Breaking the ice

In this learning sequence students investigate forces, described as a push or a pull. Find out how forces can change the shape or movement of an object. Discuss and explore deformation: breaking, bending and how different materials react under different forces. Plan and conduct a simple experiment to explore how ice and other objects behave when a force is exerted, particularly deforming and breaking. Explore how icebreakers work and relate understanding about forces, materials and deformation to the physics of icebreakers.

Note that many new terms are introduced during this sequence. Terms like ‘force, deform, observe, predict, diagram, object, properties’ will need checking for prior knowledge as they are introduced and students may need assistance to use and understand the terms. Building a word list with pictures and examples, clearly displayed in the classroom and referred to will help students remember the new words. Using the new terms in their every day and scientific contexts will help students add the words into their vocabulary and understand their meanings and when to use them.

Setting the scene

Push it! A discussion of pushing and pulling

Ask students to give examples of situations where something is being pushed or being pulled. Brainstorm as a class and record all examples. Introduce the umbrella term ‘force’, that is used for a push or a pull. Use the class examples to then discuss what the pushes and pulls (forces) do to the objects involved. Invite the class to categorise the examples into ‘changing movement’ and ‘changing shape’. Use this discussion to introduce new terms and to gauge students previous experience describing forces.

Discuss what is meant by ‘changing movement’ (e.g. going faster or slower, going around a corner) and ‘changing shape’ (e.g. flattening plasticine, squeezing a ball, sitting on a cushion). Invite students to explore these ideas in pairs, using balls – rolling, throwing, bouncing – then as a class discuss how students changed balls and what pushes and pulls were involved.

Ask students to think about a situation in the last day or so where they have used a push or a pull. Examples could include: riding a bike, pushing a trolley or pram, pulling a cart, closing a door, chewing their breakfast. Ask them to draw the situation and describe the force as a push/pull and then what the outcome of the force was. Share the drawings as the class.

Is breaking bad?

Ask students if forces can vary in how strong they are, and ask for examples.

In pairs, have them choose one example of a force that they know, and describe the difference between a small amount of force applied to the object, and a large amount of force applied. What happens to the object in each case? (Examples to stimulate discussion: closing a door or slamming door, putting a cup down on a table versus throwing a cup.) Ask each pair to share their small force/big force examples.

From this discussion, ask students what happens when so much force is applied to an object that its shape changes and it can’t change back to normal? An object is often broken if this happens. Students will readily be able to give examples of things that have broken because of too much force, invite discussion around this and objects that change shape only temporarily e.g. cushions, foam balls, or rubber bands.

But is breaking always bad? Ask students for examples where we deliberately change the shapes of objects permanently. Examples to start them off: fire alarm covers, cutting up food, mincing meat, or breaking seals on medicine.

Explore and research

Forces at sea

Show students the following images of pushing and pulling at sea:

(Plus other examples, tugs, towing, lifting)

Discuss what is applying the force and what the object is. Is it a strong force or a weak one? What is the effect of the force? What would happen if more force was applied?

Show students an image of an icebreaker in an ice sheet. Discuss the forces here – how does a boat move in ice that’s a metre or more thick?

Breaking ice

To further investigate this question, students will compare the forces needed to deform or break a block of ice and other objects by performing an investigation:

Activity for students (individually or pairs/small groups)

Introduction:

  • Give students a selection of different objects of similar sizes (around the size of a block of ice) e.g. ball of plasticine, ice block, half a Weetbix, piece of butter, small foam ball, grape.
  • Handout table with four columns, headed: object, properties, deforming weight (prediction), deforming weight (observed)
  • Explain that they will be testing the objects to see which ones will deform and which will break, when a big enough force is applied.
  • In the first column of the table, draw and write the name of the objects you will test
  • In the second column, briefly describe each object’s properties: Soft or hard? Bendy or brittle? Heavy or light?
Experiment:
  • Demonstrate: Place object under wooden board e.g. chopping board.
  • Add weights on top (sand bags, lead weights, pots filled with water).
  • Discuss: does the weight of the board count? Is the force of the weight a push or a pull? Which item will need the most force to deform? Which will need the most force to break?
  • Ask students to predict how much weight will be added before the first object’s shape changes (deforms); record in third column table
  • Have students add weights until they observe their object’s shape change. Record the weight in the fourth column. Discuss whether the change is permanent or temporary. Is the object broken?
  • Repeat for three other objects.

Explain and share

Breaking things

As a class, discuss the how the experiment went. Ask students:

  • Which items deformed and which items broke?
  • Did your results match your predictions?
  • What sorts of items are more likely to deform and then return to their original shape? Which items are more likely to break permanently?

Ask students to draw ‘before and after’ pictures of one of their items.

Elaborate and apply

Breaking the ice

Introduce the Antarctic environment, using a globe or Google Earth. Explain that in southern Antarctic waters, ice sheets can be a metre or more thick. Icebreakers are designed so that the front of the ship curves under.

Ask students why they think icebreakers are designed with curving prows. Have them discuss whether there are pushes and pulls involved, and what is pushing what, and in which directions?

Ask students to draw a diagram showing how this would work.

To help students think about what to draw and add to their diagrams ask them to:

  • Describe what happens to ice when a large weight is placed on top.
  • Explain how this helps the icebreakers and what the forces are (pushes and pulls) when an icebreaker moves over ice.

Evaluate

Use the predict, observe, explain model to help students design their own experiment.

Guide students to explore a simple ‘what if’ question; some examples are below. Ask them what they think would happen in their ‘what if’ scenarios (predict). Then help them design a simple experiment and test their idea (plan, observe). Finally ask them to share what happened with the class, and why they think it happened (explain).

‘What if’ questions:

  • What happens if the ice pieces are thinner?
  • What happens if the ice isn’t resting on a hard surface?
  • What happens if the ice isn’t pure ice? (Try adding dirt, leaves or salt to your water before you freeze it.)

Assessment ideas

At various points in the learning assess to what extent students:

  • use the properties of the objects to help them predict and explain their results
  • identify forces as pushes and pulls
  • with guidance, plan and conduct an experiment to test their ideas
  • explain their predictions and why their results varied from what they expected
  • describe and explain their results using tables and diagrams
  • describe their understanding of forces using scientific language, drawings, diagrams

Share your experience

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