This week is all about using science to make the weak stronger and the strong weaker. The experiment is a great way to explore the science of pulleys and is also lots of fun.
Start by having a tug of war. Ask your students why one team wins and the other doesn't. Can they balance the teams? You will need two long wooden poles - broom handles work well. There is no need to remove the broom heads. You will also need a candle and a long piece of thin rope.
Now ask your students whether it is possible for one child to win a tug of war against four other students. Rub the broom handles with candle wax - this reduces the friction between the rope and the broom handles.
Get two students to hold each of the broomsticks, and stand opposite each other, holding the broomsticks about 30cm apart. Tie the rope onto one of the broom handles, near the broom head, and wrap it around the two poles three times, making sure that the turns are reasonably close together. Give the loose rope end to another student. Ask the four students holding the broom handles to keep the brooms apart as the student holding the rope tries to pull them together.
Ask your class to predict what will happen, and then try the experiment.
You should find that the student pulling the rope can bring the broomsticks together. You can carry on your investigations by using fewer turns and seeing what happens. Get the students to notice how easy or difficult it is to pull the brooms together, and how much rope they have to pull through.
This experiment is a great way to introduce how pulleys work. The more times the rope is wrapped around the broom sticks the easier it is to pull them together. The only thing that may make it difficult is the friction between the rope and the broomsticks - which increases when there is more contact between the rope and the wood. The wax will help reduce this.
However, the more turns in the rope, the more the rope has to be pulled to achieve the same effect.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn