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Ellen's moments

Learning objectives

� Students will be excited by physics, and realise the usefulness of the laws of physics in extreme sports
� Students will use the principle of moments to explain why sailing boats balance

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Curriculum link

� 9L pressure and moments – use the principle of moments to explain a range of situations

Running the activity

Suggested time: 20 minutes

Possible starter:
Ask students to list, in pairs, three dangers that Ellen MacArthur faced on her record-breaking round the world trip. Take feedback. Then show students page 1, which emphasises the dangers of strong winds and instability. Page 1 can either be projected or displayed as an OHT.

Main activity:
Set the task by showing students page 2 – ask students to use the information on this page and page 3 to decide whether Ellen's rival should buy a keelboat or a multi-hull boat. As well as planning what to say, students can draw diagrams to support their decisions. They also need to plan how to answer Ellen's rival's questions, which are in 'thought bubbles' on page 3.

Plenary:
Ask some students to report on what they will say to Ellen's rival. Take a class vote on which type of boat he should buy.

Further information:
When wind blows against a boat sail, it doesn't naturally blow the boat along. It would be much more normal for the boat to blow over. The force of the wind on the sail produces a large turning moment that makes the boat rotate, and fall over. To keep the boat upright and sailing there has to be a turning moment in the other direction to keep the mast upright.
This activity highlights two types of boats:

1.Keelboats
These have a big heavy weight under the boat, made of a very dense metal such as lead. As the boat gets pushed over by the wind this gets pushed up to produce an opposite turning force to the sail and mast. So the boat doesn't fall over. If the boat does fall over, it may still be possible to right it again.

2. Multi-hull boats
These have additional buoyancy hulls on the sides of the main hull, which are held there by arms. When the boat heels over due to the wind on the sails, the buoyancy of these makes a turning force to keep the boat upright. Multi-hulls are lighter than equivalent sized keelboats, and so are faster in almost all wind conditions – sometimes by as much as 50%. If a multi-hull boat is flipped over, it cannot be righted.

Even so the skipper (Ellen) had to keep a constant watch to balance the turning effect of the wind on the mast. Disasters can strike if the wind gets too strong. Ellen monitored the pressure of the wind on the sail, and reduced the sail area if the wind got stronger.
This reduced the force pushing the boat along – but also reduced the chance of it getting pushed over.

Web links

News links

Team Ellen

There is comprehensive information about Ellen MacArthur's record-breaking journey at this site.

Mulithull pages

This site has information about multi-hull boats

The Times Herald

This has some useful information about keelboats

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