 8m high and store over one million cubic metres of water. While it is rare for a new dam in the UK to be as large as those built here in the past, overseas many very large dams are still being constructed.

11—14 science Lesson 2 11—14 science


Resources and preparationFor the plenary, you may wish to demonstrate the deforming action of a force on straight and curved pieces of thick card or plastic.

• You will need to two pieces of thick card about 5—10 cm x 30cm (e.g. cut from an A4 sheet).

• Curve one piece over a desk by pulling it over the edge while pressing down at the point where the card moves over the edge.

• Support each piece between two heavy blocks that just trap the card, then press down gently to show that one falls while the other stays in place:

Diagram of forces demonstration


Lesson plan (90 minutes)Amend the suggested timings to suit your students and delivery.

Timing Activity Learning objectives Assessment

10 minsStarterHow does the force on a dam vary with depth?If you delivered lesson 1, begin by asking students to recall and share their understanding of how pressure (and therefore the force on an area of dam wall) varies with depth, using their investigation results to illustrate their ideas.

If you did not deliver lesson 1, pose the question and use other examples (like submarines, divers or submersibles) to help students identify that pressure increases with depth.

Discussion, questioning.

5 mins What do we know about forces?Explain that in today’s lesson, students will use their understanding of forces to help them design a dam. Ask students to review what they can remember about forces and how they act, in pairs or small groups. Share ideas. Show slide 2.

Recall that forces act in pairs and can be balanced or unbalanced.


15 minsMainWhat forces act on a dam?Suggest to students that two pairs of forces act on a dam. In pairs or individually, students sketch these forces to create a simple force diagram, showing clearly how the forces act in pairs. Share ideas. Show slide 3.


Create a simple force diagram to show the main forces acting on a dam.

Written work, questioning, discussion.



10 mins What if these forces are unbalanced?Ask students: if the dam is stable, what does this suggest about each pair of forces? (They are balanced.)

Review the possible effects of an unbalanced pair of forces on an object (e.g. that unbalanced forces can produce a change in motion (speed and or direction), or deformation).

To help students, ask them to consider a boat fl oating on the water behind the dam. (You may want to sketch a fl oating boat on your board, to help your discussion.) What pair of forces act on the boat? (Gravity and up–thrust or buoyancy). Discuss what might happen to the boat if these forces stop being balanced, for example, too many people climb on board so that the downwards force of the weight of the boat and its load (caused by gravity) is greater than the up–thrust caused by the boat’s buoyancy.



5 mins Ask students to discuss what might happen to the dam if each pair of forces becomes unbalanced. (Optionally, students can illustrate their ideas by creating new force diagrams for the dam.) Share ideas. Show slide 4.

15 mins Design a damExplain that students are going to design a suitable dam for one of three scenarios. Their dam design should take into account the forces that will act on the dam, so it will remain strong and stable. Hand out one scenario to each pair or individual and review using slides 5—7, to help each group understand the environment in which they must place their dam design.

Ask students to complete two sketches to show 1) the position and shape of their dam, and 2) a simple side view to show its cross–sectional design. They should use the information given, and the map, to determine the best positions and cross–sections of each dam.

Best positions are:Sc1: on the highest point on the right hand side,within the top contour.Sc2: higher up the valley, immediately below the confl uence of the two streams. Sc3: where the valley narrows near its lower end, running from top to bottom to join the two spurs.

Observations, questioning, written work.



5 mins Invite selected students to share their ideas, using their understanding of forces to explain their design choices. (If students designed a curved dam across a valley, ask them to explain briefl y why they chose this shape — curved dams are discussed in the plenary.)


10 minsPlenaryWhy do some dams have curved walls?Show slide 8. Ask students to explain why the curved wall might be a stronger design than a straight wall.

To help students, demonstrate using the fl at and curved pieces of card, as outlined in ‘resources and preparation’ above. Press gently on each one to show how one will tend to ‘pop’ out while the other presses into place. Ask students to suggest why this is, using what they know about forces, deformation and acting in pairs (students should remember that there will be an equal reaction force to the force of the dam pressing against the sides).


Create a simple force diagram to show the main forces acting on a dam.

Questioning, discussion.

10 mins Ask students to sketch an explanation, showing the balanced forces acting on a curved dam in a narrow valley (students should use an aerial view to show the water pressure and reaction, and the pressure exerted by the dam walls on the valley sides, and again the reaction on each side).

Discuss ideas, then show slide 9.



Suggested questions• What did we learn about how pressure varies with depth?

• What does this tell you about the force of the water on two areas of dam wall, near the top and near the bottom?

• How might a dam be designed to withstand these forces?

• What can you tell me about forces?

• What’s the name of an equal and opposite force that ‘presses back’?

• What happens if the forces in a pair are not of equal strength?

• What four forces might act on a dam wall?

• If the dam is stable, what does this tell you about each pair of forces?

• What might happen if the reaction force from the ground is not equal to the dam’s weight pressing down?

• Why might this reaction force become less?

• What might happen if the dam’s reaction force isn’t equal to the force from the water pressing against it?

