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Science Y3/Y4 – Forces – week 4

PLUMB-LINES

Truth to Teach (Source)

To learn that all objects have a point where they are held in balance by the force of gravity.

This balancing point is called the centre of gravity.

To learn how to find the balancing point of regular and irregular shapes using a plumb-line.

To discuss the meaning of Amos 7:7-9 where a plumb-line is mentioned.

Way to Work (Means)

1. Review the previous work on gravity.

2. Rest a book on the edge of a table then gradually ease it over the edge. It will balance

until you push it too far and upset the balance. Explain that all objects have a point where
they are held in balance by gravity. Illustrate using a see-saw, or by discussing how a see-
saw works. Talk about the balancing point being called the centre of gravity because it is
where the whole weight of the object seems to centre.

3. Balance a number of regular shapes, eg square, circle, and rectangle on a vertical support.
Ask the children to say where the centre of gravity is for each one (the centre).

4. Put a small round tray of objects on a brick so that it balances then move the objects
around. Let a child try to move the objects so that the tray balances again.

5. Show the children the candle see-saw experiment. Put a nail through the middle of a
candle, having exposed the wick at both ends. Balance the nail horizontally on two tins
standing in a tray of water. The candle should remain horizontal. Light both ends and
notice how the centre of gravity changes as the wax drops off unevenly on both ends. The
balancing point moves from one side to the other.

6. Show the class a plumb-line and read Amos 7:7-9. Ask if they know the meaning before
explaining how plumb-lines are used. We can use them because gravity gives the stone
weight.

7. Let each child make a plumb-line with a stone and string. Let the children investigate to

see if walls, book-cases, etc are straight. Plumb-lines are also used to find the balancing
points of irregular shapes. Cut an irregular shape and hang it on a board so that it can
swivel. Do the same from two other corners of the shape, drawing a line where the plumb-
line goes each time. Where the three lines cross is the balancing point.

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