• Why might this reaction force become less?

• What shape of dam might work well in this place?

• Where will you put your dam, and why?

• What cross section would your dam have?

• Why is your dam a good design?

• What happens to the shape of the fl at card when I press down?

• What happens to the forces acting through the ends of the fl at card when I press down?

• What happens to the shape of the curved card when I press down?

• What happens to the forces acting through the ends of the curved card when I press down?

• Can you explain why a curved dam might be stronger than a straight one, in a valley?

• What kind of valley walls would you need for a curved dam to be strong?

• What if the valley walls were soft, and not hard?


Differentiation and extensionMake easier: Share some ‘true or false’ statements about forces with students, for them to sort. Provide students with the lower part of the forces worksheet, to cut out and stick on each force in the correct place to form pairs of equal and opposing forces. Omit a discussion of what might happen to the dam if each pair is unbalanced. Use one scenario (we suggest scenario 3). In the plenary, demonstrate using the straight and curved pieces of card.

Make harder: Remove the lower part of the forces worksheet with the ready–made arrows to cut and stick in place. Include a discussion of what happens if the forces are unbalanced and ask students to explain how the dam might a) move (including sliding, and tipping due to a turning moment for the most able — where might the pivot point be?) or b) deform (eg break apart). You may wish to discuss how different dam materials might behave, including concrete or clay. Use all three scenarios.

Extend: Students could build and test model dams in water troughs, using modelling clay or other materials to form them. What is the maximum volume of water they can trap using a given volume of material? Can a curved dam trap more water than a straight one? Students could develop and test a range of predictions and fi lm their dams’ performance and / or failures.

Students could use maps and other sources to locate the nearest dam or reservoir to your school and investigate its design.

Divers inspecting the underwater sections of a dam must deal with increasing pressure at depth. Students could create a simple ‘Cartesian diver’ using a sauce sachet, 1 litre bottle and water. Can students explain how this works, to move the sauce sachet ‘diver’ up or down?

https://www.youtube.com/watch?v=OM6iC6N12xY

The heavy machines that are used to prepare the ground and build dams rely on hydraulic pressure to operate their digging arms. By varying the areas of linked pistons, a hydraulic system can exert enormous force. Using their understanding of P = F/A, students could investigate how linked syringes of different diameters can be used as force multipliers.

Dams are extremely heavy structures and can only be built on suitably solid and stable ground. Can students use their understanding of forces and pressure to explain why it’s helpful that dams tend to have a large area at their base?


Curriculum linksEngland

• Science KS3

• Forces as pushes or pulls; using force arrows in diagrams; balanced and unbalanced forces; pressure in fl uids, increasing with depth; pressure measured by ratio of force over area; investigative skills; how science works.

Scotland

• Sciences

• Learners develop an understanding of how forces can change the shape or motion of an object, considering both forces in contact with objects and those which act over a distance; investigative skills.

Wales

• Science KS3

• The properties of liquids; the forces in devices; Enquiry, developing and refl ecting.

Northern Ireland

• Science KS3

• Forces and energy; practical and investigative skills.


What forces act on this simple dam?

• Draw a force diagram on the dam cross–section below.

• Include four forces.

• Think about how long you draw each force arrow.

• Label each force.

Student worksheet:Forces on a dam

Cut out each force arrow and glue it in the correct place on the dam:

Wei

ght

Forc

e fr

om w

ater

pre

ssur

e

Reac

tion

fro

m g

roun

d

Reac

tion

fro

m d

am


Build a dam: Scenario 1

A farm reservoir for irrigation

A farmer needs to store water at the top of a hill, to water crops when there is no rain. They will pump water up to the dam from a river in the valley below.

• The farmer needs to build a small square or circular reservoir with a dam wall.

• Where will your dam go? Sketch an aerial view of your reservoir dam on the map below.

• Draw a cross–section of your dam wall and label the forces acting on it.

Cross section:



A micro hydro project for generation of electricity

A village wants to generate electricity from the stream that fl ows down a nearby mountain. The water will fall down a pipe to a turbine at the bottom — the higher, the better.

• Where will your dam go? Sketch an aerial view of where you will dam the stream on the map below.


Cross section:



A new reservoir for drinking water supply

A valley is to be fl ooded to become a new reservoir that will store water for a populated area of the UK. The water company want to store as much water as possible.

• Where will your dam go? Sketch an aerial view of where you will dam the valley on the map below.


Cross section:


Forces on a curved dam

This is an aerial view of a curved dam that stores water in a steep valley. The valley sides are strong and made of solid rock.

On the picture provided, draw the forces acting on:

• The middle of the dam wall.

• Each end of the dam wall.


What is the British Dam Society?The British Dam Society (BDS) is an Associated Society of the Institution of Civil Engineers.It consists of individual and corporate members. It is open to anyone wanting to share experience or knowledge of all aspects of dams and reservoirs. BDS exists to advance the education of the public and the profession in technical subjects relating to the planning, design, construction, maintenance, operation, safety, environmental and social issues of dams and reservoirs.

11—14 science lesson 2 - british dam s

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